Hydrocyanic-acid gas fumigation in California

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Material Information

Title:
Hydrocyanic-acid gas fumigation in California
Series Title:
Bulletin / U.S. Dept. of Agriculture, Bureau of Entomology ;
Uncontrolled:
Fumigation of citrus trees
Value of sodium cyanid for fumigation purposes
Chemistry of fumigation with hydrocyanic-acid gas
Hydrocyanic acid gas fumigation in California
Physical Description:
113 p., 8 leaves of plates : ill. ; 23 cm.
Language:
English
Creator:
Woglum, R. S ( Russell Sage )
McDonnell, C. C ( Curtis Criss ), 1875-
Publisher:
U.S. Dept. of Agriculture, Bureau of Entomology
Place of Publication:
Washington, D.C
Publication Date:

Subjects

Subjects / Keywords:
Citrus -- Diseases and pests -- Control -- California   ( lcsh )
Fumigation   ( lcsh )
Genre:
federal government publication   ( marcgt )
bibliography   ( marcgt )
non-fiction   ( marcgt )

Notes

Bibliography:
Includes bibliographical references and index.
Additional Physical Form:
Also available in electronic format.

Record Information

Source Institution:
University of Florida
Rights Management:
All applicable rights reserved by the source institution and holding location.
Resource Identifier:
aleph - 029689742
oclc - 22582351
lccn - 13000011
Classification:
ddc - 632
bcl - 48.63
System ID:
AA00026004:00001

Full Text



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U. S. DEPARTMENT OF AGRICULTURE,
BEU OF ENTOMOLOGY-BULLETIN No. 90.
L O. HOWA a ief of Bureau.


SYDROCYANIC-ACID GAS FUMIGATION
IN CALIFORNIA.





I. FUMIGATION OF CITRUS TREES.
By R. S. WOGLUM, M. S. A.. 8pecialField Agent.


II. THE VALUE OF SODIUM CYANID FOR FUMIGATION PURPOSE.
By I. S. WOGLUM, M. S. A., Special Field Agent.


lL. CHEMISTRY OF FUMIGATION WITH I.YDROCYANIC-ACID GAS.
By C. C. M ONNELL Chif, Insctic rand Fungicide Laboratory,
Miscellaneous Dilision, Bureau of Chemistry.























WASHINGTON:
GOVERNMENT PRINTING OFFICE.































W. F. TASTET, Chief Clerk.

F. H. CHITTENDEN, in charge of truck crop and stored product insect investigations.
A. D. HOPKINS, in charge offorest insect investigations.
W. D. HUNTER, in charge of southern field crop insect investigations.
F. M. WEBSTER, in charge of cereal and forage insect investigations.
A. L. QUAINTANCE, in charge of deciduous fruit insect investigations.
E. F. PHILLIPS, in charge of bee culture.
D. M. ROGERS, in charge of preventing spread of moths, .field work.
ROLLA P. CURRIE, in charge of editorial sork.
MABEL COLCORD, in charge of library.

CITRUS FRUIT INSECT INVESTIGATIONS.

C. L. MARLATT, in charge.

R. S. WOGLUM, W. W. YOTHERS, E. R. SASSCER, J. R. HORTON, P. H. TIMBERLAKE,
C. E. PEMBERTON, H. L. SANFORD, entomological assistants.
J. G. SANDERS, collaborator.








,~

















LETER OF TRANSMITTAL


UNED STATE DEPARTENT OF AGRICULTURE,
BUREAU OF ENTOMOLOGY,
Washington, D. (., September 11, 1912.
Sin: I have the honor to transmit herewith, for publication as
Bulletin No. 90 of this bureau, three papers comprising a report on
Sinvestigation of hydrocyanic-acid ga fumigation of citrus ohards
in southern California. The preliminary report on the subject was
published as Bulletin No. 79 of tis bureau under the title "Fumi-
gation Investigations in California." The present final rport is
divided into three parts, which were published separately on May 10
nd 13, 1911: I, entitled "Fumigation of Citrus Trees," by R. S.
oglum, a special field agent of this bureau, containing the mai
report on the field investigations and discussing the various details
of figation procedure; II, a paper by Mr. Woglum, "The Value of
Sodium Cyanid for Fumigation Purposes"; and, III, one by Mr. C. C.
IMconnell, Chief of the Insecticide and Funicide Laboratory,
Bureau of Chemistry, "Chemistry of Fumigation with Hydrocyanic-
acid Gas."
Respectfully, L. O. HOWARD,
Entomologit and Chief oo Bureau.
Hon. JAMES WILSON,
Secretary of Agriculture.


















Digitized by the Internet Archive
in 2013












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PREFACE.


Sinvestigation into the methods of fumigating citrus tree with
hydrocyanic-acid gas was commenced by the Bureau of Entomology
United States Department of Agriculture, during the summer of
1907, and for a period of three year has been carried on in California
by the writer under the direction of Mr. C. L. Marlatt, assistant chief
of the bureau. This work was undertaken in response to urgent
requests from the horticultural commissions of the principal citrus-
fruiproducng( counties of southern California and of many active
fruit rowes. Prominent in this mvement was Mr. J. W. Jeffrey,
former secretary of the Los Angeles County horticultural co ision,
and now State commissioner of horticulture-a man entirely familiar
with the unsettled condition of fumigation practice at that time and
with the need of placing it on a more scientific basis. At the com-
mencement, the writer spent from three to four months in a thorough
field investigation to acquaint himself with the conditions of citrus
culture throghout southern California, the distribution of the dif-
fernt citrus pests and the damage caused by them, the existing
methods for their control, and the methods of fumigation practiced
ct"'|. the
in the various citrus districts.
During the early part of November, 1907, active experimental
fieldwork was commenced at Orange, CM., using an outfit belonging
to th bureau, consistincg of four tents and the other paraphernalia
ary for practical fumigation. Fiel work of this charactr
hs been continued throughout, it being the writer's effort to conduct
the investigation on s nearly a commercial bais as possible so that
the conditions and results would be those normal to the ordinary
care of citrus groves. During the work there have arisen many
problems of a laboratory nature, the solution of which would have
been most interesting, but these problems for the most part have
been set aside except in those cases where they had a direct economic
bearing on practical work in the field.
The results of this invesiation have very little of the nature of
original discoveries, although there has been acquired a vast amount
of exact information never before thoroughly understood. The
advance is largely the result of correcting, correlating, systematizing,
d p upon a more scientific as well as a more practical basis
methods which had been praticed in California or elsewhere for
many year.
V





VI HYDROCYANIC-ACID GAS FUMIGATION IN CALIFORNIA.
All information, as acquired, of direct bearing on the fumigation
practice has been given freely to the public as soon as its economic
value was established-, largely by means of addresses, demonstrations,
and printed reports.
In the present bulletin an attempt has been made to present a
succinct account of the completed results of this fumigation investi-
gation as well as a brief treatment of the salient features of fumi-
gation as practiced in California at the present time. It is of the
nature of a handbook on the most up-to-date equipment, methods,
and directions in orchard fuinigation. Full advantage has been
taken of the results of other investigators in fumigation; yet in such
cases due credit is given to the proper source. The information
given in Bulletin 79 of this bureau, which is a preliminary report on
this investigation, has been largely included in the present bulletin
in summarized form.
The writer desires to acknowledge his indebtedness to the many
people who have assisted him during this investigation and facilitated
the progress which has been made. To Mr. C. L. Marlatt, Assistant
Chief of the Bureau of Entomology, he is especially indebted for
valuable assistance and advice. Acknowledgment is also due to
Mr. Frederick Maskew, who most capably assisted him in the per-
formance of many of his experiments during the period from Decem-
ber, 1907, to August, 1909. Valuable assistance was rendered by
Mr. E. R. Sasscer during the months of August, September, and
October, '909. Mr. W. W. Yothers was engaged in the work during
November, 1909. To the Hon. J. W. Jeffrey, State commissioner of
horticulture of California, credit is due not only for his activity in
paving the way for this investigation, but also for the able support
given by him since field work was commenced. To Mr. William Wood,
of Whittier, Cal., the writer acknowledges his indebtedness for
assistance in introducing the improved system of fumigation in the
region adjacent to Whittier, as well as for practical advice with regard
to citrus insects and their control, a subject about which Mr. Wood
is especially well informed. This occasion is also taken to thank the
various horticultural officers of southern California, packing-house
managers, and the many citrus growers who have assisted and sup-
ported this investigation.
The cooperation of the Bureau of Chemistry of the United States
Department of Agriculture has been an important adjunct of this
investigation, and to Mr. J. K. Haywood, Chief of the Miscellaneous
Division of that bureau, and his assistants the writer is indebte for
the carrying out of all the chemical analyses and laboratory tests of
materials and products necessary to the working out of the field
experiments.
R. S. W..








CONTENTS.


igin of citrutrees................................... S. Woglum.. 1
H itorical ............................................................. 1
Recent renewal of interet in igation in California.................... 2
Extent to which is p ticed in Cafor .................... 3
The v ius system .............. ......................... 4
By t ................................................... 4
By ociations .................................................... 5
By counies.....................................................------ 5
By private individuals............................................. 5
Extent and cha ter of citrus orchards .................................. 6
ct enemies of citrus fruits.................................. 7
The puple scale ( pidoaphes becii Newm.) ........................ 8
The black cale ( et oe B .) .. ........................ 8
The red scale (Chrsomphau urt as ........................ 9
The yellow cale (hrysomphalu itrinus Coq.)...................... 10
The mealy-bug (Pseudococcu citri Riso).......................... 10
Apparatus............................................................. 10
Tents............................................................... 10
Poles and derricks................................................. 20
The McFadden machine............................................ 21
upplycarta nd supply wagon....................................... 22
Generating ve el ...... ............... ................... .... 24
General procedure ........... ......................................... 24

Securing the measur nt a.und and .. or. .. ... ..... . ............ .28
The old ethod of procedure.....................................
proved systeof fumigtion....................................... 32
Sof g.................... ................ 33
e s hedule ................ ....-.... ................ ..... 34
oncd r .... ...... ........................................... 34
Advantages under this system.. ....................................... 37
Experience with this system....................................... 38
The cin fu igai.. ............ .. .... ................. 40
Pot ium cyanid(KCN) ... .. .... ....... .... ........... ........ 40
phur (...i c..d...........H......................- 41
Waterasfactor in fumiation...................................... 44
The mst economical proportion f hemicals to use................. 47
The amount of chemicals in very small dosages .... .... ......... 48
Mixing the chemical........ ............................ ...... 48
Effect of the presence of sodium chlorid on the amount of gas given off. 49
Natureof theresidue---------------...................................... 50
Dosages for various scale pests----...............-----------.............------------------.......... 51
Factors which affect the dosage...................................... 52
The purple scale................................................... 53
The r d scale............ ........................................ 57
The black cale.................................................... 59
The yellow scale ................................................... 60
Dosages in general fumigation ...................................------------------------------------ 61
ST three papers constituting this bulletin were issued, the frst on May 13, 1911, and the last two on
ay 10, 1911.
V.






VIII HYDROCYANIC-ACID GAS FUMIGATION IN CALIFORNIA.

Fumigation of citrus trees-Continued. P .
Time of the year for fumigation......................................... 61
Fumigation for the mealy-bug........................................... 63
Fumigation during the blossoming period................................ 64
Fumigation while the fruit is of small size-.............................. 65
Fumigating lemons-----------------------------------------6
Fumigating lemons.................................................... 65
Effects of fumigation on unhealthy trees.....................--........... 66
Greater susceptibility to injury of some varieties than others.............. 67
The distribution of gas within a tent----...............-----------------------------. 67
Fumigation for physiological effects-----------------.................----------.... ..... 67
Effects of meteorological elements on fumigation.......................... 68
Injury to sprayed trees-................................................ 72
The appearance of fumigated trees...................................... 73
The presence of old scales on fumigated trees-.................... ........ 74
A device for covering fumigation generators. -------................... 74
The effect of climatic conditions on scale insects......................... 76
The effect of fumigation on ladybirds (Coccinellidse) and Scutellista cyanea
M otsch...............................................-............... 77
The cost of fumigation................--..------------------------...... 78
General cautions------------..................................----.....--......... 80
List of the writer's published articles and addresses on the fumigation
investigation in California ....*-......................-................. 81
The value of sodium cyanid for fumigation purposes--.------. R. S. Woglum. 83
Introduction .. --...--- --.................................................. 83
Strength of sodium cyanid expressed in terms of potaksium cyanid........ 85
Proportion of chemicals................................................ --85
Field tests...............------------...........................----.........------. 86
Action of sodium chlorid ..---................................. ..---.... 87
The kind of cyanid to purchase--............................--.. ....--. 88
Dosages with sodium cyanid........................................... 88
Dosages recommended for scale pests.......................-------... ... 89
Comparison of sodium cyanid and potassium cyanid for general fumigation. 90
Chemistry of fumigation with hydrocyanic-acid gas -....... C. C. McDonnell.. 91
Introduction..................-----------------............................---...... 91
I. Analyses of chemicals used for the production of hydrocyanic-acid gas. 91
Sulphuric acid..........---............... ........... ...........-- 91
Cyanid samples.. --..............-------.........--------------------. 92
II. Proportion of cyanid, sulphuric acid, and water for best yield of gas.. 93
Potassium cyanid..............................---------............. 93
Sodium cyanid-------.................................... ....... .. 93
III. Action of mineral acids on cyanids and hydrocyanic acid............ 96
Action of sulphuric acid on hydrocyanic acid--................ ....... 97
Action of hydrochloric acid on hydrocyanic acid...................-.. 98
Effect of the presence of sodium chlorid in cyanids on the yield of hydro-
cyanic-acid gas in fumigations ......................-....-......... .. 99
SDescription of apparatus------....................................... 10
Details of manipulation................................... -......... 100
Results of experiments.......................... .................. 1 01
Ammonia formed from the decomposition of the cyanid..................
Effect of the presence of sodium nitrate in cyanids on the yield of hydro-
cyanic-acid gas.....................................-............... 103
Summary ............. .... ........ ...................................- 104
Index....................................................................- 107











ILLUSTRATIONS.


PLATES.
Page.
TE eth ofvering small tree with bell or hoop tent ........... 18
I. Fig. -Brick furnace and drrk used in the tannin treat-
formildew, S Bernardino County, Cal. Fig. 2. achine
or coverin trees with ................ ..... 20
III Fig 1.- ethod of attaching tent to hoting pole by a half hitch of
he rope. 2.-Top of derrick, showing method of attaching
pulley. Fig. 3.-Base of derrick, showing mehod of construct-
in b~lrace...................................... ............. 22


al........................................................ 22
F 1.Sly c u with te imprv system of fumiation.
ig. 2.u wa n devied by C. E. McFadden, of Fullerton,

V. Figs. -5. ucce staes in placi a tent over a tree with
poles. Fig. 6.-A tent ree showing method of scuring the
distance around the bottom of the tent by means of a tape at-
tached to an iron rod............................ ........... 2
VI. Reoving the tent frm one tree onto another by mans of poles. 26
VII. Placing a sheett ttover a tree by mans of derricks ............. 2
VII. Fig. 1.-A ow of tented trees, with art at one end f ow, ready to
com ne osin. F. 2,-4Dsing a tre... 38
IX. a Scheule A. for gh-grade sium cyanid .............. 88
Doage Schedule -A. for hih de sdium canid ............. 88

TEXT FIGURES.

. 1. Map showing principal localities in southern California where citrus
ruit ar produc .............................. .. ..... ......- 7
2. Plan for construction of octaonal sht tent, 50 feet acro, howing
lines used in onstructing octaon................................. 14
3. ethod of attachin hos tent when coverig trees with aid of der-
ricks ......... ................................................ 1
4. Ends of hoisting poles used in placing ents over tre............... 20
5. Earthenware acid jar with attachments for field use ................. 23





8. Man carrying tray and water bucket as practiced under the old system
f ion....................... ............................. 31
9. scedulNo. 1, for potaiumcanid ...................... 34
10. Chart showing total amount of gas evolved when different proportions
of water are used................................................ 45
11. Do e schedle No. for potassium cyanid....................-- .... 59
12 A c. device attached to a fumigation generator- 75
13. Labo ry apparatus used in the decomposition of cyanids and collec-

ix
.lf th liberate hydrocyanic-acid gas--- ----- ------10%)
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U. 8. D. A., B. E. Bul. o, Part 1. Issued May 13,1911.

HYDROCYANIC-ACID GAS FUMIGATION IN CALIFORNIA.


FUMIGATION OF CITRUS TREES.
By R. S. WOOLUM,
Special Field Agent, Bureau of Entomology.
HISTORICAL.'

To Mr. D W. Coquillett, of the Bureau of Entomology, United
States Department of Agriculture, belongs the credit of first deter-
mining the great value of hydrocyanic-acid gas for destroying insect
pests on lts. During the fall of 1886, while a special agent of
what was then the Division of Entomology, experimenting upon the
cottony-cushion scale (Icerya lrclsi Mask.) in orange orchards in
California, e discovered this gas to be a most efficient insecticide for
scale-insect pests of citrus trees, and his continued experimental
work placed its use on such a practical basis that by 1890 it had com-
menced to be employed quite extensively in a commercial way.
The use of this gas was restricted to California until the winter of
1892-3, during which time Prof. Morgan gave it a trial on
orange t s in southern Louisiana. The following year, 1893, found
on trial against the San Jose scale in Virginia and against citrus
inct pests in Florida, in Montserrat, British West Indies, and in
a Colon, South Africa. Its subsequent developent and use
has been rapid as well as extensive, so that to-day fumigation of citrus
tres is carried on in California, Florida, Australia, Japan, and the
colonies of South Africa. At the time of this writing the practice is
being troduced into Spain and Poto Rico.
The great success attending the hydrocyanic-acid-gas treatment
against the scale pests of citrus trees soon brought about its intro-
duction into a broader field of activity. This gas was given its first
trial on deciduous trees by Mr. D W. Coquillett in 1894 at Char-
The Canadian Entomologist for 1877, volume 9, pages 139-140, contains mention of an experiment by
mes T. Bell which an insect cabinet was freed from insect pests by dropping sulphuric acid on
iu cyanid. This is the frst record, so far found, of the rapid development of the gas by combin-
Ing sulphuric acid with lump cyanid with the object of killing insects. The use of cyanid, however, as a
means of killing nsects in collectors' bottles Is very old. The gas liberated from moistened lump cyanid is
m that by the action of sulphuric acid or hydrochloric acid, on the authority of Dr.
J. K. Haywood, of the Bureau of of this department. The action of the acid merely hastens
the generation of the gas. It does not seem desirable or appropriate, therefore, in a discussion of the
Suse of this gas or the destruction ofnsects in orchards o in buildings, to consider
these much earlier and inor uses of the gas by collectors for killing insects, or similar limited uses. -
C. L. M.
si " 1 ^ "





2 HYDROCYANIC-ACID GAS FUMIGATION IN CALIFORNIA.

lottesville, Va. The same year it was first used in te treatment of
nursery stock, and its development along this line has been so great
and important that to-day in many States the fumigation of decidu-
ous stock before it is planted is required by law. The use of hydro-
cyanic-acid gas against insects affecting greenhouse plants has been
successfully carried on for a number of years. Among the other
important uses to which this gas has been successfully put are the
treatment of mills, various other buildings, and stored products
infested with insects. The ease with which this gas may be generated
as well as its destructive power, greater than that of any other known
insecticide, leads the writer to believe that as soon as the various
uses to which this gas may be put have been thoroughly investigated
and placed on a stable basis the future development of hydrocyanic-
acid-gas fumigation will be quite as important and extensive as has
been its past development.
RECENT RENEWAL OF INTEREST IN FUMIGATION IN
CALIFORNIA.
The hydrocyanic-acid-gas treatment of citrus trees continued to
become more widely'used and to hold general favor with the fruit
growers of southern California until about 1901, when the distillate
spray was introduced. The treatment of trees with distillate was
much cheaper than with hydrocyanic-acid gas. This fact, together
with the fact that the distillate treatment was indorsed by many of
the more prominent horticultural authorities and fruit growers led to
its widespread use during the next few years. Simultaneously the
introduction from South Africa of Scutellista cyanea Motschulsky, the.
parasite of the black scale (Saissetia olex Bern.), and its subsequent
splendid showing led many people to abandon treating their orchards
in the hope that this beneficial insect would hold the black scale in
check.
By 1903-1905 it had become very evident that the distillate spray
had not only failed to keep the scales under control, but that its con-
tinued use in many cases produced an injurious effect upon the tree
itself. The Scutellista also had failed to control the black scale,
although even a conservative must admit that its work has been of
a most praiseworthy type. Spraying rapidly sank into disuse during
1905 and 1906, until at the present time it has almost entirely given
way to fumigation. The experience of the prominent fruit growers
with the distillate spray has thoroughly satisfied them of the great
superiority of the hydrocyanic-acid-gas treatment to that with
spray for scale insects on citrs trees.
In the winter of 1903-4, Dr. G. Harold Powell, then of the Bureau of
Plant Industry, United States Department of Agriculture, com-
menced an investigation of the decay of oranges while in transit





SFUIGATION OF CITRUS TREES. 3

from alifornia. Hi efforts resulted in determining that the decay
almost entirly the outcome of mechanical inju to the skin of
hefruit during its piking and handling in the packing house.'
are washed primarily to remove the sooty-mold fungus that
in the sconeydew excreted by the black scale. Dr.
Powell demonstrated that the deca in washed fruit is much greater
in unwshed fruit. This led the fruit ower to understand
that the nesty of washing fruit should be avoided by controlling
the scale in the orchard.
As a direct result of Dr. Powell's in vestigations, and knowing from
e that the distillate spray and the Scutellista paraite
were inadequate to control the scale, fit roers took a renewed
int t in fuiation. This led to a demand for an investigation
of this process, tobe conductedby the United States Department of Agri-
culture, and the following ea, 1907, the writer was detailed to this
field. The fumigation practice was then in a very chaotic condition
as the outroth of years of use without any secial effort to have
the pstandardized. Indeed, it was a favorite pos of ma
professional fumigators to veil their operations in mystery in order
to secure the reputation of being authorities in a practice which they
made to appear complicated and difficult of understanding. Conse-
quently the growes, for the ost part, although arranin to have
their orchards fumigated, took no interest in a procedure which they
little understood.
In the fac of this situation the first reports of this investiation
give out in 1908 attracted the immediate attention of the fruit
grwers. After gaining a general understanding of the process of
orchd fumigation the growers in many localities have become much
ineted and subequenty have adopted or have caused to be
Sapted the more important recommendations of this investigaton.
Ts adoption of better methods has led to mor satisfactory work
The grower has immediately seen the advantage of better
meh with the result that where formerly many wr with diffi-
culty induced to have their trees fumigated, to-day the successful
t needs no inducement whatever, but, on the contrary, e-
quires that his trees be treated whenever their condition appears to
demand it. This public interest in fumigation has made it one of
the very live topics in the horticultural field in southern California




California, viz, Ventura, Los Angeles, Orange, Riverside,
San Bernardo, and San Diego. In these counties about 85 different
Bul. 123 Bur. Plant "Ind. U. Dept. Ag, 1908.





4 HYIDROCYANIC-ACID GAS FUMIGATION IN CALIFORNIA.

parties, including contractors, associations, county horticultural
commissions, and private individuals, owned approximately 5,150
tents on June 1, 1910, the date on which the securing of these data
was completed.
In order to ascertain the extent to which fumigation is now prac-
ticed, as well as the tax which this procedure annually places.on
citrus fruit growers, a careful canvass of the different parties operating
tents has been made. This canvass has resulted in showing that
approximately 36,000 acres were treated during the year from July,
1909, to July, 1910. Many fumigators gave the number of trees
which they treated; others the acreage alone. The average orchard
will approximate 90 trees to the acre, and in those cases in which
estimates were returned in acreage alone, this number has been
considered to comprise an acre. Wherever not known, the cost of
fumigating a tree has been placed at 30 cents, which price ap-
proximates very closely the cost of fumigating the average-sized
citrus tree in California. Calculated on this basis, the cost of fumiga-
tion of the citrus orchards of southern California during the season
1909-1910 approximated $1,000,000.
THE VARIOUS SYSTEMS OF FUMIGATION.
Each of the citrus-fruit-producing counties of southern California
has a board of horticultural commissioners consisting of three mem-
bers whose duties are to supervise the destruction of insect pests,
plant diseases, and noxious weeds within their respective counties.
In the three greatest citrus-fruit-producing counties-Los Angeles,
Riverside, and San Bernardino-numerous inspectors are also
employed to assist in carrying on this important work. As a matter
of convenience the counties are usually divided into three districts,
each of which is supervised by one of the commissioners. If inspectors
are employed, usually each is allotted a limited portion of one of
these districts, and is held responsible for the proper control of pests
therein. He advises when the trees shall be fumigated, and, after
arranging for the-execution of the work, is supposed to see that it is
properly carried out. There are several different systems under
which the work may be done.
BY CONTRACT.

The larger part of fumigation is carried out under the contract
system. Individuals or firms that possess complete equipment for
commercial fumigation and practice fumigation as a business, enter
into an agreement with the grower, who desires to have his orchard
treated, to do the work for a certain sum. The rate is seldom uni-
form but varies with such factors as the character of the ground, the
acreage, and the size and arranement of trees. Usually the cyanid





FUMIGATION OF CITRUS TREES. 5

and acid are furnished by the contractor at a certain price per pound,
although sometimes the grower himself supplies them. In the latter
the sole consideration is the cost of covering per tree.
BY ASSOCIATIONS.

A citrus tio is composed of a large number of growers from
same district organized for the purpose of cooperation in the
handling of their fruit. Some of these associations own fumigating
outs which are utilized in t treatment of orchards belonin to
its members. The manager of the association looks after the pur-
chasing of chemicals and supplies, and also selts competent men to
run the outfits. The inspector of the district usually directs the
movements of the outfit from one orchard to another. Under this
system the chemicals and labor are supplied at actual cost, plus a
light allowance for the purchase as well as wear and tear of equip-
ment. In ort, this system i supposed to be merely self-supporting.
BY COUNTIES.

Each of the county boards of horticulture owns a greater or
smler number of fumigation tents. In San Bernarino County this
system has reached its greatest development, for here the horticul-
tural commission owns fully 500 tents and carries on mor work
nnually than all other systems combined. This fumigation is under
personal direction of a county horticultural officer. The cost to
the grer of treatment by these outfits is usually what it actually
Sthe county to perform the wor. An important consideration
in avor of the system of county owned tents is that it readily enables
th-treatment of trees on city lots and in small orchards in out-of-the-
way places which otherwise would in all probability be neglected.
BY PRIVATE INDIVIDUALS.

Many trus fruit growers who control a considerable acreage have
fumigation outfits for their own work. In a few cases two or three
growers in a locality combine in owning an outfit. The private
ownership of tents is rapidly gaining in favor and well merits this
increased popularity, as it p decided advantages.
Excepting private ownership, it would be scarcely possible to say
which of these systems is superior. Each has its advantages. While
one system may prove uperior in one locality it might prove less
in another. The reason for success or lure lies not in
itself but largely in the personal element directing and
conducting the procedure. A reckless, uneconomical, or unreliable
director of any one system will achieve inferior results and give less
satisfaction than a carefulo economical and perfectly reliable one
a1 ck i ,l p ~ t ~jr e





6 HYDROCYANIC-ACID GAS FUMIGATIONI IN CALIFORNIA.

under any of the others. This is mainly due to the f that directors
of the former class are likely to employ field men of inferior qualifi-
cations. Efficient fumigation at the present time means, for the
most part, that the men in the field performing the operations are
careful, conscientious, and reliable. Otherwise the work is likely to
be performed in a slipshod, hasty manner, along lines of least resist-
ance. Work of this character, combined with the element of guess
work in deciding the dosages and proportions of chemicals to be
used, has been responsible for most of the unsuccessful results. If
perfectly reliable men are employed to carry on the actual work in
the field, using the most approved methods, success will be as marked
with one system as with another.
The recent horticultural ordinances of Los Angeles, San Bernardino,
and Riverside Counties requiring fumigators to be licensed are a step
in the direction of more efficient results. Such ordinances offer a
means of debarring outfits which perform unsatisfactory work.

EXTENT AND CHARACTER OF CITRUS ORCHARDS.
The production of citrus fruits in southern California is confined to
the narrow stretch of land south and west of the Sierra Madre Range,
extending from Santa Barbara on the north to the Mexican border.
Although citrus plantings are located here and there throughout this
territory, in reality only a small proportion of the land capable of
cultivation is devoted to this industry. The most prominent centers
of.production (see fig. 1) are in the foothills region and lower land of
the San Gabriel Valley; the corresponding regions of the San Ber-
nardino Valley, including the Redlands-Highland, Riverside, and
Corona districts, and the coast region of Orange and Los Angeles
Counties. Regions of smaller production are found in southern
Santa Barbara and Ventura Counties, in the San Fernando Valley,
and in western San Diego County.
The groves vary in size, the majority probably averaging between
5 and 15 acres. Some fruit growers have from 50 to 100 acres or
more, while a few fruit companies control from several hundred up
to about 3,000 acres. The trees for the most part are budded varie-
ties which average less than 20 feet in height. In some districts a
few groves of seedling trees 30 to 35 feet in height still exist. The
trees in most of the groves, especially those of more recent planting,
are regularly arranged, averaging from about 22 to 24 feet apart.
Some of the older groves are less uniform, either because they were
not arranged after the "block" system, or, if so, additional alternte
rows of trees were interset, which broke up the continuous open space
between two rows of regularly set trees, thus rendering it confusing
as well as difficult to work freely therein.






FUMIGATION OF CITRUS TREES. 7

The and on which the orchards occur is for the most par flat or
on gently sloping, and in a state of frequent tillage-conditions
which obtain because of the necessity of irriatin during much of
the year. At Redlands, in San Bernardino County, a considerable
areage of ornges is found on terraced land. Fumigation of such
trees is slow and difficult, t, fortunately, the comprise a very small
percentage of the groves in that county requiring treatment.
INSECT ENE OF CITRUS FRUITS.1
The laer number of pests most injrious to citrus fruits in south-
er California belongs to the Coccide, a group of insects popularly





VENTURA LOS ANGEL5











SAN DIEGO
a vo 0 B Oo












Ir.1.-Mip 3hOwing principal localities In southern California where citrus fruits are produced.
(Author's illustration.)
SAN DIEG






wn scale insects Among the scale insects wich are generally
so destructive as to require extended efforts for their control are the
rple scale (Lepidos es beckii Newm.), the red scale (Chrysom-
pkalus aurantii Mask.), and the black scale (Saissetia olew Bern.).
The yellow scale (Chrysomphas citrinus Coq.), considered a variety
of the red scale, is uch less detructive generally, though sufficiently
trublesome in some loclities to be considered a pest of primary
iportance. The citrus mealy bug (Psedococcus citri Risso) has
reently beenvery iurious in certain sections. Other insect pests
a kcitrus trees to a greater r less extent, but those jt mentioned
ar generally the most injurious, and the principal method of their
control is fumigation with hydrocyanic-acid gas.
SSee Bul 79, Bur. Ent., U. Dept. Agr., 1908, p. 10.
67330-Bull. 90-12-2





8 HYDROCYANIC-ACID GAS FUMIGATION IN CALIFORNIA.
THE PURPLE SCALE.
(Lepidosaphes beckii Newm.)
The purple scale appears to prefer the more moist regions in the
vicinity of the ocean, as its distribution is confined largely to this
part of the citrus belt. This insect confines its attacks to citrus trees,
infesting not only the leaves and branches but also the fruit. Much
injury results. The young purple scale insects hatch from eggs
deposited by the adult. The number of broods of this insect in
southern California has never been exactly determined. Prof. H. J.
Quayle, -of the University of California, is at present investigating the
life history of this, as well as the other injurious citrus scale pests. In a
climate like that of southern California, which is never severe at any
time of the year, there is much overlapping of broods, so that scales in
all'stages of development can be found at almost any time of the year.
The writer's own observations in the field have shown that there are
two very noticeable general broods, onre appearing in the early spring
during March or April, the other in the fall, usually about October.
These broods are much earlier some years than others, depending on
the nature of the weather. The fall brood of the scale is the most
injurious, as shown by the fact that trees which at a distance may
appear entirely healthy one month may have the leaves of a large
area, or even an entire side, turn yellow and drop off the next month.
The orchardists speak of this as "firing." It is due to the attacks of
the enormous number of young insects which, on hatching, have
spread about and settled down on those branches immediately
adjoining the ones previously infested. Trees infested with the
purple scale seldom present a diseased appearance on all sides. The
habit of this insect is to frequent the inner and shadier portions of
the tree, so that sometimes severely infested trees may present no
visible appearance of this condition on the outside. In the majority
of cases where the infestation appears on the outside of the tree it
will be found that it is at or near the northwest corner, which is the
shadiest part during the day. The attacks are also confined largely to
the lower part rather than the top of the tree. In long and seriously
infested trees the insects may spread throughout.
THE BLACK SCALE.
(Saissetia olee Bern.)
The black scale is found more or less throughout southern Cali-
fornia, yet matures more freely and causes more injury in the region
adjacent to the ocean than in the hot interior valleys. It occurs on
a wide range of hosts, including trees, shrubs, and herbaceous plants.
The commercial importance of the black scale arises largely from






FUMIGATION OF CITRUS TREES. 9

its habit of secreting honeydew, which spreads over the leaves, fruit,
and branches, furnishing a growing medium for a black or sooty
mold fungus, resulting in a blck coating throughout the tree. This
coating is removed from the fruit by washing, or in light attacks by
bruhing. In the investiations by Dr. G. Harold Powelll of the
causes of decay of oranes while in transit from California, it wa
shown that the decay was greater in washed than in unwashed fruit.
To avoid the ashing of fruit it is necessary to destroy the scale
in the orchards. The black scale appears generally to have little
effect on the vitality of the tree. Its attacks are confined mainly
to the branches, yet it is commonly found on the leaves during its
earler stages of development, and sometimes it matures in this situa-
tion. Seldom does it mature on the fruit. The young of the black scale
insects hatch from eggs deposited by the adult. They can be readily
destroyed by fumigation in the early stages of development. Ap-
proaching maturity they become tough and leathery, and in this
condtion they are capable of resisting very heavy dosages of gas.
The breeding of the black scale in southern California has never been
closely investigated, so the exact number of broods is not known.
When this has been done undoubtedly it will be found to be very
variable with different hosts, or even on the same host. The scales
on "sucker" shoots will mature much more rapidly than those on
other parts of the tree, There is one noticeable general brood which
Susually largely hatched by the first part of September. In the
warmer and ier parts of the citrus belt, remote from the coast, the
hatching of this brood is quite distinct, so that in most instances all
the insects may be fomud in the early stages of development at the
same time. In the immediate vicinity of the coast, and especially
on recently budded trees, one frequently finds the scale in all stages
of development on the same tree. In these latter instances fumi-
gation will prove less satisfactory than in the former.

TIHE RED SCALE.
(Chrysomphalus aurantii Mask.)
The red scale, althoug its injuries are more severe in some local-
ities than in others, has the limits of its distribution very much the
same as has the black scale. It can be found within a few miles of
the ocean or as far inland as Redlands. This inset occurs on many
host plants besides citrus trees. It attacks the fruit, leaves, and
branches. In point of destructiveness it excels all other citrus scale
insects in this State, destroying not only branches, but sometimes
entire trees by its attacks. The young are born alive. It has at
least t e broods and is very prolific.
I Bul. 12, Bur. Pant Ind., U. S. Det. Agr., 1908.






.10 HYDROCYANIC-ACID GAS FUMIGATION IN CALIFORNIA.

THE YELLOW SCALE.
(Chrysomphalus citrinus Coq.)
Infestations of the yellow scale appear to be most marked in the
foothills region of the San Gabriel Valley and along the Sierra Madre
Range through Upland and Cucamonga. It causes considerable dam-
age at Redlands, San Bernardino County, yet elsewhere is not
regarded as a very serious pest. This insect infests the leaves and
fruit, seldom occurring on the branches. The young are born alive,
as in the case of the red scale, to which it is closely related.

THE MEALY BUG.
(Pseudococcus citri Risso.)
The mealy bug occurs in various sections of the southern part of
California. The districts of greatest injury are in southern San Diego
County and at Santa Paula, in Ventura County. Little effort for its
control has been made except in these two places. This insect at-
tacks fruit, leaves, and branches, secreting a honeydew, which is fol-
lowed by a black fungus, as in the case of the black scale. Its injury
is much greater than that of the black scale because it discolors and
weakens the rind of the fruit at those places where it extracts the
juice. The cottony secretion in which the eggs are deposited is
difficult to remove. The severe washing which this fruit requires,
combined with its weakened rind in certain places, produces a heavy
decay in such fruits as are treated in this manner. The mealy bug
occurs on several hosts beside citrus trees. The young are hatched
from eggs deposited by the adult.
APPARATUS.

TENTS,
When hydrocyanic-acid gas was first employed in treating orchards
the apparatus used in the process was of a very cumbersome nature.1
The most popular apparatus consisted of tents more or less of a bell-
shapled nature manipulated by a high derrick mounted on a wagon.
The wagon was drawn between the rows and the tents lowered over
or raised from the trees by means of ropes attached to the derricks.
The use of suclh apparatus'was difficult, slow, and costly.
SHEET TENTS.
During 1892 Mr. C. W. Finch, a fumigator at Riverside' Cal.,
d(evised a much simpler and cheaper apparatus than those theretofore
used, which consisted of flat sheet tents, octagonal in outline. (Fig. 7,
p. 29.) This sinplified tent was rapidly adopted, and now sheet
I Heprt of Eritntologist, 1'. S. Dept. Agr., 1887, p. 126.





FUMIGATION OF CITRUS TREES. 11

Sa exclusivel (parts of two outfits excepted) used in southern
Clif ia. A tent of this character is easy to construct, easy to
ra, and its manipulation has been so perfeted by years of use
that it is very easily handed in the field by intelligent workmen.
Siz.-The standard sizes of sheet tents are 17, 24, 30, 36, 41, 43,
45, 48, 52, 55, and 64 feet, but larger ones up to 72 and 84 feet have
been employed. The size of ths style of tent is properly based on
the distance between parallel sides, not on the distance between oppo-
site corners.
aterials used.-The materials now generally used for sheet tents
in southern California are 64-ounce or 7-ounce spBecial drills and
8-ounce special army duck, though 10-ounce army duck is sometimes
usd in very large tents. These cloths are spoken of in ounces,
meaning such a weight per yard 30 inches wide. Drills are used as
fre as ducks. In some other countries where fumigation is prac-
ticed, notably South Africa, even heavier than 10-ounce cloth is
sometimes used. This is largely because of its strength and tightness
of texture. The tendency in California has been to sacrifice tight-
in favor of litness, as the lter tents are so much more easily
manipulated in the field. The main reason for this tendency is prob-
ably that the practice has been largely in the hands of contract fumi-
gators rather than in the hands of the growers themselves. The con-
cers furnishing the fumigation tents apparently have made no special
fort to supply the very tightest goos available on the market, prob-
y because the profits on these goods would be smaller than on
the cheaper and more porous cloths. Several of these firms have
agoods which they reconnend for fumigation use. For the
m t part these goods are about on a par as regards the requirements
Sfumigation. Only one grade istincty superior to the others has
been seen; i is used solely by private outfits, and is slightly more
expesive than the other grades. The results secured depend di-
rectl on the tightness of the cloth; in fact, this consideration of
tightnes tenting is one of the most iportant factor in the entire
fumigatn procedure. On it depends not only the ellicacy of the
tment but also to some extent the cost of the operation. A
doge recommended as securing certain results with tents of a given
d eof tightness will not produce te same results with tents of
ls closely woven material. Even though the initial cost is greater,
tightly woven material is the most economical in the long run.
New tenting material.-Considerable attention has been given dur-
Sinvestigation to te character of cloth used i fumigating
an attempt has been made to secure the most suitable
material possible. The leading manufacturers and dealers in cotton
ducks and drills in the United States were consulted and samples of
their tightest cloths secured. Many of the nearly two hundred sam-






12 HYDROCYANIC-ACID GAS FUMIGATION IN ICALIFORNIA.

ples secured were very superior in tightness to the grades now gen-
erally supplied in California for the fumigation trade. Although this
collection contained samples of the very tightest ducks and drills
manufactured, it was decided to accept the offer of one of the largest
cotton-goods concerns to carry on special experiments in weaving for
the benefit of this investigation. As a result of these experiments
samples of 6-ounce. 7-ounce, and 8-ounce drills were furnished.
These are easily the most tightly woven drills the writer has ever
seen and some local cloth experts are of the same opinion. Each of
these samples was made very tight by forcing in more threads than
are found in the tightest drills on the market.
Experiments with new tent material.-Tents were made of each of
these 7-ounce and 8-ounce special drills and tested in the field. Part
of an orange orchard of trees from 10 to 15 feet tall which were
severely infested with the purple scale was fumigated during Sep-
tember, 1909. The point was to determine at what strength eradi-
cation would occur with these new tents. Both potassium cyanid
and sodium cyanid were used. On examining the results with these
new tents it was found that schedule No. 1 (see p. 34) with potas-
sium cyanid produced eradication, whereas with the ordinary fumiga-
tion tents a 1- schedule was required to secure the same results.
With the sodium cyanid it was found that the equivalency of between
a three-fourths schedule and a No. 1 schedule produced eradication,
whereas it requires the equivalent of a 11 schedule with ordinary
tents to reach the same degree of efficiency. These results with both
potassium cyanid and sodium cyanid show that this new tenting
material requires at least one-fourth less of these chemicals than the
regular tents now largely used. This would mean a saving of fully
25 per cent on the amount of cyanid required in field work if the
tighter tents are used.
What clothi to use.-Cyanid is the most expensive element in fumi-
gation work. A saving of 25 per cent on this article means a materi-
ally lessened cost for the process. This better and tighter special
tenting material may be somewhat more expensive than the present
inferior goods used, yet the amount of cyanid saved as well as the
superior results secured from its use will in the long run many times
offset the additional initial cost. 'The writer, would advise either a
7-ounce or lan S-ounce weight of these new goods for commercial
fumigatioun as superior to any cloth he has ever seen. There appears
to be little difference in tightness between the two weights. By
reason of its greater weight the 8-ounce drill might prove more-dura-
ble, whereas on the other hand the 7-ounce weight is easier of manip-
ulation in the field. This special grade of drill advised by this inves-
tigation can be purchased at any of the dealers in fumigation tents,
in Los Angeles, Cal. Anyone making an investment of the amount






FUMIGATION OF CITRUS TREES. 13

iny in the purchasing of a fumigation outfit usually is better
satisfied if has exercised, or had the opportunity to exercise, his
ow judgment in the selection of the cloth, even if experience has
proved one particular kind to be superior to all others. To satisfy
this demand of independent people it is suggested that anyone con-
templating the purchase of a fumigation outfit procure as many
samples as possible of 61-ounce to 8-ounce drills and 8-ounce ducks
and compare tightness of these samples with that of samples of
the 7-ounce and 8-ounce special dills recommended by this investi-
gation. This can be done by holding the various samples at different
angles between the eye and the sun, noting the comparative number
of light rays penetrating the different samples.
Cosrtion.-In California the demand for fumigation tents is so
great that several cincerns make a special business of meeting it.
The man contemplating the purchase of an outfit visits one or all of
these different dealers, and, having selected the cloth which meets
with his approval, places his order for the number of tents of the
size desired. These are shipped to him ready for use. A method of
constrcting tents will be explained, however, for the benefit of
people more distant from the sources of supply th a the people
of southern California. As previously explained, the tents are flat
het, octagonal in shape. The ducks and drills are usually 30
inches wide, although sometimes they measure 29 or 291 inches.
The sides of the strips are sewed together so that the strips all run
in a parallel direction. Before attempting to cut the cloth for a
tent it is well to construct a diagram having therein an exact rep-
resentation of the number of strips required as well as their length and
shape. Such a diagram is shown in figure 2 (p. 14), and was originally
prested by Dr. A. W. Morrill,' who was en ged in fumigation
work aain citrus pests in Florida from 1907 to 1909. Each side
of the tent or octagon, when constructed, will be equal, approximately,
to two-fifths of the distance between the parallel sides. In explainin
the construction Dr. Morrill states:
The stps whe cut should be overlapped three-eighth or one-half inch and double
stitched, and all raw edges should be hemmed. In calculating the number and
length of strip the overlapping ill reduce e width of e cloth from three-fourths
inch to inch. As an illustration of the method of calculating the length of the
strips used in making an octagonal tent of 8-ounce duck, 50 feet may be taken as the
desired size. This is equal to 600 inches and the width of the cloth, if 29.5 inches,
will be reduced to 28.5 if verlapped one-half inch at the seams. By dividing 28.5
inches into 600 inches the nearest multiple is found to be 598.5 inches, or 49 feet
10 inches, which is sufficiently close to the desired width for practical urpoes.
.T er of strips in a tent 598.5 inches wide is 21. The middle section B [fig. 2]
tely wo-fifth the entire width, or 239.5 inches. Deducting this from
.5 the entire width, the remainder, 359, equals the sum of the widths o
secons A and C. These sections being equal, the width of each is 179.5 inches
I Bul. 76, Bur. Ent., U. S. Det. Agr., p. 16,1908.






14 JfYDROCYANIC-ACID GAS FUMIGATION IN CALIFORNIA

The number of strips in each section can now be readily calculated. The 21 strips
should be numbered on the diagram from left to right. Section A requires six strips
and 8.5 inches of the seventh. Similarly, section C requires six strips beginning at
the right (twenty-first to sixteenth, inclusive) and 8.5 inches of the fifteenth. Sec-
tion B requires the remaining 20 inches of strip No. 7, 20 inches of strip No. 15, and
seven entire widths, thus making the total of 21 strips required.
The cutting of the cloth can be done without waste if the details of construction
are well planned. In the above tent seven strips 50 feet long (49 feet 10 -inches)
should first be cut out for section B. Strips Nos. 7 and 15 are next cut and the out-
side corners cut at an angle of 45 degrees, as indicated in the diagram.- Each strip
for sections A and C is cut shorter by its own width outside at each end than the strip
preceding it. Thus the
------------- required lengths of the
", 1 I / side strips are found by
-__ / matching the inner edge
Sof the new one to the
,, \j outer edge of the one be-
Sfore it. It is desirable to
have the central section,
"' B, made up entirely of
,I full-length strips so that
,. / the stress will not be
SI S across seams. The stress
,/ I is so slight, compara-
S 1" tively, in the side sec-
12 3 45 6 7 1011 12 13 i 16 17 1819 20 21 tions A and C thatthis is
tions A and C that this is
"..--..-. not an important point.
i Such is the con-
\ struction of a 50-foot
,- c: \ tent. The method of
/ "\ constructing a tent
....r......... -...........-..........- -- of any other size is
E similar. Tents up to
FIG. 2.-Plan for construction of octagonal sheet tent 50 feet across, s. u
showing lines used in constructing octagon: A, C, side sections; B, 45-foot size are con-
central section of full-length strips; E, E, so-called "ends" of tent; structed throughout
S, S, so-called "sides" of tent; R, R, reinforcements; 1-21, strips
of duck 29) inches wide overlapped j an inch at the seams. (From of either of the drills
Morrill.) or of 8-ounce duck.
Larger size tents should have the full length strips of 8-ounce duck,
while the shorter side strips, or "skirts," as they are sometimes
called, are made of a light drill. The main strain and wear falls on
the middle, heavier and stouter, long strips; hence the use of the
lighter material for the "skirts" decreases the weight of the tent
without affecting its durability. The duck used in such tents should
be of the very tightest grade available, while the 61-ounce or 7-ounce
special drill previously recommended is most suitable for the "skirts"
Amount of cloth requiredfor different-sized tents.-It is very essential
in constructing tents to know the amount of cloth required for the
size which it is intended to make. The writer has calculated this for
the regular sizes and gives below the results. Calculations are based






UMIGAON OF C ITRS TR S. 15

30 inhes wide, nd represent the number of linear yards
cloth required-not square yards Allowance of an inch to
each strip has been made for overlapping edges. These figures are
on the assumption tha the cloth is cut without waste

Cloth Cloth
Size of tent. Size of tent.
wie.) wide.)

Yards. Yard,.
24 feet... ................................ 70 48 feet................................... 2
30 feet.................................... 1B 5 ................................. 315
36 feet ........................... 1.55 feet ................................ 345
41feet.. ................................ 195 feet ................................... 4-
43 feet ................................... 215 72 feet ...................................
45 fee ................................... 23)

Siz to purchase.-Most of the tents in souther California are
of either the 36, 41, 43, or 45 foot sizes. Few tents of less than 36-
foot size are constructed. Outfits or parts of outfits having tents of
48, 50, 52, and various other sizes as high as 84-foot are known. The
number, however, is comparatively smal. The size of tent required
depends on the size of trees in the orchard or orchards to be fumi-
gated. The tents should b large enough to cover the tallest trees.
A eas method of accomplishing this, as suggested by Morill, is by
throwing a tape attached to a reel over the top of the tallest te and
measuring from ground to ground. Although the weight of the tent
reduces the height of the tree to some extent, nevertheless it is policy
to add from 2 to 4 feet to the ditance measured by the tape so as to
be ured of havin the edges of the tent rest well on the ground.
If an outfit is to be procured for use in a young orchard, the tents
purch hould be large enough to allow for 5 or 6 years extra
wth. The average age of a fumigation outfit is frm 3 to 5 years,
ding, of course, on the amount and character of usage which it
undergone. A well-cared-for outfit used onlby a private
grower in vering his own orchard should last through 5 or 6 seasons
of work.
Ring atachment and reefrcene .-Small iron rings are some-
times attached to tents as catch places for the poles or derrick hooks
used in throwing them over trees. These ring attachments are most
convepient on tents above 45 feet in diameter, but unnecessary on
r sizes. An easy and satisfactory method of attaching rings
to thecloth, as proved by many years of experience in California and
is shown in figure 3. It consists in gathering the clo
f te tt about some object or material, binding the same in place
bt cord, which also passes through the ring. A tightly rolled
wad of some cloth suchasburlap is commonly used. Another method
well worth mentioning is by meansof a piece ofmanilarope from





16 HYDROCYANIC-ACID GAS FUMIGATION IN CALIFORNIA.

3 to 4 feet long sewed to the tent in the form of a right angle, as shown
in figure 7, page 29. As fumigation tents should be pulled onto or off
the trees in the direction in which the strips of cloth run, the rings
should be so placed as to make this method of manipulation
possible. This is accomplished by having two rings at either end of
the tent and apart by about the width of the average-size tree treated.
They should be placed from 3 to 5 feet back from the edge, the dis-
tance depending on the size of the tent. A small link of chain
called a "jingler" is usually attached to the ring, the sole purpose
of which is to direct the operator to its location. By merely giving
the sheet a shake this simple device enables the tent pullers to
easily locate the rings on the darkest nights.
Such a great strain is localized at the place
where the rings are attached that it is well
to have this part reenforced by stitching
on an extra thickness of cloth. The same
material of which the tent is constructed
I f is very suitable. The strip used should be 3
ror 4 feet long.
SBELL TENTS.
Originally the bell, or hoop, tent was the
kind in use in California, and .even now it en-
joys a limited use in some countries. This
FIG. 3.-Method of attaching tent is dome shaped, having the mouth held
hooks to tent when covernig open by a circle of I-inch gas pipe. It is
trees with aid of derricks: a, s d o f rin mll tr Plt
Tent gathered around ball of Suted only for covering small trees. Plat I
burlap or other suitable ob- illustrates the character of this tent and the
ject; b, stout cord for attach- m of its ip tio. Expers
ing ring; c, catch ring; d, hook method of its manipulation i
on pulley block; e, lap link California have resulted in the disuse of bell
or"jinglr." (From Morrill.) tents in favor of sheet tents, the latter style
being not only easier of construction and manipulation, but also
more easily kept in repair.
GAS-PROOFING.
The treatment of covers with various substances to increase their
tightness has been in practice to a greater or less extent since the
beginning of fumigation. Linseed oil was one of he first tried.
It renders the tents perfectly tight but greatly increases their weight.
Experience has proved that tents so treated are lable to burning and
rotting under the conditions to which they are subjected in the fild.
Treating tents with the mucilaginous concoction resulting fom
soaking the common cactus (Opuntia engelmani) in water for
two to four days was practiced to some extenf during the nineties.'
Numerous other methods have been tried, such as painting with a
flexible paint; treating with glue dissolved in water; treating with
I1







Bul. 90, Part I, Bureau of Entomology, U. S. Dept. of Agriculture. PLATE I.



































































FlIS. 1-3.-METHOD OF COVERING SMALL TREE WITH BELL OR HooP TENT. (FROM
SMORRILL.)





FUMIGATION OF CITRUS THEES. 17

actl s juice cd with linsee d oil, or glue, or tannin, etc. Expe-
ce with man substances has resulted in discarding all of
them in this State, so that to-day no effort is made to render tents
g-tigt by the use of any liquid substance.
It is well that a decidedly smaller dosage would be required
it ai covers than is nesessary wth tents of the present
aracter. This has led many writers on fmigation to advise the
treatment of the cloth with some of the substances just mentioned to
accomplish that end. Had these writers undergone a considerable
in te actual handling of tents in the field their expressions
o particular subject might have been somewhat modified.
Tents treated by sme of these methods will be rendered gs-tight, or
early so, but for certain practical reasons they are not now used and
never will be used on a large commercial scale.
Experiients have been made with many different substances in
attempting to render cloth gas-tight, and several samples of gas-tight
or almost gas-tight cloth have been received from dealers. This
experience in all cases has shown that to render a cloth very much
more nearly gas-tight than is possible in weaving, some treatment
must be used which materially increaes the weight of the tent as
well as rendering it somewhat stiff. Both of these conditions should
be avoided as much as possible. Heavy tents are not only dificult
to manipulate, but also destroy fruit and break branches while being
hauled over trees. Stiff tents will not lie close to the ground, thus
allowing the escape of gas. Tents must be constantly overhauled to
mend the holes which result from acid burns as well as other causes.
The mending of such treated clot is so difficult as to be impracticable,
especially in large-sized tents.
fter considering both sides of the question experience leads to
the conclusion that the economy of gas resulting from gas-tight tents
is more than offset by the many difficulties experienced in the use of
such stiff heavy covers in the field. The writer advises the purchase
of the most closely woven untreated cloth obtainable, of the char-
acter and weight previously mentioned (p. 12), believing such to be
superior for orchard wor to cloths which have unergone a treatment
to render them gas-tight.
Small tents used in treatin nursery stock, and especially covers of
cloth made over a fraework in the shape of a box having one end
open so as to be easily placed over nursery trees or such small plants,
can be rendered gas-tight without experiencing some of the more
ius objections to their practical use that exist in the case of large
covers in orchard work. Linseed oil is very suitable for this purpose.
The preparation and application of a linseed-oil varnish, which is
used by the War Department in the treatment of cloth for balloon
Si oted below It renders the cloth as-tight and at the





18 HIYDROCYANIC-ACID GAS FUMIGATION IN CALIFORNIA.

same time leaves it more pliable than any other gas-tight treatment
experienced. The Acting Secretary of War has described this
method as follows:
In order to render the cloth gas-tight a linseed-oil varnish is applied. The varnish
is made in proportion of 100 pounds pure linseed oil, 4 pounds litharge, and 1 pound
of umber. This should be heated to a temperature of 130 to 2000 C. for six or seven
hours and constantly stirred during that time. A sponge or wad of cloth is ordinarily
used for applying the varnish, which should be put on in very thin coats and well
rubbed in by hand. The addition of one coat of this varnish about once a year will
be found of great value in preserving the impermeability of the material.
One coat of this oil on each side of the tenting will prove adequate
with most cloths. Treated cloth should be hung up to dry for
about two weeks, and if not entirely tight at the end of that time a
second coat should be applied. The cloth should be thoroughly dry
before it is used.
MILDEW-PROOFING.
The treatment of tents with substances to render them proof
against mildew is practiced to some extent. In San Bernardino and
Riverside Counties probably the majority of tents are dipped in a
solution of tannin, while in Los Angeles and Orange Counties, which
are much nearer the coast and consequently have a more generally
moist climate, tents for the most part are used without any treatment
whatever. This localizing of treatment to the dryer sections demon-
strates that for a climate like that of southern California the mildew-
proofing of tents is not absolutely essential. The covers used in this
investigation have been in use mostly in the more moist coastal
region for three seasons, yet they have never been affected with
mildew. Neither has any mildew injury to other tents been seen or
heard of meanwhile. The life of untreated tents in this State appears
to be as long as that of those which have been mildew-proofed-at
least this is the case with tents that are properly cared for in the
field as well as in storage.
Long-used tents are now cast aside, not because of weakness due
to deterioration of cloth from mildew, but largely from weakness due
to extensive mending of holes, resulting principally from acid burns,
but to some extent also from use on trees containing dead branches.
This necessary patching, combined with general wear, limits the life
of the average fumigating tent to 3 or 4 years. Judging from the
experience of California fumigators, as well as from that of the writer
himself, it appears unnecessary in California to treat tents for mildew
if proper precautions are taken for drying them. Wet tents shud
be spread out during the day on the ground between the trees, so
that the sun may reach them as much as possible. At the end of a
season's work they should be thoroughly dried, rolled up, and stored
in a dry room.






FUMIGATION OF CITRUS TREES. 19

In places likeFlorida, as well as in tropical countries where tents
become wet every night, treatment to prevent mildew would seem
advible. Even in California it will act as a guarantee to those
Swho e little care in the drying of their outfit.
The dipping and boiling of tents in a solution of tannin is the only
method now practiced there to render them proof against mildew.
This tannin treatment has been in use for a long time, and is very
satisfactory. Contrary to the belief of many, tannin does not render
the tents any tighter. It merely shrinks them, which can be as well
accomplished by ipping in water, or a few ights' exposure to heavy
dews will produce the same results. The tannin treatment, as prac-
ticed by Mr. S. A. Pease, horticultural commissioner of San Bernar-
dino County, is as follows:
A brick furnace [Plate II, fig. 1] constructed so that the upper half partially
incloe a tank, 3 by 10 feet and 3 feet deep, made of No. 16 galvanized iron. This i
filled with water to within 8 inches of the top, which would be about 500 galons, and
about 200 pounds of extract of oak bark is added. This mixture is raised to a tem-
as high as the hands o the opertor will stand. A tent stetched out in as
loosened a conditionaspoible ina ) is thenintroduced into the at. It i
around and kept submerged by means of w en paddle manipulated by the
crew. Afterorminuts of thistreatm t e tnt israised to the top of a derrick
above the tank and suspended for a few minutes until well drained, after hich it is
lowered on a ck, moved away, and spread out on the ground to dry. Twenty-five
ons of water and 20 pounds of extt are now ded to the tank before another
tent is introduced, and this is repeted for each succeeding one.
The above recommendations are for 45-foot tents. Larger tents
require more material and smaller ones less. Oak-bark extract costs
about 10 cents per pound by the barrel.
MARKING.

On pages 29-30 of this bulletin is explained a method of marking
tents which is used in the ost improved fumigation procedure.
Tents should have been thoroughly wet at least once before being
mark as new cloth is subject to considerable shrinkage, and if
marked before shrinka the measurements will be erroneous. Meas-
urements made of sev tets of 6-ounce drill, before and after
shrinkage, showed that 45-foot covers shrink about 3 feet length-
wise of the strips of cloth. The crosswise shrinkage is much less. A
convenient method of shrinking untreated tents is to spread them
out on the lawn and wet with a hose or spriner. After being dried
y are ready for marking. Tents treatd with tannin should be
marked after the treatment.
The best method for marking tents is to place them on a smooth
floor. If this is not possible, spread them out on the smoothest
ground available. A tapeline, brush, and marking fluid are required.
Printer's diluted is the best marking material although some






20 HYDROCYANIC-ACID GAS FUMIGATION IN CALIFORNIA.

have used lampblack and turpentine, or a soft, flexible paint, with
satisfaction. The first line marked should be the one running
through the center of the tent. When many tents are to be prepared,
the use of a stencil large enough to include a complete line of figures
on one side will facilitate the operation. The numerals should be
not less than 5 or 6 inches long.
POLES AND DERRICKS.
Two wooden poles or derricks are used in placing tents over trees.
No absolute statement can be made as to when poles should be
employed or when derricks. The practice is to use poles when-
ever possible.
-a This has resulted,
in general, in the
use of poles with
tents up to 45 feet
[i1 in size and of
i| 'II derricks with the
i larger sizes. Some-
times poles are
used with tents of
48-foot or 50-foot
sizes, but this is
difficult, especially
if the trees are tall
I and narrow rather
ithan low and
t l broad. Preferably
Sthe poles should
Sbe 6 inches to a
FIG. 4.-Ends of hoisting poles used in placing tents over trees: a, Used foot longer than
with tents where rings are present: b, used with tents having no rings. the height of the
(Original.)
(Original.) trees. The two
lengths of poles in most common use are 14 feet and 16 feet. Twenty-
foot poles are occasionally required. These poles average from 2 inches
to 21 inches in diameter, are rounded, and made of straight-grained
Oregon pine. The lower end is slightly sharpened to secure a ready
hold in the ground. The upper end, to which a rope is attached for
erecting the poles, preferably is also bluntly narrowed after one of
the methods shown in figure 4. This figure also shows two convenient
methods of attaching the rope. The end of a is narrowed about e-
half inch on all sides for 3 or 4 inches. This allows it to slip easily
through the rings in tents. The rope is tied in.a shallow furrow 6 or
7 inches distant from the end. In b the end of the pole is merely
rounded, while the rope occupies an auger hole through the center







Bul. 90, Part I, Bureau of Entomology, U. S. Dept. of Agriculture. PLATE II.
































FIG. 1.-BRICK FURNACE, TANK, AND DERRICK USED IN THE
TANNIN TREATMENT OF TENTS TO PREVENT MILDEW,
SAN BERNARDINO COUNTY, CAL (ORIGINAL)































FIG. 2.-MACHINE FOR COVERING TREE WITH SHEET TENTS. DEVISED BY C. E.
MCFADDEN, OF FULLERTON, CAL (ORIGINAL.)





FUMIGATION OF CITRUS T 21

of the pole and at the same distance from the end as is the furrow
in a. A knot at the end of the rope prevents its removal from the

Most of the wear on the rope falls on the first 2 or 3 feet adjacent
to the pole, for this part is alf-hitched each time over the tent.
(See P. I, fig. 1.) Some fumigators substitute a stout piece of
rawhide about 3 feet long to occupy this region between the end of
the rope and the pole. The rope should be j-inch or j-inch, and
about 3 feet longer than the pole.
Derricks are necessary for very tall trees, which can not be covered
conveniently with poles. Theyconsist of long poles having a frame-
work attached to the bottom by which to prevent slipping, as well
Sto confine their movement to either of two directions while stand-
ing erect. There is a rope and pulley arrangment at the top for
raising the tent. The length of the uprights depends on the height
of the trees to be fumigated. They should be fully a foot higher than
the tallest trees. In California derricks aver between 25 and 35
feet tall, having the top 21 to 3& inches and the bottom 3J to 4|
inches in diameter. Straight-grained Orego pine is used Their
construction is well showin Plate III, figures2 and 3. The frae-
work at the bottom (Pl. III, fig. 3) is held together by bolts. The
tackle attached to the derrick in part consists of a pulley block fixed
to the top of the pole as shown in Plate III, figure 2. A j-inch to
-inch rope is attached to this pulley, passing through another pulley
block which is free, and thence back through the fixed one (see Pl.
VII). The rope used should be about three times the length of the
pole. When the derrick is to be moved from one tree to aother the
fe pulley should be hooked to a rope or ring on the standard, the
rope pulled taut, and the free end tied temporarily, as shown in
te III, figure 3 This will prevent the rope from becoming
ted. The movable pulley should have a hook at the bottom
by which can be attached to a ring on the tent. A guy rope several
feet longer than the derrick i fastened at the top of each upright
and is used in its manipulation.
THE M' FADDEN MACHINE.

Mr. C. E. McFadden, of Fullerton, Cal., has devised an elaborate
and ingenious machine for placing tents on trees. A picture of this
mahineis shown in Plate I, figure 2. In brief it consists of a iron
framwork mounted on a pair of trucks. At the center of either end
othe fraework is attached a long arm made of iron tubing.
These arms are comparable to a pair of long hoisting poles. Each of
Ithese a is raised or lowered by a system of steel cables passing
hro plle attached to the arms and two high iron standards
nipulated by a gasoline engine, which also oper-
ates another pair of cables used to raise or lower the tents to or from






2 HYDROCYANIC-ACID GAS FUMIGATION IN CALIFORNIA.

the end of the poles. When ready to throw a cover, the machine is
drawn opposite the tree, the arms are lowered until their ends are
suspended beyond the outer edge of the tree, cables are then let down
from the end pulleys and run through two series of rings in the tent,
after which the tent is raised to the end of the derricks. These rings
are so placed that when the cable is raised about one-third of the tent
is gathered up in a series of folds. The derricks are then erected
and the tent cables released, when the tent will fall over the tree.
This operation is quite rapid as well as less wearing to the trees
and tents than the use of poles or derricks. Although slower and more,
expensive than the use of poles in covering small trees, it is easily
superior to derricks in covering large ones, such as seedlings, especially
where so closely set that the branches interlace.
SUPPLY CART AND SUPPLY WAGON.

An apparatus of some sort is required in carrying from tree to tree
the chemicals necessary in fumigation. The idea of using a two-
wheeled pushcart originated with the San Bernardino County outfits,
where this method has been used for several years. Observation of
its use convinced the writer that in most places the employment of
a properly, equipped handcart is the most practical method available
for carrying the chemicals. Extended effort has been made to equip
such a cart in a manner suitable for convenient use in the field. The
result of this effort is shown in Plate IV, figure 1, the make-up of
which has been so improved over the original as to resemble it but
little. As purchased, the cart-bed consists of a plain box fitted with
a two-shaft handle. This handle is removed, and is replaced by a
tongue having an enlarged link-shaped iron about a foot long firmly
attached at the end. This link-shaped handle is very convenient in
field work. The scales for weighing the chemicals are placed on a
platform above the center of the box. The ordinary kind having a
free scoop and using weights is most convenient; 1, 2, 4, 8, and 16
ounce weights are required. The cyanid is contained in a tin-lined
box in the rear half of the cart, while the acid and water are placed
in the front end. A 10-gallon keg firmly attached in a horizontal
position to the bed of the cart is a very convenient receptacle for the
water. A galvanized-iron basin, like that shown above the keg,
having an opening at the bottom fitting into the bung of the keg,
makes a very satisfactory funnel for filling the latter. The aci may
be held in an earthenware jar or a lead-lined tank, with cover firmly
attached to prevent slopping.
By way of a cover for the earthenware jar, a lead-lined lid (fig. 5)
which fits tightly within the top has been used. At the center of this
lid is an opening about 6 inches in diameter, around the ciru eree
of which is attached a leaden tube which extends downward several
it "






Bul. 90, Part I, Buieau of Entomology, U. S. Dept. of Agriculture. PLATE 111.

















































FIG. 1.-METHOD OF ATTACHING TENT TO HOISTING POLE BY A HALF HITCH OF THE
ROPE. THE EDGE OF THE TENT IS DOUBLE-LAPPED. (ORIGINAL.) FIG. 2.-TOP OF
DERRICK, SHOWING METHOD OF ATTACHING PULLEY AND GUY ROPE. FIG. 3.-BASE
OF DERRICK. SHOWING METHOD OF CONSTRUCTING BRACES. (FROM MORRILL.)
-Alt-l











4ii iiiii~ ~ iiiii iiiiiiiii!ii


!! ..... ........








Bul. 90, Part I, Bureau of Entomology, U. S. Dept. of Agriculture. PLATE IV.




























FIG. 1.-SUPPLY CART USED WITH THE IMPROVED SYSTEM OF FUMIGATION. (AUTHOR'S
ILLUSTRATION.)




































FIG. 2.-SUPPLY WAGON DEVISED BY C. E. MCFADDEN, OF FULLERTON, CAL. (ORIGINAL.)






FUMIGATION OF CITRUS TREES. 23

inches and prevents the slopping of acid through the hole. A lead-
lined cover fits into the top of this tube. This opening in the cover
is for us in filling the jar. Very few metals will withstand sulphuric
acid without corroding. For this reason all the common types of
facets are practicall worthless for drawing acid. In fact there is
no faucet on the market that is altogether satisfactory for this pur-
ose. A manufacturing firm on the Pacific coast has expermented
extensively along this line, but without any marked success. This
difficulty has been met in an entirely practical manner by attaching a
three-quarter-inch copper or iron pipe to the lower side of the jar and
regulating the flow of acid by means
of a large pincock placed on a hort
piece of rubber tubing at the end of 4
the pipe (fig..5,1,, and 5). The flow -
of acid is rapid and easy to control.
Pure rubber is most satisfactory, and
a fresh piece should be substituted
about every other night.
The water is drawn from a faucet.
In order that ts may be drawn on
the same side of the cart as the acid,
an elbow pipe of the character shown
in Plate IV, fiure is required. he
faucet should have an opening of about
three-fourths inch to allow a heavv
flow, and should be of such a type that
a half-turn will give it a full opening.
As fumigation is usually conducted
at night, a torch is placed on the 5.-Ee cid jar with attach-
t of the cart to furnish a light by ments forfielduse; ,Jarcomple; inside
Sviewof lead-lined cover showing tube at
which to measure the acid and water: n; top or in cover;
another, othe elevated platform, is in hck; 5, method of attahing iron
jpape t -,and rubber tube on end of pipe
convenient for the man weighing the wite attached. mus
cyanid. tration.)
This style of cart is entirely practicable for almost all fumigation
work. The chemicals can be measured quickly and accurately with-
out any slopping of acid or water. A glass graduate with a capacity
of 16 or 32 ounces, preferably the later, is essential for measuring
acid and water. A kind having elevated rings in the glass has been
found most satisfactory for night work. The man handling the acid
should wear rubber gloves. Cotton gloves are convenient for hand-

Sfumigators have preferred to use a horse-drawn wagon
equip for carrying the chemicals rather tha a handcart. This
has d in a number of very original combinations. The most
67330o-Bull. 90-12---3






24 HIYDROCYANIC-ACID GAS FUMIGATION IN CALIFORNIA.

elaborately equipped of these recent innovations was devised by Mr.
C. E. McFadden, of Fullerton, Cal., and is shown in Plate IV, figure 2.
Some of its devices are very ingenious and well worthy of mention.
The large box (a) in the middle of the wagon contains an acid carboy
whose neck can be seen projecting above and into which is inserted
a rubber hose (b) which leads backward and downward to the rear of
the wagon. Pieces of lead pipe are attached to the ends of this hose,
the free end of which (g) is equipped with a piece of rubber tubing
and cut-off for regulating the flow of acid. The water is contained
in the barrel (c) at the front of the wagon. A hose leads from the
bottom of this back to the large vertical cylindrical object (d) to the
left of the acid hose and adjacent to the wagon wheel. This cylinder
is the graduate used for measuring water. The slender iron rod (e)
seen projecting from the top of this cylinder is attached to a movable
float. This rod is graduated so that each graduation is equivalent to
3 ounces of water. The turning of a valve at the bottom of this
cylinder allows the water to flow in, raising up the movable float.
When the graduation on the iron rod shows that the cylinder contains
the required amount this valve is closed and another opened which
directs the water through a downward-projecting pipe into the gen-
erator placed beneath. The cyanid is contained in the box (h) at the
rear of the wagon. The wagon is also equipped with a thermometer
and hydrometer. The upper and larger (f) of the two horizontal
cylinders above the scales is a rotary device for reading the dosage
schedule. The lower cylinder (kc) contains a sheet of paper on which
a record of the dosages used is kept.

GENERATING VESSELS.

In California earthenware vessels of the type shown in figure 12
(p. 75) are made especially for, and are almost exclusively used in, gen-
erating the gas. They are sold without a cover. The average capacity
is 2 gallons, although a 1-gallon size is sometimes used for small trees
and a 3-gallon size for very large ones. A 11-gallon generator will
serve for a dosage of about 15 ounces of cyanid without boiling over,
a 2-gallon generator for approximately 20 ounces, and a 3-gallon one
for about 30 ounces, provided the cyanid is in average-sized lumps
and not powdered. Where dosages larger than 30 ounces are required,
use two generators, or three if necessary,

GENERAL PROCEDURE.
The process of fumigation consists of covering trees with cloth tents
and generating beneath them a very poisonous gas called hydrocyanic-
acid gas. As previously mentioned, sheet tents exclusively are used
in California. After exposing a tree to the gas for a definite time,






FUMIGATION OF CITRUS TREES. 25

usually an hour or thereabouts, the tent is removed to the next tree
and the process repeate. The work is carried on at night.
Let it be supposed that a man owing an orchard of several acres
has made arrangements to have this fumigated. Before entering on
the actual work certain preliminaries are attended to. The fumigator
prefers and usually requires that the orchard shall have been culti-
vated recently so that the ground shall be clean and smooth. This
condition of the soil is not only an advantage to the fumigator but to
the grower as well, for in loose level soil the tents will lie closer to the
ground and thus allow less escape of gas underneath than would be
possible on weedy or roughly furrowed land. The moving of the
chemical cart or wagon is also more difficult on the rougher ground.
An outfit usually consists of about 30 tents. Before placing the
outfit in the field the fumigator makes a survey of the orchard in
order to determine in what manner the tents can be used to best
advantage. This depends on such considerationas the arrangement
of the trees, the lengh of the rows in different directions, slope of the
soi, whether irrigation furrows are present, location of water supply,
and similar factors. Having decided the direction in which the tents
shall be pulled, the wagon which has moved the outfit from the pre-
coding field to the present one is driven along the first row to be
fumigated and a tent and generating pot dropped off at each tree.
The "commissary," or place where the supply of chemicals and water
is located, sould preferably be near one end of the row of tents.
The location of the source of water will, of course, determine this
position. If no source is bordeing the field, barrels should be pro-
vided for this purpose. The acid is usually furnished in large iron
drums. It is convenient to remove the acid from the drums into
10-gallon glass carboys of the nature shown in figure 6. These car-
boys are easy to handle and two or three hold enough for a full
night's work.
Immediately preceding the treatment the tent-pullers unfold the
tents and have them in position for covering the trees. This position
should be with one end facing the tree on the side away from the
direction in which the are to be moved. Covering the trees is
commenced at one end of. the row. Two poles of the character
described on pages 20-21 are required, one for either side of the tree.
If rings are in the tents the ends of the poles are attached to the
rings. However, it is very much easier and more satisfactory not
to use rings on tents manipulated poles, b pout to double-lap the
edge of the tent over the end of the pole and attach it by a half hitch
of the pulling rope (Plate III, fig. 1). This is quickly done, does
not subject the tent to undue wear, and prevents detaching, as
sometimes occurs with rings, but the greatest advantage is that the
distance between the poles can be gauged in accordance wth the






26 HYDROCYANIC-ACID GAS FUMIGATION IN CALIFORNIA.

width of the tree, which frequently makes tent pulling much easier
than when the poles are more broadly separated. The tent should
always be moved in the direction of the strips of cloth so as to prevent
pulling the seams apart.
The successive stages in the covering of a tree are shown in Plate
V. In brief, they are as follows: When the tops of the two poles
have been attached to the edge of the tent the width of the tree
distant from each other, the bottoms are placed at the sides of the
tree opposite the trunk, as shown in Plate V, figure 1. Each tent-
puller then places one foot
on the end of his pole to pre-
vent it from slipping and
S'. pulls on the guy-rope, thus
raising the upper end of
the pole and the tent (see
Plate V, fig. 2). When
erected to such an angle
that the poles no longer
slip, the puller removes his
foot from the bottom and
runs away from the pole so
as to secure a greater lever-
age on the rope (see Plate
V, fig. 3). The direction
of the pulling should be
not only forward but also
somewhat to the side so
as to keep the tent taut
between the ends of the
poles and thus prevent it
from being caught in the
FIG. 6 -Carboy with handles attached to facilitate pour- top -of the tree by sagging.
ing the acid and carrying the carboy. (Author's illus- After covering the tree the
tration.) edges should be kicked in-
ward so that the tent hangs straight from the tree to the ground,
thus preventing unnecessary space underneath and making the tent
lie close to the soil.
The removal of tents from one tree onto another is done directly
without first lhving to pull them off onto the ground. In fact, it is
easier to (raw a tent off of one tree onto another than to raise it from
the ground onto the tree. Attach the poles to the edge of the tent
as previously explained. The poles can then be laid flat on the
groundl, as ini Plate VI, figure la, or the end with the tent attached
raised up an(d leaned jaainst the tented tree, as shown in Plate VI,













w
-o
-J
a-












0
4-8~






4-d
01





















FIGS. 1-.-SUCCESSIVE STAGES IN PLACING A TENT OVER A TREE WITH POLES. FlQ. 6.-A TENTED TREE, SHOWING METHOD OF SECURING THE DISTANCE
AROUND THE BOTTOM OF THE TENT BY MEANS OF A TAPE ATTACHED TO AN IRON ROD. (ORIGINAL.)
E i i iiiiiiiil~ i i ii iiiii iiiiiii iiiiii~ ~i iiiiiiiiiiiiiiiiii iiii iiiiiiiiiiii iiiiii !i iil!ii liiii~ iiiiiii iii iiiil!i ii i i i i i iiiiiiiii iiii ii ii i iiiii iiii i iiiiiiii iii iii iiiii iiiiiiiiiiiiii ~ iii i ii iiiiiiiii i i iii~ iiii i i i ii ~
0( ) ( ( ) ( ) ( ( ( .............................. .i ) ))) ) )))% )))) ) )) )))U i i i i i i i i i i i i i i i i i i i i i~ i i )
w! i i ......
o0............................................................................................. . . . . ... . . . ..... .. "ii i ii iiiiiiii % . . . . . . ..... .. . ... .. ... ii iii iiiiiii ....














AROUND THE BOTTOM OF THE TENT BY MEANS OF A TAPE ATTACHED TO AN IRON ROD. (ORIGINAL,)







w






















60
2
Q 4;

a












00









REMOVING A TENT FROM ONE TREE ONTO ANOTHER BY MEANS OF POLES.
Figs. 1a and lb show two different methods of the first stage in removing a tent: in 1l poles are flat on the ground: in lb the ends are raised n nd le7and oi .i,
of tented tree. The successive stages following figure 3 are the same as 4, 5, and i6, of Plate V. (Original.)






FUMIGATION OF CITRUS TREES. 27

figure ib. The second step in the procedure is shown in figure 2 of
Plate VI, while the remaining steps are the same as in Plate V,
figures 4, 5, and 6.
In covering very large trees derricks of the nature described on
page 21 are used. Four men are required for their manipulation,
which is shown in Plate VII. A derrick is raised to a nearly upright
position at each side of the tree (Plate VII, figs. 1-3), leaning at a
slight angle backward and held in this position by the guy-rope
attached to its top (Plate VII, fig. 3). The movable pulley of each
derrick is then attaced to a ring in the tent (Plate VII, fig. 3) and
pulled up to he top of the derrick, where it is held (late VII, fig. 4).
By puLling on the guy-rope the derrick is caused to fall forward,
drawing the tent over the tree.

CALCULATING THE DOSAGE.

Having covered the trees, the next requirement is the amount of
chemicals to use, or the dosage. The dosage i the most important
consideration in the gas process. It varie not only with the size of
the tree but also with the character of insect to be destroyed. Spe-
cific recommendations of dosage for the principal isects injurious
to citr trees in California are iven elsewhere in this bulletin
(pp. 51-61).
The first requirement in calculating the dosage for a tree is to
compute the cubic contents inclosed by the tent when in position
over the tree. Although most citrus trees posses a certain gneral
similarity in shape, they are nevertheless somewhat irregular, no two
ver being iden ain atic ll respects. This irregularity renders it
impracticable to determine the contents to within a cubic foot or so
of its actual volume; yet it can be approxiated with a sufficient
degree of accuracy for such practical work as fumigation. In order
to calculate the cubic contents of an object it must be considered as
shaped like som regular geomietrical figure or figures. The figure
which most closely approximates in shape an orange or lemon tree
before it has been pruned is a cylinder surmounted by a hemisphere,
and in computing the volume of citrus trees they are considered to
be of this shape.
If the height and iidth of a tree covered with a tent is known, it
is a comparatively simple matter to calculate its contents. In the
past work in California the dosage has been based upon these two
measurements. After a tree i ere tent it is a matter of
some difficulty to determine its height and width. By using as fac-
tors the distance around the bottom of the tent and the longest dis-
tance over the top of the tent we arrive at a more practicable method
by whic to compute the cubic contents of a given tree. Usng these

Li






28 HYDROCYANIC-ACID GAS FUMIGATION IN CALIFORNIA.

measurements as a basis the writer has invented a formula 1 by
means of which the cubic contents of a tree may be computed.
To avoid computation work in the field, in so far as possible, the
writer has formulated a table approximating the cubic contents of
trees of different dimensions, which is sufficiently extensive to include
any citrus tree in southern California. During this investigation no
tree has been found whose dimensions did not fall within the limits
given in this table. The distance around and over a given tree being
known, the table will show the approximate cubic contents of the
tented tree. The dosage can then be applied in proportion to the.
contents and at any strength desired.
A lemon tree, after being pruned, is flat on the top. Therefore
the geometrical figure which is applicable to an orange orunpruned
lemon tree can not be considered as applicable to a pruned or flat-
topped lemon tree. The figure which approximates the latter is a
cylinder. Now it so happens that the contents of a cylinder having
certain dimensions over its top and around its bottom are almost
the same as for a figure of the same dimensions composed of a cyl-
inder surmounted by a hemisphere. This is a great advantage, for
the schedule of dosage proposed for orange trees may also be used
for all lemon trees, thus obviating the necessity of preparing two
different schedules.

SECURING THE MEASUREMENTS AROUND AND OVER.

The distance around the bottom of a tent is easily secured by the
use of a tapeline, or by pacing. The distance over the top, however,
was much more difficult to determine until Dr A. W. Morrill,2 in
the course of his work for the Bureau of Entomology against the citrus
white fly (Aleyrodes citri R. & H.) in Florida, invented a method of
marking tents for this purpose. The Morrill method renders the
securing of the distance over the top of the tent as easy as that
around the bottom.
1 Prof. Woodworth (Bul. 152, Univ. of Cal. Agr. Exp. Sta., p. 5, 1903) was not only the first to suggest the
measurements around the bottom and over the top of tented trees, but also was the first to propose'a for-
mula for obtaining the contents of tented trees based on a knowledge of these distances. An analysis of
this formula during the early part of the writer's field work proved that it was inaccurate, thus necessitat-
ing the determination of a new formula. The writer has worked out a formula based on th ttwo measure
ments above mentioned. It is as follows:
C2 (O C(3x-4)\
4i. 2 12- /
In this formula C equals the circumference of the tree.
0 equals the distance over the top of the tree.
C* C(3-r-4)
IT a person works out and notes down in a chart the values of i and 12- for different vaues of C of
which he is apt to make common use, It is possible by its use in connection with the formula to determine
thel cwnentls of troes with fair rapidity.
a Bul. 70, Bur. Ent., U. S. Dept. Agr., pp. 31-34, 1908.











LU









o
I-















4-







bj




Eo
o

C
















PLACING A SHEET TENT OVER A TREE BY MEANS OF DERRICKS.

Fig. 1.-Derricks in position on opposite side of tree from tent. Fig. 2.-Erecting the derricks. Fig. 3.-Derricks erect. Hook of movabe pulley attached to
ring in tent ready for raising it to tot of derrick. Fig. 4,-Edge of tent raised toopof derrick. By pulling on the guy rope the derricks fall forward, bring-
itn. the tent over the tree. ( rivialn )






FUMIGATION OF CITUS TRE. 29
THE MORRILL METHOD.

In figure 7 is shown an outline of a regulation fumiatin tent
marked after the Mrrill system. Three parallel lines and one line
at riht anles to them are indicated on the tent: The middle one
of the three arallel lines passes through the central point in the tent
c a, nning lengthwise of the central section or strip of which
the tent is made and passing over the top of the tent from the edge

Z/-- l-- Z--
59 91 91
9- 9/-- 9--














n oe se/- th- os- -
686

9 .9 9








the dc in whc te tn su e p *n o
S-i e- t m
Fort-- t-- ao -
z- -- Z--












9 9- 9



14 14 --14
i/S /5 --/
-/ --/7 --7







.7.-Outline of a shet furnigation tent marked according to the Morrill meth od. (Author's illustration.)
on one side to the ede on the opposite side; these lines also run in
the direction in which he tent should be pulled on or off a tee.
Beginning at the center these lines are graduated in feet toward
either edge of the tent, after the manner shown in the diagram.
For tents above 36 feet (averagre size) it is unnecessary to commence
the graduation nearer than 5 feet from the center of the canvas.
When one of these lines is over the middle f the tre the dis -tance
over can be calculated by merely adding together the two numbers
n the opposite sides of the tent where the ede touches the ound.






30 HYDROCYANIC-ACID GAS FUMIGATION IN CALIFORNIA.

For instance, suppose that on the line over the center of the tree 12
is nearest the ground on one side and 15 on the other. The distance
over the center of this tree would be the sum of these numbers, which
is 27 feet. With the lines graduated after this manner it makes little
difference in determining the distance over the top of the tree whether
or not the geometrical center of the tent is at the center of the tree,
the single requirement being that some part of one of the graduated
lines approximates the center of the tree.
The two lines running parallel to this central line- should be about
4 feet distant from it in the larger fumigating tents. The reason for
using these auxiliary lines is, that in practice the center of the tent
is very often pulled considerably to one side, especially in covering
small trees. If the middle line does not fall immediately over the
center of the tree, one of the other two lines is quite likely to do so,
and that one should be used in obtaining the distance over.
The cross line running at right angles to the three parallel lines also
passes through the center of the tent and is marked, like the others.
The idea of this cross line is that in case of an irregularly shaped
tree the distance over can be taken in two different directions and the
average taken for use in determining the cubic contents. For experi-
mental purposes with a few tents this line is an advantage, but in
practical operations it is unnecessary and should never be placed on
the tent, as measurement over the top in one direction is sufficient.
The presence of so many lines tends to confuse the operator.
Having calculated the volume of a tree from the two measurements,
around and over, it is possible to dose the tree at any strength per
unit volume desired. When the dosage has been determined the
chemicals are measured out and placed underneath the tented trees.
THE OLD METHOD OF PROCEDURE.

When this investigation was started, a system of fumigation was
used exclusively in which the dosage given the trees was based
entirely on guesswork. The estimator, who ordinarily is the man in
charge of the outfit, starts out in an orchard equipped with a blank
schedule sheet of cross-section paper. He walks between two rows
of trees, jotting down in the corresponding squares of the schedule
sheet the dosage which he believes the trees should receive. This
dosage is based on his eyesight supported by his past experience.
If he is a careful scheduler, he will look at the trees from different
si(des before ilndicating the dosage, as trees are sometimes more com-
I)pact on one side than on another. Less careful men set down the
I,,sdage flor tlie two rows of trees while moving along as fast as they
CLn walk. The writer has seen some schedulers walk through the
or:chard at a rapid pace, taking four rows at a time.






FUMIGATION OF CITRUS TREES. 31

The inaccuracy of such a method is at once apparent. Measure-
ments made after many estimators have shown that the most careful
are very irregular in their scheduling. No one has been found who
does not at times vary as much as 50 per cent in dosage estimates
for trees containing exactly the same cubic contents after being
covered with tents.' This variation in the scheduling of an indi-
vidual fumigator is not all, but the general average dosage used
by one man has

one-fourth to one-
half more and
so0et ies even
twice that used by
anot her for the
very same insect.
This chart of dos-
age for the trees in
an orchard is taken
into the field at
night. Before dos-
ing a row of trees
the common meth-
od is to first meas-
ure out the dosages
for the trees in this
row into small cans
and pitchers, which
are placed in a
hand tray, as shown
in figure 8. This
tray is then carried
from one tr to the FIG. 8.-Man rrying tray and water bucket as practiced under old
next down the row. system of fumigation. (Author's illustration.)
The water is carried in a pail and measured at each tree. The
instruments used for measuring the water have been found to vary
all the way from graduated dippers to quart pitchers or old tin cans.
Under this old method the general results secured by a few of the more
careful and expert fumigators have been fairly good. However, the
work in the majority of cases has been irregular and poor. This old
system is rapidly sinking into disuse, being replaced by an improve(d
procedure which has resulted from the present investigations.
SSee Bul. 79, Bur. Ent., U. S. Dept. Agr., pp. 23-24 1909.





32 'HYDROCYANIC-ACID GAS FUMIGATION IN CALIFORNIA.
AN IMPROVED SYSTEM OF FUMIGATION.
After becoming acquainted with the chaotic condition of the fumi-
gation practice as it existed at the commencement of this investigation,
it was very evident that some system should be perfected which
would entirely eliminate the guess features and provide for a calcu-
lation of the dosage based directly on the size of the tree. As the
result of extended observation, experimentation, and devising, a
system of fumigation having decided advantages over the old method
was introduced into California during the month of July, 1908. This
system is by no means original, but is largely the result of utilizing,
correcting, correlating, systematizing, and making entirely practical
the best methods and ideas which either had been in practice or had
been suggested in the State before this investigation was begun. The
method of procedure was copied largely after that of the San Ber-
nardino County outfits, while the method of calculating the dosage
was suggested by Prof. Woodworth, of the University of California,
and made practical by the adoption of the Morrill method of marking
tents. A method quite similar in general features to the California
improved system was introduced into Florida by Dr. A. W. Morrill,
during the winter of 1907-8. Each of these systems was in the process
of evolution at the same time, yet almost entirely independent of
each other. To Dr. Morrill, however, greatest credit is probably due
for the present advance in procedure, as the inventing of his method
of marking tents was the turning point between impractical and
practical scientific field calculations.
The tents should be marked after the Morrill method described on
pages 29-30. oOnly three parallel lines are used, the crossline being not
only unnecessary but a disadvantage in practical work. This sup-
plies an easy and rapid means of determining the distance over the
top of the tree. The distance around the tented tree can be measured
accurately by pacing if this is done in a careful manner. Experience
with men on the outfit used in this investigation as well as on some
of the practical outfits which first adopted the improved system
demonstrated that after some practice in pacing around tents some
men could so regulate their pace as to be sufficiently accurate for
practical purposes. This resulted in at first advising the method of
pacing to secure this distance. The broad adoption of this improved
system, as well as the frequent changes that take place in the personnel
of a crew, resulted in the pacing being assigned to various types of
men1, some of whom have been known to be hasty and careless.
Another discouraging feature is that the tents are very often not
properly kicked in around the bottom of the tree, which interferes
with accurate pacing. To eliminate the possibility of irregularity
due to the above causes it is now advised that pacing be discontinued






FUMIGATION OF CITRUS TREES. 33

and that the distance around the tent be secured by means of a tape.
To meet this requirement, a scheme has been devised by Messrs.
Griffin and Gray, of hittier, Cal, which renders the securing of the
distance around the tent not onl absolutely accurate but also more
rapid and easy than by pacing. The apparatus consists (1) of a
straight iron rod 3 or 4 feet long and about one-half inch in diameter,
havin the lower end sharpened while the upper end is made in the
form of a loop, and (2) a strong tapeline having a snap at one end by
which it is fastened to the loop of the iron rod.
To secure the distance arond a tent the iron rod is stuck into the
ground at one end of the marked line on the tent which runs over the
top of the tree (Pl. V, fig. 6). The operator then moves around
the tree, allowing the tape to slip through his hand as he moves.
When he has obtained the distance around he drops the tape, takes
the iron rod, with the tape attached, to the next tree, and continues
as before. In this manner the operator is required to move only
once around each tree. This method is entirely practical, as proved
by experience, and in havig their work done the growers should
demand its use. It reduces variation resulting from the work of
careless operators to a minimum. From these two measurements
(the distance around and the distance over) it is possible to approxi-
mate the cubic coenents of the tree and thereby calculate the dosage.
This iht be done in the field and the trees then doed in pro-
portion to the contents. However, the time required for the cal-
culation of the dosage, even after determining the cubic contents
of the tree, woud not only prevet rapid field work and allow an
opportunity for error, but would cause a lack of uniformity in dosage
from the consideration of the cubic contents alone, as will be explained
later. This difficult has been obviated by preparig a dosage
scedule from which the quired dosage may be calculated without
any figuring as soon as the measurements of the tree are known.
LEAKAGE OF GAS.1

One of the most important questions relating to the proper dosage
in fumigation is that of leakage of gas through the tent. In fact,
with the present character of tenting, where the gas has usually all
escaped by the end of an hour, the dosage depends directly on the
amount of this leak In figures which approximate a citrus tree
in shape the volume decreases at a more apid rate than does the
surface Computation shows that a tree 20 feet around by 12 feet
over has 0.86 of a square foot of tent surface for each cubic foot of
to escape through, whereas a tree 79 by 54 feet has only
0.22 of a square foot of tent surface for each cubic foot of gas to
escape through. This would mean that there is about four times as
SSee BuL 79, Bur. Ent., U. S. Dept. Agr., p. 47,1909.







34 HYDROCYANIC-ACID GAS FUMIGATION IN CALIFORNIA.

great an opportunity for leakage, or that the leakage would be approx-
imately four times as rapid in the smaller tent as in the larger one.
There can be little doubt that the leakage of gas through most of the
tenting materials used in this State is nearly in accordance with these
figures. In order to secure uniformity of results this leakage must
be taken into consideration and small trees must.receive more cyanid
to 100 cubic feet than the larger trees. The correctness of the fore-
going statement has been repeatedly demonstrated during the work
in the field. Reference to the leakage of gas through tents was first

D/S7TANCE ARPOU/ND, IN/ FEET.
16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 5860 62 64 66 68
10 Z 2 2 & 10
12 2 3 3 4 12
14 3 J 33 44 4556 S S5 61 "" 14
16 3 3 4 4 S 55 55 6 6 6 71 1 16
18344 5r l 5 65 6 666677 61 18
20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68
20 ,4 S ~5 6 6 6 6 7 7 8s8 9' 9 /0 /0 20
R 22 6 6s 6s 7 7 7 8 8 ,9 910 1o 0 0 9/ 22
S241 6 7 7 8 6 9 /o0 /o // / /2 12 24
S26 7 II8 8 / 9 /I/ N /2 /33 /3 41 i/s i 26
28 8s 9 /Oo /0 // /12 /1 13 14 /4 4 /f / 6 /16 1 28
30 32 343638 40 42 44 46 48 50 52 5456 58 60 62 64 6668
"30 9 /0 // /2/ 12 /1 3 r/4 /515 iS /6 17 1/8 /8 9 20 1 2/ 3
32- 13 114 1 /2 /3 // /6 /6 /6 /77 /7 /8 //92 o 0 2// 2s 32
34 17/3 /s4 /5 //7 /7 /818 /8 /9 /9 0s 2/ 2/ s 22 j 34
36 /4 /s /6 /77 /7/8 /91 o 3 o 202/ 2 22e as a23 4 s 36
38 / /S /7 / I /9o 202/ 1 2R ,3 23 24 2 4 s s6 3
40 42 44 46 48 50 52 54 56 58 60 62 64 6668
,40 /6 /7 / //9 o 20 2/ 2 23 24 R4 s 26 262 7 40
S41 -/ /9 20 o20 2/ 22R23R24 24so72 s266s272 2 41
42- --o 20 202/ 2223,s4 2262as s s627 68 -a 42
431 1 R 2 sW as A Rs 06 2s 7, 47 7s a9 432
44 1s Zs3 s3 2S s 26 26 7 S 8 9 s29 44
50 52 54 56 58 60 62 64 6668
45 23 s 2526 26 27 228 92930 0 45
46 4 -22626 27 27 eB s9 9 30 3/1 46
47 e 267 7 s 72 so 2 03/ .32 47
1481 R6 2 26 2728 29 9 30 / 32 32 48
49 25 27s2gas as so so 3/ as A 49

FIG. 9.-Dosage schedule No. 1, for potassium cyanid. (Original.)

made by Prof. Woodworth,' and in a recent publication Dr. A. W.
Morrill 2 has given it a very thorough treatment.

DOSAGE SCHEDULE.

After having performed a large number of experiments against the
purple scale, and determined the dosage required to destroy this in-
sect on different sized trees, the writer utilized these results in pre-
paring a schedule of dosage to be used in fumigation. This schedule
(see fig. 9) has been designated "dosage schedule No. 1.I" The
doages used in this schedule are entirely original with the writer,
being the result of experimental work supplemented by calculations
as explained in the following paragraphs. After the dosages were

Sceo Bul. 152, Univ. of (al. Agr. Exp. Sta., 190. 2 See Bul. 76, Bur. Ent., U. S. Dept. Agr., 1908.






FUMIGATION OF CITRUS TREES. 35

determined they were introduced into a chart of the same general
form as that used by Dr. Morril. Such a chart form has been known
in scientific work formany years and was first introduced into fumi-
gation scheduling by Prof. Woodworth.'
A averae-sized orange tree, one 41 feet in circumference by 28
feet over, was taken as a basis in the preparation of the schedule.
The cubic contents of the tree was determined and a dosage calcu-
lated which would give it 1 ounce to each 100 cubic feet. Trees of
other dimensions, both larger and smaller, were then considered and
their contents determined. In working out the dosage for these other
trees not only was the cubic contents taken into consideration but
also the rate of leakage as compared with that of the tree 41 by 28
feet in size. Trees which were smaller than tlis would have a greater
proportional leakage rate while the larger ones would have less, as ex-
plained on pages 33-34. In securing the dosage for various trees, those
smaller than 41 by 28 were given sufficient cyanid in excess of 1 ounce
per 100 cubic feet to offset the increased leakage, while the dosages for
larger trees were proportionately decreased below the 1-ounce rate.
This allowance for leakage so odified the schedule that some of the
smaller trees were receiving in excss of 1 ounces per 100 cubic feet,
hile trees as large as 60 by 44 were receiving only about three-fourths
of an ounce to the same space. It thus can be seen that each dosage
ws worked out independently and so correlated to the other dosages
that when placed in schedule No. 1 the ultimate result was that of a
schedule which should approximate uniform results throughout.
ow to e the chart.-Referrin ain to figure 9, th ttop line of
numbers, commencing at 16 and continuing up to 68, repreents the
distance, in feet, around the bottom of the tent. The outer vertical
columns of numbers, on either side, commencing at 10 and increasing
regularly to 49, represent the distance, in feet, over the top of the tent.
The dosage of a tree of known dimensions is found in that square where
the vertical column headed by the distance around the tree intersects
the horizontal line of figures corresponding to the distance over. For
instance, in the case of a tree 40 feet around by 28 feet over, in the top
line of numbers 40 is next after the third heavy vertical line. The
dosages computed for trees 40 feet around are to be found in the ver-
tical column headed by this number, which commences with 7 and
ends with 16. Then the vertical column of large figures at either
margin is followed down until 28 is reached. All dosages computed
for trees 28 feet over are found in this horizontal line of figures, which
commences with 8 and ends at 16. The dosage for a tree 40 by
28 feet is found at the intersection of this line with the vertical col-
umn headed with 40, that number being 11, the required dosage of
cyanid in ounce. Before the numbers 20, 30, 40, and 45 in the lines
Bul. 152 of Cal. Ag. Exp. Sta., p. 1903.






36 HYDROCYANIC-ACID GAS FUMIGATION IN CALIFORNIA.

at the right and left margins are to be found blank spaces, and in the
horizontal lines corresponding to these the numbers at the top of the
chart are repeated in that part of the chart containing dosage figures.
These numbers, repeated in this manner, make it easier for the eye
to locate with certainty the dosage figures sought. In the chart used
by the writer the figures representing distances around and over are
printed in red. The lines bounding these columns of figures are also
red. All the rest of the lines and figures are black.
The writer does not maintain that this table is accurate to the
minutest part of an ounce for every dosage, but that such variations
as do exist are so small that in practical work in the field the results
in killing scale insects will be found uniformly satisfactory through-
out. Two years of experience with the outfits belonging to this
investigation, as well as with many practical outfits, in which work
thousands of acres have been fumigated, have proved that this
belief is well founded.
It is a common practice with tent-pullers in covering small trees
to kick in the edge of the tents little if any so as to prevent as much
as possible the generator as well as the escaping gas from coming in
contact with the tent. This common practice leaves much more
:space under the tent and incidentally makes more leakage surface in
.small trees than was provided for in the original chart. (Bulletin
No. 79, fig. 28, p. 65.) To correct this feature of the fumigation
practice, the original chart has been revised by increasing the smaller
.dosages to the extent which in field practice has been found necessary.
In this revised chart the half ounces are not used as in the original.
Field experience has taught that it is desirable to have schedules as
simplified as possible. The writer would now advise all fumigators
to discard all old schedules and to use the revised one entirely. The
.dosage strength on which this schedule was based was calculated for
use against the purple scale. However, this does not imply that it may
not be used against other insects; in fact, the greatest advantage
of it is that it can be manipulated so as to meet the requirements for
use against any insect. The schedule in its original form is not recom-
mended for use against all insects under all conditions, as many have
believed. Some of the scale pests frequenting citrus trees require
a heavier dosage for their destruction than others. The first point
to be determined is the strength of gas required for a particular pest
under its special conditions. When this is known, then schedule
No. 1 can be manipulated to meet the requirements, provided it is
not already of the proper strength. This change is secured by
increasing or decreasing all the dosages throughout schedule No. 1 at
the samne rate, i. e., , 1, 1, etc. The resultant schedule will be one
of uniformity even as is the original. So in treating an orchard the
first point to determine is what species of insect has to be combated.






FUMIGATION OF CITRUS TREES. 37

avin determined this, the dosage strength to use must next be
learned. On this latter point the writer has spent much time, and
elsewhere in this bulletin will be found information as to what dosages
should be used for most of the common scale pests.
PROCEDURE.
Five men are required to operate this system to advantage. Two
men pull the tents and kick in the edges around the bottom of the
tree. One man takes the measurements of the tree, and should also
empty the generator to be used for that tree and have it in readiness
by the time the supply cart arrives. He should empty the generator
with one and the same hand at all times, and with this hand he should
never touch the tent. He should also be careful not to slop any of the
residue on his clothes or shoes lest it be rubbed off on the tent and thus
produce acid holes. The supply cart, described on pages 22-23, is most
convenient for carrying the chemicals from tree to tree. Two men
look after the chemicals-one measures the water and acid, the other
weighs the cyanid. The latter then holds up the edge of the tent while
the acid man places the charge beneath the tree. (See Pl. VIII,
fig. 2.)
-In actual field practice, after the tent-pullers have commenced
removing the tents the cart is brought to one end of the row which
Sto befumigated. (See Pl. VIII, fig. 1.) The estimator secures
his measurements and calls them out to the cyanid weigher at the rear
of the cart, who then determines the dosage from a schedule which is
fastened to the raised platform. The required amount of chemicals
is then measured and the tree dosed. While the handler of cyanid
and acid are thus engaged the estimator has moved on to the next
tree, secured his measurements, and hols his generator in readiness
when the cart is brought up. This tree is dosed in the same manner
as the first, and thus the procedure continues until the entire row has
been fumigated.
The above procedure is such as the writer has used in the field and
as has been followed by most outfits using the improved system.
In the procedures adopted by some other outfits there are marked
differences in the duties of the different men.
ADVANTAGES UNDER THIS SYSTEM.
This improved system possesses decided advantages over all
others The element of guess in estimating dosage and the conse-
quent waste of cyanid under the old method are eliminated. With
the use of a known dosage strength certain definite and uniform
results occur. The chemicals are measured accurately and the most
economical proportion used at all times. Each tree gets the dosage
scheduled for it-a result which did not always happen under the
old method, owing to confusion of the cans on the tray. The tent-






38 HYDROCYANIC-ACID GAS FUMIGATION IN CALIFORNIA.

pullers seldom get more than one or two trees ahead of the cart,
and thus all trees receive the same length of exposure. The work of
the men who dose the trees is easier than where the heavy trays are
used. Another decided advantage is that the orchardist can at any
time determine what character of treatment is being given his trees
and see that the work is properly carried out. In the past many
persons have been prone to look upon fumigation as a process that
is complex and more or less mysterious. In some cases fumigators
of years' experience have encouraged this widely prevailing opinion,
so that they might themselves be looked upon as experts in a prac-
tice difficult to understand and only capable of being successfully
performed by men of long experience and special qualifications.
This is, of course, erroneous. The improved system outlined in
these pages shows how simple the practice of fumigation may be
made. Careful men who have never before heard of fumigation
can begin the practice of this system and are competent, after a
few hours of instruction, to secure even better results than were to
be expected from the most expert fumigator in California under the
old method. This system reduces fumigation to a matter of simple
mechanical operation, entirely intelligible to the average man,
wherein the operator, to obtain the best results, is required merely
to proceed according to the formulas and directions given. Hence,
orchardists are enabled to own their own outfits and carry on their
work after the most economical as well as successful manner. Owner-
ship of tents is to be advised for several reasons, which are fully
explained elsewhere in this bulletin. Fumigation then can be con-
ducted at the most opportune time, good and careful work is assured,
while the expense is at a minimum. An orchardist owning his own
tents can keep his fruit clean at all times, which otherwise might be
impossible, because the number of fumigation outfits in southern
California at the present time is insufficient to cope properly with
the situation.
EXPERIENCE WITH THIS SYSTEM.

The introduction of this improved system was made with two
outfits of the Whittier Citrus Association, at Whittier, Cal., during
the latter part of July, 1908, using dosage schedule No. 1 for the
purplle scale. On August 15 a demonstration was made before a
nmeting of fruit growers and fumigators in Los Angeles and.pro-
voked deep interest. The very successful work against the purple
scae at NWhittier had begun to be evident by this time and led to
the prompt adoption of the system by the orchardists thereabout.
Growers from other localities, inquiring into the experience with
the recent innovation at Whittier, commenced to sanction its adop-
tion in their respective districts, so that by the end of the fumiga-
j,'







Bu 90 Part I, Bureau of Entomo .og U. S Dept. of Agricultue PLATE VIII.




























FIG. 1.-A Row OF TENTED TREES AND CART AT ONE END OF Row, READY TO COMMENCE
DOSING. (ORIGINAL.






























FIG. 2.-DOSING A TREE. THIS VIEW ALSO SHOWS THE SCHEDULER SECURING THE
MEASUREMENTS OF THE NEXT TREE IN THLe Row. (ORIGINAL.)






FMGATION OF CITRUS TREES. 39

Ss of 1 fully a dozen outfits in various parts of Los
and Counties were using the new method in prefer
e to th e o Theexe of the firstseason hsled to th
rapiand sucess introduction of the new system quite generally,
that it now has adopted by many outfits in Los Ang
Orange, Ventura, Riverside Counties, while in San Bernardino
Couny it is used almost exclusively.
Naturally there was considerable opposition, at the commenc-
ment of this investigation, on the part of the professional fumigators.
hr prjuice has been overcome to a large extent by demonstra-
tions and pe rona cooperation, and many of them are now endorsing
the new methods. The chief means of exploitation have been lee-
tures, demonstrations, and personal contact with the fruit growers.
Sthis educational campaign the assistance of many county horticul-
tural officers and managers of citrus associations has made success
far easier than it otherwise would have been.
The rapid and general adoption of the new method indicates its
practical economy, for new ideas are not adopted by California
horticulturists mere for the sake of novelty. The primary ques-
tion before the grower is whether in the long run the new system of
tmigation is more economical than the old one. The new system
s been usd in and about Whittier for nearl two years. Having
been located in tht region, the writer has been able to keep in touch
ith the c on of fumigation thereabouts.
A y ago oall fumigation in the Whittier and Rivera dis-
was e on under the new method. Packinghouse statis-
ties of last year's crop at the Whittier and Rivera Citrus Associations,
hich handle m f fruit from this section of several thousand
acres, showed that a considerably smaller percentage of fruit was
scarded because of being infested with scale than during any
se n when the old method of fumigation was practiced. Such
statistics are conclusive and their significance is plain.
One of the writer's early contentions was that, after one or two suc-
cese thorough treatments under the new method using the proper
og, most orchards would be in such a clean condition that they
could go without treatment at least every other year. Indicating
the correctness of this belief, Mr. William Wood, the very efficient
formehorticultural officer f the Whittier district, states that
many more orchards in his district which were treated during 1908
under the new system were sufficiently clean not to need fumigation
the following season than has been the case under the old method
t a t ithin his experience. To show the general attitude of
the growers it is only necessary to say that they are sosatisfiedas
to refuse to have their orchards treatedexcept underthe newpro-
cedure. This condition at Whittier is illustrative of what is taking
673300-Bull. 90-12--4
an ao OB
~;""" the






40 HYDROCYANIC-ACID GAS FUMIGATION IN CALIFORNIA.

place in many other districts in southern California. In short, the
experience with this new method of fumigation has been so success-
ful throughout the southern fruit-growing sections that it is only a
matter of time when it necessarily must entirely supplant the old
methods.
THE CHEMICALS IN FUMIGATION.
For the generation of hydrocyanic-acid gas in fumigation potas-
sium cyanid,. sulphuric acid, and water are necessary. The hydro-
cyanic-acid gas is produced by the action of the sulphuric acid on
the cyanid of potassium. Under the early methods of generating
hydrocyanic-acid gas the cyanid was dissolved in water before being
used. At the present time cyanid is used entirely in the crystal
form. The water is first measured and poured into the generating
vessel. The required amount of acid is then added to the water,
producing a great increase of the temperature of the mixture. While
the mixture is hot it should be placed beneath the tree and the
cyanid added. If permitted to cool before the cyanid. is added,
the generation of gas will not only be slower than with the heated
mixture, but the amount of available gas will be decreased, thus
making the operation more expensive, and necessarily less efficient.

POTASSIUM CYANID (KCN).

An imported potassium cyanid designated as 98 to 99 per cent
pure is used almost exclusively for fumigation purposes in southern
California. This imported potassium cyanid has been employed
throughout the field investigations of this bureau and is referred to
in this bulletin whenever cyanid is mentioned, unless specific men-
tion of another grade is given. Analyses of seven samples taken
promiscuously in the field during this investigation averaged 98.1
per cent pure, the poorest sample being 97.28 per cent. The gen-
eral funigation experience with such a high grade of potassium
cyanid has been very satisfactory. This cyanid is purchased in
cases containing approximately 200 pounds.
The potassium cyanid most frequently used in California for
fumigation purpose s is commonly spoken of as "German" cyanid, it
being generally known that this chemical is imported' from Ger-
many. This term "German" has been used in distinguishing the
regular potassium cyanid from another kind popularly knon as
"Ameriean cyanid, which has had a very limited and unsatisfactory
usage for a number of years. This "American" cyanid ws known
to ie of a generally much harder composition and slower in gener-
at ion. Because this latter cyanid is made in America the general
belief 1is prevailed among fumigators that it is impossible to manu-
I For Ithf in of lo im n va;r id, sr e iPart II of this Bulletin.






FUMIGATION OF CITRUS TREES. 41

facture as suitable a grade of fumigating cyanid in this country as in
Ger y. Such a belief is, of course, erroneous. Chemical analysis
f thi o-called ericn" anid has so it to be not potas-
sum cyanid but sodium cyanid, which is a very different article.
N potassium cyanid is manufactured in this country.
A potassium cyanid guaranteed to be 98-99 per cent pure should
e used, as experience gaied during this investigation, as well as
that from commercial operations, has proved this grade of cyanid
to be uniformly successful. Moreover, such a highgrade article is
quite free of sodium chorid co on salt), the detrimental action
of which is explained later.
Cyalid should be exposed to damp air as little as possible, as it is
decomposed by moisture. alysis of a sample exposedt to the air
for a few months showed it to be several per cent less pure than
originally. Such a cyanid, or even one which has become moistened
by ony a few days' exp sure to the weather, is slower in generating
its gas, and this is an objectionable feature in fumigation. After
opening in the field, the case of cyanid should be protected by a tight
cover which will ward off the action of de or rain.

SULPl URIC A~CD (11s04).

A ommercialsulphuric acid ( 0 ), f L Baum, which is approxi-
mtel 93 per cent pure, should be used. Sulphur is the basic
edient in the manufctre of sulphuric acid. Generally speaking,
nl acid in Which the sulphur is obtained frm bristone is preferable
to one ade from ir pyrites. The rea is that those which are
lma'de from a brimstone base usually contain fewer impurities than
those iade from iron pyrites. If the impurities be eliminated,
owever, the sulphuric acid made from the e is as satisfactory in
fumigation that made from the other.
The commonest impurity in sulphuric acid is sulphate of iron
(FeSO). This often occurs in acids made from pyrites, and some-
times to a very great extent. It adds a milky appearance to the
acid. The action of acid on long-used iron drums also causes the
formation of sulphate of iron, evidenced by the whitish ppearance
of the "settlings" or t c t the bottom of the drum. The
writer has used acid contaiinin considerable sulphate of iron without
any apparent injury to citrus trees or uit. Nevertheless acid con-
SExperimetation during this investigation has shown that a high grade of sodium cyanid will produce
extl as satisfactory resuls as a high-grade potassium cyanid. No sodium cyanidless than 128-30 per
cent pure (as reckoed in terms of a potassiu cyanid)shold be used. If a suitablepotassium cyanid is
not available then purchase a sodium cyanid of the purity mentioned. A pound of this sodium cyanid
contains approximately one-fourth more available gas than a pound of potasium eyanid. Hence, if used,
the dosages employed should be one-fourth less than those mentioned in this bulletin. The roportlon of
chemicals alsodifferent.A 1--2 formula is advised; that is, to avee) of 12130 per
cent Nodium eyanid use 11 ounces (liquid measure) of sulphuric acid and 2 ounces of water.
For a thorough treatment of sodium cyaid in relation to fumigation see Part II of this Bulletin.






42 HYDROCYANIC-ACID GAS FUMIGATION IN CALIFORNIA.

taing large quantities of sulphate of iron should be avoided for
the same reasons that cyanid containing large quantitie of impu-
rities should be avoided, even though the impurities are apparently
harmless.
Traces of nitric acid (HNO,) are sometimes present in sulphuric
acid. For several years an opinion has been current in California
that nitric acid when present in sulphuric acid used in fumigaion
would result in the burning of fruit. Burning of fruit has occurred
to a greater or less extent throughout the history of fumigation,
yet in recent years, because this damage has sometimes taken
when an acid made from pyrites was being used, and in which a
trace of nitric acid was sometimes present, the belief has become
quite general among fumigators that such an acid was unsafe.
The theory proposed as the cause was that the heat produced in
generating the hydrocyanic-ai d gas drove off the nitric acid in the
form of a vapor, which, coing in contact with the cooler surfaceof
the fruit, condensed, resulting in a burn or pit. Careful experiments
were recently carried out in order to decide this point. Eight orange
and lemon trees well laden with fruit were treated on three different
nights, using sulphuric acid containing from 1 to 10 per cent of pure
nitric acid. Dosage schedule No. 1 was followed. The exposure
was one hour. No pitting or burning resulted with any of the
strengths used. As these amounts of nitric acid are far in excess of
the quantities ever found in commercial sulphuric acid it can be
safely concluded that there is no danger of burning as a result of
the presence of nitric acid in the commercial sulphuric acid.
Traces of arsenic, lead, or zinc are sometimes found in commercial
sulphuric acid, yet in all samples of acid analyzed during this inves-
tigation the quantity, when present, has been so small as to demand
no consideration as a source of injury.
Sulphuric acid is purchased largely in iron drums containing from
1,500 to 2,000 pounds. Glass carboys of about 10 gallons' capacity
are sometimes used. The drums, because of their great weight, are
seldom taken into the field. A common and convenient method
is to roll the drums onto an elevated platform at the source of sup-
plies. The acid is then removed into glass carboys or some other
receptacle for carriage into the field. Two or three carboys usually
will contain enough acid for one night's run of an outfit of tents.
C('re should be observed in handling this acid: Rubber oves
are advisable. If some acid accidentally reaches the flesh hasten to
wash t he affected parts with water.
THE AMO1UNT OF SULPHURIC ACID NECESSARY.
Chemical combinations take place with definiteness under the
same conditions; that is, given thle sanme condition, when one
chemical acts upon1 another in the production of a third substance,
4.






FUIGATION OF CITRUS TREES. 48

the proportion between the first two chemicals is practically the
same. Such is the case when sulphuric acid acts upon potassium
yanid in producing hydrocyanic-acid gas. A given amount of
yanid requires a given amount of sulphuri acid of a fixed degree
Spurity in order to thoroughly utilize the quantity of cyanid
employed, evolving the maximum amount of gas, and carrying the
n toion. A quotation from a letter received from
~ K. Hof the Bureau of Chemisty, of this department,

In c acid on po ium cyanid approximately four-fifth of
an of 93 per cent acid is used up for every ounce of 98 per cent

Expressed in fuid ounces four-ffths of an ounce avoirdupois equals about 0.42 of
a fluid ou We my sortically I ounce avoirdupois of 98 per cent
0 ounce of ordinary commercial sulphuric acid
to hydrocyanic acid Since iis always best to
Sthe rcton to completion, it is probable that
id o e f eria sulphuric acid is ample in practice to
convert ounce avoirdupois of 98 per cent potiu cyanid td hydrocyanic acid.
Sfluid ounce of the acid is used it will certainly leave a
of acid. It is perfectly posible, however, that this
f phu d is ofl in eatingup the ture so that moref the
hydrocyanic acid is liberated and not absorbed by the liquid.
Two series of field experiments were performed which were identical
in l r ct ecept that in the first i ounce of sulphuric acid was
u e in the oter ounces were used to eac ounce of potas-
sium cyanid. Analysis of the residue by the Bureau of Chemistry,
tis d ment, showed that raion w ass perfect with the
smaller proportion of acid as with the larger. The addition of a
great excess of acid might even result in an impediment to rapid work
in the field. As an explanation of this condition it might be state
that the residue always contain su ance which is soluble in
water alone, but the presence of a large of sulphuri acid wil
cause it to crystallize and solidify. This latter condition will fre-
quently occur if more than equal parts of acid to cyanid are used,
especially so )ith the smaller dosages. The removal of solidified
residue necessitates loss of time.
Summing up, it may be said that 1 fluid ounce of commercial
sulphuric acid (93 per cent) to 1 ounce (avoirdupois) of 98 per cent
potassium cyanid is certainly enough to carry the reaction to com-
pletion in the liberation of hydrocyanic-acid gas and is perhaps an
unnecessarily large amount. In practical field work, where dosages
of varying sizes are constantly bein used, it is very convenient to
reckou the acid in the same number of parts as the cyanid. The
use of 1 part (fluid measure) of acid to each part (1 ounce avoirdu-
pois) of cyanid is therefore recommended.
The reaction is as follows: 2 KCN+SO= K +2 HCN.





44 HYDROCYANIC-ACID GAS FUMIGATION IN CALIFORNIA.

WATER AS A FACTOR IN FUMIGATION.

There are several reasons why water should always be employed
in fumigation. It is very useful in dissolving the potassium cyanid
and hastening and completing the chemical reaction with the acid.
A piece of cyanid thrown into a mixture of acid and water immediately
gives up a portion of its mass in solution. Scarcely has the cyanid
dissolved when it is partially converted into gas. The heat liberated
during this process assists in forcing the solution of more cyanid
which is also partially converted into gas. This continues until
the chemicals are exhausted and the reaction ceases.
Potassium sulphate, a solid, is the by-product resulting from the
reaction by which hydrocyanic-acid gas is produced. Water dissolves
the potassium sulphate as it forms and prevents it from coating the
cyanid not yet in solution. In the presence of an insufficient amount
of water, the potassium sulphate is not completely dissolved, but
forms a coating on the pieces of cyanid, preventing the sulphuric
acid from penetrating to it, and thereby retarding, or even in part
preventing, the reaction. In such cases this undissolved potassium
sulphate usually solidifies, causing the pots to freeze." This
phenomenon always occurs where the formula is 1-1-1, or where
the same amounts of water, acid, and cyanid are used. On agitating
the residue by stirring, it is almost always possible to find small
pieces of undissolved cyanid enveloped in a coating of the potassium
sulphate. Ordinarily, when the residue is stirred the particles of
cyanid are removed, to some extent, from this envelope of potassium
sulphate, allowing some of the unused acid to reach them, and thus
evolving a small amount of gas without the addition of more acid.
Under these conditions, however, the reaction is never complete,
and it is highly desirable, therefore, to add sufficient water at the
beginning to dissolve all the potassium sulphate.
Recalling the statements made in discussing the amount of sul-
phuric acid to use, it is seen that the "congealing" or "freezing"
of the residue in the generating jars is due to either or both of two
conditions: (1) An insufficient amount of water to completely dis-
solve the sulphate of potassium, or (2) a large excess of sulphuric
acid, whereby the water is rendered less capable of taking into
solution the same amount of sulphate as it otherwise would.
Another very important function of the water in the reaction is
the heat produced by the union of the sulphuric acid and Water.
Potassium cyanid introduced into this heated mixture gives off
hydrocyanic-acid gas much more quickly and thoroughly" than at a
lower temperature, and in field work rapid generation of gas is
essential.
IThe action of iure or highly concentrated sulphuric acid on pota -
si5um (cyanil results in a very different chemical reaction than when





SUMIGATION OF CITRUS TREES. 45
the aci h ben diluted with water. With very dilute sulphuric
id and up to a strength of to 1 part of water, which is
as concentrated a mixture as is ever used in fumigation work, nearly
pure hydrocyanic-acid gas is given off. By decreasing the propor-
ti of water used below 1 part, the amount of hydrocyanic-acid
gas resulting is also decreased until, when concentr ated sulphuric
acid acts on a cyanid, hydrocyanic-acid gas is not given off, but rater
an entirely different gas called carbon monoxid.
THE EFFECT, OF DIFFERENT PROPORTIONS OF WATER.

On tenpemture i ofgas.-Anyone who has watched the escaping gas
and steam from the reaction of potassium cyanid a sulphuric acid
wherein diferent proportions of water were used could not fail to
otice that the violence with which the generation starts and the
ais given off is apparently greatest with the smaller roportions of
water. Experiments carried on by this investigation showed that

PER C.EMNT OF GA$ G/vVLW or, y a'
M 4AC1I0WrR 10%o 20% 30%1 s 40% 50% 60% 70% 80% 90% 100%
I 1 87.84

i 1 3 89.95
1 1 4 86.25

1 6 79.65
11 7 73.47

10.-Chart showing total amount of g evolved when d rent proportions of wa a used.
(Author's llu straton.)
the temperatre of the escapng as was coideraly hierith
smaller proportions of water than with the larger proportions. Inone
eperiment the highest temperature of the escaping gas was 1240 F.
with I part of water, but only 900 F. with 8 parts. The temperature
was approximately uniform with from 1 to 4 parts of water.
On anmount of arailable gas.-The Bureau of Chemistry of this
department, at the request of the Bureau of Entomology, performed
an experiment to determine the amount of hydrocyanic-acid gas
vailable when generated with proportions of water varying from I

art (fig. 10).
In these experiments commercial sulphuric acid 660 Baume, analyz-
ing 92.77 per cent pure, and potassium cyanid 97.12 per cent pure
Iere used. Three ounces (fluid) of sulphuric acid and 3 ounces

See Part IIIof this Bulleti.





46 HYDROCYANIC-ACID GAS FUMIGATION IN CALIFORNIA.

(avoirdupois) of potassium cyanid were employed in each experi-
ment, and 3, 6, 9, 12, 15, 18, 21, and 24 ounces, respectively, of
water were used in the different experiments.
From this chart it is evident that with the grades of acid and
cyanid mentioned the largest amount of gas is available from 2 parts
of water. As the proportion of water is increased above 2 parts the
available gas is decreased, until with 8 parts of water we obtain only
about 43 per cent of gas, or less than one-half as much as with 2
parts. In other words, 1 ounce of cyanid and 1 ounce of acid in
combination with 2 ounces of water will produce much more avail-
able gas than 2 ounces of cyanid and 2 ounces of acid with 16 ounces
of water.
The cause for the smaller amount of gas with 1 part of water has
been explained on page 44. One of the principal reasons for the
decrease of the amount of ga a as we go above 2 parts of water is that
the temperature of the acid-water mixture decreases as the pro-
portion of water increases. With dosages of 5 ounces of cyanid the
temperature was found to be 1900 F. where two parts- of water were
used, but only 1250 F. with 8 parts of water. The hotter the acid-
water mixture the quicker and more violent the reaction with the
cyanid will be. Secondly, hydrocyanic-acid gas is very solubl in
water. As the cyanid is immersed during the reaction, the gas has
to rise through the liquid in order to escape. Less gas will be
absorbed by rapid evolution through a small amount of water than
by slower rise through a large amount.
The proportion of water used by different fumigators under the old
system has varied all the way from 2 to 8 parts, some men even
varying widely in their individual work. In brief, the miethod fol-
lowed by the "generator" man in dosing has been that on coming to
a tree he first looks at his can of cyanid for that tree and then makes
a guess as to how many ounces it contains. If he is using 2 parts of
water he will use twice the amount that he thinks there is cyanidin
the can; if 8 parts, then 8 times the amount of cyanid he thks
there is in the can. A very few'outfits have measured the water in.
graduated beakers; the majority of receptacles used have varied all
the way from half-pint dippers to quart dippers, quart pitchers, or
even old tin cans. Think of measuring with accuracy the amount of
water for a tree requiring 4 ounces of cyanid with a quart pither!
The writer has frequently seen fumigators, in measuring the wter
for a tree, first measure out what they thought to be the proper
amount, then hesitate as to whether it was enough, and finally dip
out a second or even a third portion. Those second and third dips
meant less available gas, and the common multifold guessing in the
measure of water under the old system has been directly responsible
for inegular results.






FIGATION OF CITRUS TREES. 47

It has been a common practice among fumigators to increase the
dosage when fumigating a tree severely infested with scale. It also
been a common practicein fact so comon as to be almost
niversal-to increase the proportion of water when using such heavy
ages. It ved that this extra water reduced the tempera-
ture of the ga, thereby preventing the burning of the foliage. Very
naturally, the use of extra water might produce less injury, but this
would not be due to the reduction of temperature, as has been
lived, but to the decrease of the am of gas given off. This
practice has caused a great waste of cyanid and wide disparity in
lt. Indeed, the iter believes that no one factor has h mor
to do with the wide variation in results secured in fumigating citrus
trees than has this erratic use of water.
THE CORRECT PROPORTION OF WATER.

It been shown that 2of water to part each of cyanid
d sulphuric acid will produce the maximum amount of available gas.
is impractical, however, to use 2 parts of water in field work

are used, will tly solidify within one hour's time,



S to hold the s long h at to
prevent "freezing." This phenomenon is an impedimient to rapid
lfor se lite time is ruird to ove this co
residue from the constricted-neck pots in common use.
It is evident in this instance that a "fro generator does not
imply an incomplete eneration, althoughin some other cases the resi-
due left ma be ealed the eneration incomplete. With 3

recommended by and used in all the field work of the writer. With
dosages of 12 ounces of cyanid or above, a 2-ounce ratio of water can
be used without danger of "freezing." The water should be measured
carefully with a glass or dipper graduated to ounces.

THE MOST ECONOMICAL PROPORTION OF CHEMICALS TO USE.
In the preceding discussion it has been shown that for various
reasons 1 fluid ounce of commercial sulphuric acid and 1 ounce
(avoirdupois) of 96 to 100 per cent potassium cyanid in combination
with 3 fluid ounces of water give a complete reaction. Thus the
1-1-3 formula, hitherto recommended by the Bureau of Entomology,
Sfully indorsed for fumigation workinthe field.
A review of the use of hydrocyanic-acid gas for fumigation, both in
Ciforia and elsewhere shows frequent divergence -from the more





48 HYDROCYANIC-ACID GAS FUMIGATION IN CALIFORNIA.

economical and satisfactory proportion of chemicals indicated above.
However, since the results of this investigation have been given out
the former erratic methods of measuring water have almost entirely
disappeared. The usual practice now is to use 3 parts of water
which is generally measured in.graduated receptacles. The systema-
tizing of the use of water has been one of the greatest accomplish- a
ments of the present investigation.
In such special treatments as that of nursery stock, mills, houses,
and the like, where the extra time required to remove the congealed
residue would in no way interfere with the rapid and economical
progress of the work, 2 parts (ounces) of water to each part (ounce)
of cyanid is recommended.
THE AMOUNT OF CHEMICALS IN VERY SMALL DOSAGES.
The results in the fumigation of small trees requiring from 1 to 3 .or
4 ounces of cyanid have generally been much less satisfactory than for
the larger sizes. If the amounts of chemicals used for such small
dosages in large generating pots are always in proportion to the 1-1-3
formula the reaction will sometimes be slow and incomplete. This is
especially the case if pieces of cyanid of such size as to project above
the surface of tAe liquid are used. In order that the cyanid may be
entirely covered by the liquid the entire dosage should be not in one
piece but preferably in two or more smaller pieces. It is also advis-
able to increase the amount of the acid-water mixture to a slight
extent in such cases. An extra ounce of acid and 3 extra ounces
of water will usually suffice.
MIXING THE CHEMICALS.
It is preferable to pour the water into the generator first and then
add the acid. The pouring of the water onto the acid is more likely
to cause splashing of the acid from the jar onto the fumigator. When
the acid and water are in readiness for generating the gas the fumi-
gator adds the pieces of cyanid to the mixture and hastily retreats.
As already stated, the cyanid should be added while the- mixture of
water and acid is hot. Other investigators have called attention
to this, while experiments performed by the Bureau of Chemistry of
this department show that the reaction with a cool solution is very
inferior to one when the heat is great. Potassium cyanid added
to the mixture of acid and water when hot lost 10.68 per ct of
hydrocyanic-acid gas, while the same cyanid added to a mixture of
acid andl water while cold lost 23.25 per cent, a difference of more
than 12 pIer cent. The cyanid should never be placed in the water
before the acid is added. If the acid is added to the cyanid in solu-
tioIn, a very violent reaction takes place, which will sometimes throw
I Gossard, Bul. 67, Fla. Agr. Exp. Sta.





-UMIGATION OF CIR TREES. 49

mueh of the liquid from the vessel. In one instance about 1 pound
of eanid was dissved in water in a 2-gallon enerator. Acid was
then added, producing a disturbance so violent as to throw some of
the uid almost to the top of a 2-story barn.
The cyanid should be in pieces anywhere from the size of an English
walnut to that of a good-sized lemon. The smaller pieces should be
ud in the sall dosages. Powdered cyanid should be avoided in so
far as posibl. Where purchased in large boxes there is always a
considerable quantity of fine material at the bottom. Entire dosages
for a tree should never be composed entirely of this character of
cyanid or a violent reaction will take place, blowing uch of the
fine particles out of the generator ad endangering the tent as well
as the operator. This fine cyanid i most economically and satis-
factorily disposed of by using it in small quantities along with lumps.
The generation of gas has practically ceased at the expiration of
from three to five minutes.
Mny writers on fumigation recommend the use of paper bags for
holding each dosage wen placed in the generating pot. These bags
reused larely to retard the reaction so that the operator may
etreat to some distance before the eneration comences or else
to prevent slopping. The writer's own experience, as well as some
principles previously mentioned in this chemical discussion, would
lead to advising, against the use of paper bags. The retardation of
generation is so marked in the case of small dosages in heavy paper
b that the ount of gas resulting must be considerably less
than if the cyanid had been introduced in a free state. By the exer-
cising of -a slight aount of care in introducing cyanid in the free
state into a generating vessel there is no danger of the operator
being affectd by the gas or of the acid being slopped out. Neither
will the nerating pots boil over if the amounts scheduled on page
24 are used. Fine or powdered cyanid should never be used in
houses. In household work sheets of heavy paper should be placed
underneath the generators.
EFFECT OF THE PRESENCE OF SODIUM CHLORID ON THE AMOUNT OF
GAS GIVEN OFF.

Pratiall all commercial cyanid contains more or less common
t, technically known as sodium chlorid. The action of this salt
in connection with fumigation demands consideration. It has been
found, when sodium chlorid is present in the reaction of sulphuric
acid on a cyanid in the production of hydrocyanic-acid gas, that
this chlorid salt produces a secondary reaction which liberates an
acid called hydrochloric acid, and that this liberated hydrochloric acid
SIf paper sacks are employed, they should be of thin paper, or slit to allow the free action of the acid on
the evanid.-C. L. M.






50 HYDROCYANIC-ACID GAS FUMIGATION IN CALIFOR IA.

immediately attacks the hydrocyanic-acid gas and decomposes it to a
great extent. Hence, as the presence of sodium chlori
produces a partial decomposition of the hydroc yic-
liberated, the ultimate result is that less gas i ~f t
a cyanid of the same degree of strength wich
sive experiments carried out by the Bureu emistry of this
department showed that the presence of sodium d in a reaction
causes a very marked decomposition of the ydrocyanic acid.
Experiments with two different cyanids each of which has had a
limited usage in California showed that the amount of sodium
chlorid in one caused a decomposition of 9.76 per cent of the total
hydrocyanic acid, the other of 34.07 per cent. An experiment per-
formed with a cyanid having a very large quantity of sodium chlorid
in the reaction resulted in a decomposition of over 92 per cent of the
total amount of gas, only a little over 7 per cent being evolved
The results of thee experiments bring to ourattention a second
requirement in the purchasing of a cyanid. That it be of a certain
degree of purity is no longer the only consideration. It is of equal
importance that the cyanid be practically free of sodium chlorid.
Possibly extensive and expensive refining would be necessary to
eliminate all traces of sodium chlorid from a cyanid. Such a condi-
tion would be preferable but can not be demanded at the risk of
increased cost. We can, however, reasonably expect a high degree
of purity, and the writer would condemn as unsuitable for use in
'fumigation any cyanid containing in excess of 1 per cent of sodium
chlorid. This does not mean that every cyanid used should be
examined to determine if it contains in excess of this quantity of
sodium chlorid. A potassium cyanid 98-99 per cent pure has such
a small margin for impurities that it will not contain any objection
able quantity. A potassium cyanid guaranteed as 98-99 per cent
pure can be used with entire safety provided its purity measures up
with the guarantee.
NATURE OF THE RESIDUE.

The residue resulting from the generation of hydrocyanic-acid g a
is usually a bluish or greenish colored liquid consisting for the most
part of water. It also contains sulphate of potassiu, more orless
sulphuric acid, and some hydrocyanic acid held in solution. This
combination of substances is of a very poisonous nature. Ne t
less, some witers on fumigation, considering the plant-food elements
whiich this residue contains, have advised that the residue was of
mu1ch importance as a fertilizer and should be spread over the ground
for such a purpose. This is an instance of the too frequent tendency
STh domposang act of soium chld on cyanid used i uas first m
N*I li in uifletin 15, (oorgia State Board of Entomology, 1905.






U GATON OF CITRUS TREES. 51

of many writers to advise a practice based on theory aone The
wrr has vegetation destroyed by the action of this residue,
pecially where the amount of residue was large. Its injurious
effect on cover crops has been caled to his attention by one orchard-
ist One of the most striking examples of its injrious effect that
the writer has ever seen was in an orchard of large tree on liht
sany soi. The residue was emptied at the trunks of the trees, with
the result that portions of the root systems of some t were
destroyed. These examples go to show ttthe residue produces an
im diate injurious effect on vegetation wherever it may come into
cwith the same.
The rsiue should never be emptied near the base of the tree, but
ot in the middle of the row. It shouldbe so placed that the tents
do not into contact with it while being moved from one tree to
nothr. Te common practice in California is to empty it midway
between the two rows of trees in the opposite direction from which
the tents are being moved. This prevents their bein dragged over




, and ellow scales ere the insect pests of citrus trees
against which fuigation was generally practiced. In some instances
thistreatment has been tried against the mealy-bug. The distri-
bution of these insects is such that in one locality the purple scale
might be the principal problem of control, whereas in another it
might be the black scale, red scale, or yellow scale. Usually a single
speies will predominate in any one orchard, yet sometimes two or
even all require attention at the same time.
he t ency of citrusfruit growem is to overlook the fact that
problem of control is one wherein different pecie of insects are
coeed and to believe tat whatever treatment the fumigator
aplies should accomplis the same results in all cases. Only the
tent itself is consied, not the strenth of dosage, ad this
s led *any ohadiss to complain because fumigation by the out-
fits owned by associations or counties sometimes costs as much or
even a little more than that by contractors. If one party performs
the work much cheaper than another, the real basis of this cheapness
is that less cyanid is used. The desired results can not be accom-
plised unless thcorrect dosage requirements are met.
Many dosage tables for the different scale pests of citrus trees
have een publised in California an elsewhere, based on a consid-
eration of he ight and width of the trees. These dosage tables for
the most art are very erratic, being calculated largely from hearsay






52 HYDROCYANIC-ACID GAS FUMIGATION IN CALIFORNIA.

rather than from actual experience in the field.1 The practice among
commercial fumigators has been to absolutely ignore these tables,
depending, instead, on their own judgment. The result is that
their scheduling differs markedly from that of the published
schedules.
FACTORS WHICH AFFECT THE DOSAGE.

Although citrus-fruit growing in southern California is restricted
to a limited area the climatic conditions are not uniform in all sec-
tions. The region adjacent to the coast generally is cooler and much
damper at night than in the interior valleys. This situation has led to
a diversity of opinion among fumigators as to the comparative
dosage for the two sections. Some hold that a heavier dosage is
required near the coast on the ground that the gas is absorbed by
the dampness; others, that the drier and lighter air in the interior
valleys allows a more rapid escape of gas through the tent, which
necessitates more eyanid than for the heavier air of the coast. Set-
ting aside these opinions and examining the situation as it actually
is, we find that the general dosage strength for a particular insect is
approximately the same throughout southern California regardless
of nearness to or remoteness from the ocean. Of course, there are a
few striking individual variations from this general statement, but
these variations are as noticeable in one place as in another.
Personal experience in all sections has taught the writer that the
leakage of gas is for the most part noticeably greater in the dryer and
warmer interior sections than near the coast. Despite this.condition
any given dosage appears to be as efficient in one place as in another.
This marked efficiency in the dryer and warmer sections regardless
of the greater leakage might possibly be due to the fact that the scale
insects are more susceptible to the gas in the higher temperatures
general there than in the cooler temperatures of the coast. It is
known among entomologists that insects are active at high tem-
peratures but become dormant at low temperatures and in the latter
condition are more difficult to destroy. Prof. Woodworth, of the
University of California, has informed the writer that laboratory
experiments performed by him have shown this condition to exist
naonlg scale insects and that the temperature at which they become
dormant is relatively high. This interrelation of temperature and
activity has a very important bearing on the fumigation treat nt
iand demands much further experimentation in the field as well as in
the laboratory.
The old conception that an increase of dosage was required near
the coast to offset the loss of gas from absorption by moisture is also
no longer tenable. Experience has shown that the results during
JBul. 79, Bur. Ent., U. S. Det. Agr., pp. 20-22 199.






FUMIGATION OF CITRUS TREES. 53

damp nights near the coast are exactly as satisfactory as on dry ones.
Even if the gas is absorbed by moisture the tents become so much
t ter from bein moist that an negative effect from the dampness
is offset.
The character of the tenting material used directly affects the dosage
required. Most of the ducks and drills now used in California (see
p. 11) are about equaliv gas-tight. The recommendations of dosage
given in this bulletin are for these tenting materials. With the
special new drill experimented with during these investigations (see
pp. 11-12) one-fourth less dosage is required. Any tenting tighter
than the cloth commonly used for this purpose in California will also
require less dosage.
THE PURPLE SCALE.

Preliinary experiments to deterine the os required for the
destruction of the purple scale were ndertaken at Orange, Cal.,
during the month of November, 1907. Oange trees severely infested
with the purple scale in ll stages of development were treated with
e rates va ing from three-fourths of an ounce of cyanid per
100 cubic feet up to 21 ounces per 100 cubic feet. The cubic contents
of the trees varied but little, the trees raging from 11 to 14 feet in
ight. The 1-1-3 formula wa followed. Exposure lasted one hour.
After a period 4f about two months an examination of the results of
this experiment was made. To show the care th which the exam-
ination was conducted in this as well as all other experiments against
the purple scale it might be mentioed that in each case the scales
were overturne and examined with a high-power hand lens.In
those instances in whichof th t ctte scale were not at
once revealed the delicate ventral scale was ruptured and the con-
tents scraed out. Through this method not a single egg could
escape observation.
Aa result of this experiment it was found that all insects were
destroyed on the leaves and branches by a 3-ounce dosage rate,
that all insects and over 99 per cent of the eggs were destroyed at a
1-ounce dosage rate and that all eggs on the leaves and branches were
destroyed at a 1-o nce dosage rate. Very little fruitwas on the trees,
yet, where present, normal eggs were found on the fruit after a dosage
as high as a -ounce rate
In another experiment in which the trees were considerably smaller
s being not more than 7 feet tall, it required a 2-0oune rate to
eradicate the eggs on the leaves and branches, even though the lenth
of exposure was one and one-half hours This condition shows that
smaller trees require a much heavier dosage proportionally than large
trees to offset the leakage of gas, as explained on pages 33-34.
During July, 1908, an orane orchard near Whittier, consisting of
about two acres of trees averain about 7 to 9 feet tall which were






54 HYDROCYANIC-ACID GAS FUMIGATION IN CALIFORNIA.

severely infested with purple scale, was fumigated with the 1-1-3
formula with an exposure of one hour. Dosages varying fro 1
to 2) ounces per 100 cubic feet were used. The result was that
eradication occurred on the leaves and branches at a rate of 2 ounces
per 100 cubic feet, thus corroborating the work on such small trees
previously carried on at Orange.
Eradication of an insect would be preferable, yet experience during
these experiments just mentioned demonstrated that the dosage
required for eradication might result in injury to the fruit. A rate
of 1 ounce per 100 cubic feet for trees about 11 to 12 feet high was safe
under almost all conditions and such a rate was adopted for general
work. Dosage schedule No. 1, based on such a strength of gas, was
prepared at this time.
This schedule No. 1 was used during the autumn of 1908 by several
practical outfits in the vicinity of Whittier, and has since been fol-
lowed in other sections in which the purple scale occurs. Thousands
of acres have been fumigated after this schedule. The general result,
when the work has been carefully done, has corroborated the writer's
own experiments in that all live insects and in excess of 99 per cent
of the eggs were destroyed on the leaves and branches. Such a
killing is entirely satisfactory. In some cases a slight amount of
pitting of fruit has occurred, especially in the top of the trees, and has
caused some growers to complain. This slight amount of pitting can
be overlooked by reason of the superior killing which has resulted.
To use a dosage sufficient to control the scale and at the same time
entirely avoid pitting throughout a fumigating season is a practical
impossibility.
The first season the improved system of fumigation was adopted
schedule No. 1 was used almost universally. As this schedule gives
dosages considerably in excess of those formerly used, fumigators in
general became somewhat uneasy about using it, with the result that
during the season of 1909 a three-fourths schedule, rather than full
schedule No. 1, was used by the majority of outfits. Although the
results with the three-fourths schedule have been very good, this
schedule is far less satisfactory than the full schedule No. 1.
The writer advises the use of full schedule No. 1 (see fig. 9, p. 34)
for the purple scale. The results with this dosage are so superior, as
shown by experience, that most orchards are rendered so clean that
I hereafter they do not require fumigation oftener than once itwo
years. It is more economical to use schedule No. 1 and escape treat-
iment alternate years even if a little fruit is pitted in the operation
Shan to use a smaller dosage and be obliged to treat an orchard every
vyear. It is seldom, however, that any marked degree of pitting takes
place with schedule No. 1, if proper care is exercised during the
I)J~e1.~1 1011






FUMIGATION OF CITRUS TREES. 55

LENGTH OF EXPOSURE.

Experiments against the purple scale showed that in using a 2-ounce
dosae rate eradication occurred on the leaves and branches with a
30-minutes exposure, whereas with a one-hour exposure it was pos-
sible to acomplish the same results by using a 1-ounce dosage rate.
This deonstrates that decidedly better results can be secured by
leaving the tents on the trees one hour than is possible with 30
minutes of gssing. With the present character of tents in use
practically all gas has escaped on most nights by the end of an hour.
This frnishes sufficient evidence that a longer exposure would be
unnecessary. However, experiments have been carried on in which
exposures of greater duration than one hour were made, but no
better killing esulted. From all the experimental evi(ldnce at hand,
an exposure of one hour is advised for the purple scale. This length
of time readily enables an outfit to go through the complete operation
of preparing the chemicals and dosing the trees, with a few minutes
to spare for rest.
ERADICATION.

Experiments during the earlier part of the investigation showed that
the purple scale could be eradicated from the leaves and branches of
trees by using a dosage equivalent to a -dosage schedule. During
the first part of September, 1908, an isolated orange orchard containing
about 1 acre of trees from 10 to 18 feet tall and severely infested with
the purple scale was fumigated, using a I -dosage schedule (dosage
schedule No. 1 increased one-half). No old scaly fruit was left on the
trees. e results were as follows: An inspection of this orchard
during the latter part of the autumn failed to reveal any live insects.
The crop of fruit on the trees was entirely free of scale for the first time
in the memory of the owner. Many examinations have been made
since, et without the finding of a single live insect.
This experiment has shown that eradication of the purple scale on
trees free of infested fruit is possible with a 14-dosage schedule, if
the work be carefully done. In small isolated orchards it might be
practicable at certin times to use this dosage. For general work
the employment of this erication dosage is not advised. The
writer's experience has assured him that careful work under the most
favorable conditions would largely avoid pitting of fruit even with
thshigh dosage. But, a matter of fact, the work in the field is not
ways carefully done, nor are the most favorable conditions always
ten advantage of. Experience has shown that the pitting of fruit
whregular schedule No. I sometimes causes a slight dissatisfaction
ong growers. If the injury from shedule No. 1 sometimes pro-
ducedissatisfaction, it is very evident that the greater risk with a
laredosage is too great to justify its general adoption.
673300-Bull. 90-1 --5





56 HYDROCYANIC-ACID GAS FUMIGATION IN CALIFORNIA.

Other conditions exist which take part in prohibiting this greater
dosage. If this dosage were used in general orchard work, it is doubt-
ful if eradication would occur in all cases. Tents not properly
pulled down on all sides of the tree, a hole in the tent, mistake in
measuring the trees or in reading the dosage from the schedule,
erroneous measuring of chemicals, boiling over of a generator, over-
turning of a generator, and numerous other considerations which will
sometimes escape even the most careful manipulator, make the differ-
ence between eradication and noneradication more variable in prac-
tice than in theory. If the fumigator is inclined to be a little careless,
some of the above errors will frequently creep in.
Moreover, unless compelled to do so the orchardists in any one
locality would not all use this dosage, while possibly some would not
fumigate at all. To go to the extra expense required in an eradication
dosage and then be subject to reinfestation from one's neighbors
presents no special attractiveness to the grower. Supposing that
the growers in any one locality were willing to use an eradication
dosage, the present number of fumigation outfits is inadequate to
meet this requirement within the limited time necessary in order to
prevent reinfestation. These practical considerations demonstrate
that the eradication of the purple scale from any large district is im-
practicable at the present time.
DIFFICULTY OF DESTROYING THE SCALE ON THE FRUIT.
There is one more important point which must be considered in
connection with fumigation for the purple scale. In experiments
to which attention has been called it has been shown that destruction
of scale is much more difficult on the fruit than on the leaves and
branches. Careful investigation of this point for about two years
has also shown that the susceptibility of the scale on some fruit is
much greater than on others. Hence, no exact standard of destruc-
tion for the scale on fruit is possible. When scales become matured
and deposit eggs the dosage required for eradication is very much
greater on the fruit than on the leaves and branches. It may require
a one-fourth to one-half or in some cases an even greater increase.
A dosage sufficient for eradication of the scale on the fruit is impracti-
cal for the very same reasons that make eradication on the leaves
and branches commercially impractical. A grower possessing a
few trees on which he intends to eradicate the scale at one fumigation
should remove all infested fruit before the operation and then, use a
1 1 schedule. It is advisable to remove the old scaly fruit in any fumi-
gation. At picking, fruit badly infested with scale is usually left on
the tree, and frequently from one to a half dozen or more old, scale-
infested oranges per tree remain throughout an orchard. Even after
a good fumigation one of these old fruits might carry more healthy






FUMIGATION OF CITRUS TREES. 57

rple-sale eggs than all the rest of the tree, and on the hatching
of these eggs the insects will spread to other parts of the tree. The
danger from old scaly fruit is evident, and all such should be re-
moved from the trees before fuigatin an orchard.
There are times in which a scale-infested orchard to be treated
contain some sale on the reen fruit. During the autumn season
when fumigation is most practiced the purple scale is largely in its
earlier stages of development, in which it may be destroyed by the
employment of schedule No. 1. The immature fruit which is scale
infestd can be left on the tre. It is the old scaly fruit which requires
removal at the time of fumigation.
TWO SUCCESSIVE TREATMENTS.
A few growers whose groves are severely infested with the purple
scale will desire to have the scale eradicated if posible, even though
the initial expense is considerably above the cost of a regular treat-
ment, yet hey do not care to assume the risk of having any fruit
on the trees injured. In such cases some authorities advise two
successive treatments d(uring the early autumn and about five or six
weeks apart. The dsages used should be sufficient to destroy the
mature isects. The first treatent would destroy all the insects,
leaving only es on the trees The time elapsing between this and
the second treatment should be just long enough to allow all the eggs
to hatch. Abt five weeks is supposed to be sulficient unless the
weather be exceptionally cool. Careful inspection will settle this
point. If the first treatment has been thorough and there are no egs
present at the secod, eradication should result. A three-fourths
schedule should be used in each treatment. The first fuigation
should be in the autumn, not later than the first part of October.
Double fumigation is seldom resorted to, as its economy in the lon
un is somewhat questionable.
THE RED SCALE.
The red scale is generally held as the most dificult of all citrus scales
destroy. Extensive experiments during this investigation, carried
ut in many sectio of southern California, have proved it to be one
fthe easiest to destroy. It is, however, the most difficult insect to
out of an orchard when once it has become established in a com-
ty, and this may be the basis for the opinion as to its greater
iant power to hydrocyanc-acid gas. By reason of its great pro-
ess, its infestation of some weeds common about citrus orchards
well as many trees and s bs which are sometimes planted on
veways or about the buildings on the premises, ad the ease with
it spreads, this insect frequently will quickly reinfest an orchard
Shas been treated. ive insects left on a few trees in an orchard
ckly multily and infest the others. The author has eradicated






58 HYDROCYANIC-ACID GAS FUMIGATION IN CALIFORNIA.
the red scale in orchards and yet these have become reinfested within
a year. In one orchard the reinfestation was traced to some fig trees
on one side which had not been fumigated; in another, the scale
spread from a neighboring orchard across the way; while in a third,
the scala came from nightshade (Solanum sp.) which had not been
destroyed. The insect is distributed by the wind, by birds, and
especially by clinging to the bodies of the hordes of insects which
frequent citrus trees and carry them to other trees. Foremost among
these insects are the ladybirds (Coccinellide), of which there are
numerous species as well as vast quantities of individuals. Before
fumigating for the red scale care should be taken that host weeds
along irrigation flumes, ditches, and fences are destroyed so far
as possible and all neighboring trees subject to its attacks cleaned up.
Dosage.-The first orchard treated for the red scale (December,
1907) was a severely infested one of between 2 and 3 acres of trees
in an unhealthy condition, and was located at Sierra Madre. The
height of the trees was about 10 to 14 feet. The 1-1-3 formula
was used. Exposure lasted one hour. Dosages of from one-half
to 3 ounces per 100 cubic feet were used. Eradication took place
with all strengths.
In September, 1908, about 1 acre of trees about 10 feet tall,
located at Whittier, was treated with dosages of from of 1 to 1 ounces
per 100 cubic feet. Exposure lasted one hour. Eradication resulted.
During April, 1909, 4 to 5 acres of unhealthy orange and lemon
trees at Villa Park, Orange County, were fumigated with dosages of
from one-half to 1 of schedule No. 1. The exposures lasted 45
minutes, 1 hour, and 1 hours. Complete eradication occurred.
An acre of entirely healthy orange trees severely infested with the
red scale-fruit as well as leaves and branches-was treated during
September, 1909. The results of this experiment showed that a
one-half schedule usually would destroy the scale on the leaves and
branches, but that it required a three-fourths schedule to accomplish
this on the fruit. As satisfactory work was done with an exposure
of 45 minutes as with 1 hour.
The examination of much work carried on by practical outfits using
both a three-fourths schedule and a No. 1 schedule has demonstrated
that eradication would result when careful work was done.
Results from these extensive observations show that the red scale
is more easily destroyed on unhealthy than on healthy trees, and
that it is slightly more difficult to destroy on the fruit than on the
leaves and branches. The dosage used must be based on a strength
sufficient to destroy the scale on all parts of all trees; thus it is
apparent that a three-fourths schedule is the most economical for
the red scale. In all fumigation work against this insect it is advised
that a three-fourths schedule (three-fourths of schedule No. 1) see
fig. 11) be used. An exposure of 45 minutes is sufficient.






FUMIGATION OF CITRUS TREES. 59

THE BLACK SCALE.

To specify a certain dosag for use at all times against the black
scale is impractical. The reason is that tis insect is more difficult
to destroy in some stages of its developent than in others. ile
young and in a soft condition it can be destroyed by a light dosage.
As the insect approaches maturity its body becomes leathery and
tough, which renders it difficult to destroy. The eggs require even
a heavir dosage. The great variation in development among the
black cale results that at most times of the year trees will contain
insects in all stages of development, from those recently hatched to

1.A57AA .C- 4O//O0, /A/ FE--T
16 1820 22 2426 28 30 323436 38404244 4648 50 5254156 58 60 6264 66 68

T4-14 s a 3 E j 3 3 f 44 4 4_S 1412
163 3J 3 4 4 4 4 44 5 16
18 J 3 -? 14 4 4-- -T s -- S S 1 _1



26 S6s7s 7 1 e .ld _ I
830 32134 36 38 40142 44445 48 50 S2154156 58 60 62164 66 68
0 a7gsJs gs s //// // ///21t 28

S7 818 9 9 / 1 13 1 44 1 9 16 A6 3
1 0 o01 1/ // 12 /2/3 1s 14 5/ 16 17 32
1g /0 /v n lI 13 13.14 14 o e14 s/6- 1 6 16 7 17 /7 34
36 // /7 13 /a ? M 1 4 / /56 /7 1/7 /7 17/1 / /9 36
3~8 1r 113 1 1 r4 1i /i / /7 /7 / 78 18 d 1 9 e20 38
40 424446 48 50 52154 56 58 60 62 6466 68
40 12 13 1 4 IS IS 16 17 1/7 I/ e1 9 t20o o 2040
41 Ve 4 IS o t6 171 7 7/8 /6 s1 20 0eo t20 / 41
y42 / es 15/ /7 h7 /10_ / /920 20 2o so0 2/. 42t
S43 /7 /16 1 7 1 7 i i so so 1o o s R 43
44 17 17 1a tur to S12 sOa t / ae tu n r






d4 this a bo 9o f at e h f s as 45er
45 117 /3 201RO 20 20 Z2 2P 23 23145
% 10 so '0o so o W sP 20s a 2a s22 46
w7 92 0o v2so s2 2a -2 z3 2s v 47
i 20 so 2o OR e s ss R3 R 04 48
L tv -o aI 1/ e/ as ss 23 a s4 RS49
FIG. 11.-Dosage schedule No. for potassium cyanid. (Original.)


destroy the more resistant individuals is necessary.
Experiments during this investigation as well as observation of the
work of practical outfits, has demonstrated that in the younger and
tenderer stages this insect can be destroyed by one-half of schedule
No. 1. Very few of those in the later stages of development are
affected by such a dosage. A three-fourths schedule not only
destroyed those in the earlier stages of development, but also those
of a leathery nature and many that are tough and full grown, as
evidenced by their size, together with the absence of the character-
c letter "H" on the upper sde. A considerable percentage of
is also destroed at this strength. A full schedule No. 1 dosage






60 HYDROCYANIC-ACID GAS FUMIGATION IN CALIFORNIA.

will destroy all insects except a few of the toughest of the matured
scales. It will also destroy a large percentage of the eggs.
In fumigating for the black scale the treatment should be applied at
the time when the insects are largely or all, if possible, in the early stages
of development. Close watch of the condition of the scale in his
orchard will enable the grower to determine the proper time. There
is a general breeding period during late summer or early autumn
when this condition usually exists. Wait until the eggs of the old
brood have all hatched, even if by this time some of the earliest
issuing insects are becoming leathery. This is the time at which
fumigation is advised and a three-fourths schedule (see fig. 11, p. 59)
should be used.
If it be desirable to fumigate at a time when the insects are in all
stages, including the mature and tough individuals as well as those
of the younger generations, a full schedule No. 1 (see fig. 9, p. 34)
should be used.
It would be difficult to advise a dosage sufficient to destroy the
eggs, because of the wide variability in the action of such a dosage.
The writer has seen the eggs of some species of scale insects destroyed
by employing schedule No. 1, whereas in other instances the eggs
have been unaffected even where a very much heavier dosage was
used. This is partly attributable to the closer attachment to the
host plant of the different individual scales and partly to a small
parasitic insect (Scutellista cyanea Motsch.) which attacks the eggs
of the black scale very freely. The larvae of this parasite cements
the edges of the scale to the host plant, making it more difficult for
the gas to penetrate to the eggs under such scales than under those
not parasitized. The exposure should last one hour.
THE YELLOW SCALE.

The yellow scale is comparable in almost all respects with the red
scale except that it is much less destructive. The region in and about
Redlands, San Bernardino County, is the principal section in southern
California in which special effort for the control of this insect alone
is required.
During September, 1909, 2 to 3 acres of orange trees at Redlands
infested with the yellow scale were treated with dosage rates of from
one-half to 1 of schedule No. 1. The insects were destroyed on all
trees except those on which a one-half schedule was used. A very
few survived on these. No difference in effect was apparent between
an exposure of 45 minutes and one of an hour.
A three-fourths schedule (see fig. 11, p. 59) is recommended for
the yellow scale. The section in which the yellow scale is most
serious is about Redlands, where a great acreage is on terraced land.
In such fumigation it is believed that a full' schedule No. 1 should be






FUMIGATION OF CITRUS TREES. 61

used, to make sufficient allowance for the irregularity in the surface
of the ground, which renders the possibility of leakage from under-
neath greater than in regular orchard work.
DOSAGES IN GENERAL FUMIGATION.

If the treatment is for any one of the insects mentioned previously,
the dosage recommended for that particular isect should be used.
Frequently, however, two, or even three, different species may be
found in the same grove. In such cases use that dosage which is
heavy enough to destroy the most resistant one. For instance, if
the purle and red scales, or the purple and black scales, occur in
the same orchard use dosage schedule No. 1 for an exposure of one
hour. If the re and black scales, or the yellow and black scales,
occur simultaneously, us a tre-fourts schedule for one hour,
unless the black scale is in an advanced or matured stage, in which
case a full No. 1 schedule is required.
TIE OF THE YEAR FOR FUMIGATION.

Although fumigation is carried on in California at all ties of the
year, there are certain periods in which the operations are more
general. There are two main factors to be taken into consideration
in fumigating, i. e., the species of scale insect and the condition of
the tree. As to the latter, it may be said that at certain periods of
the year the fruit is in such a tender conition that it can not with-
stand a heavy dosage without injury. This period is while the fruit
is of small size, usually from April until about August. The bulk of
fumigation in California at the present time is carried on between the
latter part of August and December. Probably the principal reason
for treating during tis period is that the black scale is usually at
that time most successfully reached. Although the life history of
the black scale has never been thoroughly worked out for the region
in which these investigations were made, it is generally understood
that the majority of the insects of the largest and most regular brood
are hatched and in their least resistant stage during the months of
Sptember and October. In some favorable seasons the eggs are
almost all hatched in August.
The black scale occurs in practicaly every citrus-growing locality
in southern California, while the purple, red, and yellow scales are
more localized. Where any of these other species occur in orchards
infested with the black ale it is a common practice to fumigate
during the regular black-scale period, using the dosage necessary to
destroy the most resistant species. The majority of these scale
Insects can thus be caught at one time. When fumigating for the
pu e scale alone, operations can be commenced as early in the
season as the trees are in a condition to wthstand the heavy dosage






62 HYDROCYANIC-ACID GAS FUMIGATION IN CALIFORNIA.

required without injury to the fruit, although probably it would be
preferable to fumigate a little later in the fall. The purple scale can
be found in the egg stage throughout the year. There is, however,
a period in the fall during which the smallest proportion of eggs is
to be found. With dosages lower than that of eradication the best
results can be accomplished at this time, which usually is somewhere
about October. The red and yellow scales are born alive and can
be successfully destroyed throughout the year.
In fumigating for any of the scale insects there is one point worthy
of consideration. Aside from trying to save the tree from destruc-
tion or from having its vitality impaired by the attack of scale pests,
the orchardist fumigates principally in order to have his fruit come
into the packing house as clean as possible. It would be well, there-
fore, to fumigate as nearly as possible at the time which would insure
him the cleanest fruit. Although lemons are gathered throughout
the entire year, the bulk of the orange crop is taken during the first
six months. Thus fumigation during the fall and early winter
would be sure to place the cleanest fruit in the packing house. If
carried on in the late spring or early summer, such insects as remain
undestroyed would have the opportunity to breed throughout a period
of several months and infest much fruit.
The small size of the fruit and to some extent the heat renders
fumigation generally unsafe before the month of August. From
this time up to December the weather is usually quite dry and not
especially cold. December is usually the month during which cold,
rainy, and generally unsettled weather may be expected to com-
mence and to continue with more or less irregularity into the follow-
ing March. Serious injury to the trees may result unless the greatest
care is exercised in treating during these winter months. At this
period most fruit is almost fully grown, which adds a great weight
to the tree. Covering heavily laden trees with fumigation tents not
only tears off and scars a considerable amount of fruit, but also
breaks off heavy branches. The writer does not believe it advisable
to treat trees in such condition.
The labor question, at all times a perplexing one in fumigation
operations, is especially serious where the work is carried on in the
winter. The frequent delays, possibly for several days at a time,
necessitated by the rainy and damp weather render it difficult to
secure good and careful men who will remain continuously with the
outfits. As they are paid only for actual work performed, operators
earn little more than their living expenses during the irregular hours
when work is possible in the winter season.
After viewing all sides of the question, it is advised that the prefer-
able months for general fumigation are from August to December.
lThe treatment can, however, be carried on with both safety and






SUMIGATION OF CITRUS TREES. 63

efficiency between December and April, provided the work be per-
formed by careful men who observe the various factors affecting
these two considerations. These factors are given at various places
throughout this bulletin.
FUMIGATION FOR THE MEALY BUG.
Considerable excitement has been aroused in some parts of southern
California during the last year or so over a so-called threatened
invasion of the citrus mealy bug (Pseudoccus citri Risso). This
insect has been known in this region for at least 15 years. Its
eatest njury hs been done in the vicinity of National City, San
ego County; along the Sierra Madre foothills, near Monrovia, Ls
Aneles County; and very recently at Santa Paula, Ventura County.
There have also been sporadic occurrences in many other sections.
This sporadie activity has been of a somewhat puzzling nature. A
tree severely infested with the mealy bug one day might be found to
be practically free of this insect a month later, though no artificial
measures of control had been applied. The mealy bug might reap-
pear on this same tree the following year, or even sooner. The
writer is inlined to attribute this disappearance largely to the
activity of parasitic and predaceous insect enemies. The general
appearance of the remains of the mealy bug, as well as the rear-
ing of several bneficial insects therefrom, including two or three
species of Ilynmenoptera, a brown lacewing (Hemerobiuis sl.), and a
syrphus fly, lead the writer to this conclusion. Several species of
occinellida, also have been found present in severe infestations of
this insect. Undoubtedly climatic conditions as well as fungous and
bacterial diseases produce some effect.
Te recent proninence of this insect in the immediate vicinity of
Santa Paula, Ventura County, is well worthy of mention. Its infes-
tation here has been so severe that the proportion of fruit in some
orchards ruined by this insect during 1909 was very great. The
large amount of damage caused there, together with the difficulties
experienced in destroying the mealy bug, has led to agitation for its
control in several other localities.
At various times during this investigation a few trees infested with
mealy bugs have been treated. The usual dosage has been 1 to 2-
times schedule No. 1. The results have been variable. Some trees
would appear to be entirely freed, while on others many live insects
would remain.
During the autmn of 1909 a prominent orchardist of Santa Paula
carried on some very extensive fumiation work against this insect,
partly according to the writer's directions. Some trees were treated
i2, 2A, and 3 times schedule No. 1 for from one to one and one-
alf hours without eradicating the scale in any case. The results






64 IIYDROCYANIC-ACID GAS FUMIGATION IN CALIFORNIA.

with these different dosages were about the same. The very young
insects were destroyed, but a large proportion of the matured ones
and eggs escaped. A few trees were then covered with double tents
(two tents, one over the other) in order to render them tighter and
twice schedule No. 1 was used. An examination-of these results a few
days after treatment showed eradication on some trees, while others
contained a small percentage of live adult insects. A 10-acre block
of trees was then fumigated with double tents (two tents, one over
the other) and twice schedule No. 1. An examination of this work
showed a considerable percentage of live matured insects and eggs.
From the results of the aforementioned fumigation against the
mealy bug it is seen that the early stages of this insect are destroyed
by the use of twice dosage schedule No. 1. If a study of the life
history should reveal that at some certain time they may all be
found in this early stage of development fumigation might then prove
of some avail. The failure, however, to secure eradication of the
mealy-bug in its more mature stages, even where dosages as high as
three times schedule No. 1 were used, indicates that general com-
mercial fumigation for the mealy bug is impractical.'

FUMIGATION DURING THE BLOSSOMING PERIOD.

The statements by experts on fumigation as to the amount of
injury resulting from work while the trees are in blossom are very
conflicting. Some fumigators hold that a very light dosage will
destroy tender blossoms, while others believe that the blossoms will
stand a heavy dosage. In order to decide this point, much experi-
mentation has been carried on and many observations made through-
out this investigation. To attempt to relate the details of the scores
of experiments and observations made along this line in all parts of
southern California would require too much space, so mention will
be made here only of general results. Fumigation observations by
the writer have covered the entire period from the time the blossoms
first appear until the petals drop. In not a single instance during this
period has any serious injury resulted even though dosages as high as
11 and 2 times schedule No. 1 have sometimes been used.
Occasionally some blossoms were affected, and dropped off soon
after the treatment, yet these blossoms were normally weak and
would have fallen without fumigation. The fumigation merely
hastened their shedding. Even if a heavy shedding took place
(which never happens), there would be no cause for alarm, as the
settinlg of only a small percentage of the blossoms on a citrus tree is
necessary to produce a full crop of fruit.
1 Thi' sanWe conclusion has Ion reached by Mr E. .Essig, horticultural commissioner of Ventura
Co unt See l'ro onaft College, Journal ol Entomology, Deinmier, 1909.)






FUMIGATION OF CITRUS TREES. 65

The blossoms appear to be much more resistant to the gas than
the young leaves or leaf shoots. Trees in which there are blossom
hoots and tender leaf shoots side by side will have the leaf shoots
burned back, while the blossoms remain unafected. This shows that
te blossoms wil stand a much heavier dosage than the tender leaves
and leaf shoots.
The young leaf shoots grow so rapidly in certain varieties of trees,
or even in the same variety of tree in different localities, that during
the latter part of the blossoming period they almost obliterate the
blossoms. Although fumigation of these trees will not injure the
blossoms it will frequently burn back these leaf branches very severely.
In such cases the growershould not become alarned by the burning,
as 1s trees and future crop of fruit are in no way endangered. Re-
newed growth will soon take place, while the crop of fruit will be
exactly a large as if the trees were untreated.
In conclusion, it might be said that experience in this investigation
has showf that fulligation can be safely conductel dur1ing the
blossoming period with the dosages at present generally employed
Sby fumiators, namely, schedule No. 1.

FUMIGATION WHILE THE FRUIT IS OF SMALL SIZE.

The recor of several experiments during 1908 in fumiating
while the fruit was of small size are given in Bulletin 79 of this bureau.
The results of these experiments demonstrated tlhat heavy dosages
can not be used while the fruit is small without m or less injury.
Additional evidence has been secured during the latter part of this
ivestigation which has entirely corroborated the earlier experiments.
Thus it may be stated that the most critical period for conducting
fumigation is between the time the fruit sets and the time it attains
a diameter of about an inch. This period occurs during the late
sprig and summer. It is advsed not to fumigateduring this period,
which is usually fron April to August. Although in some cases an
orchar may be treated during this period with a light dosage without
injury, yet the risk is too great to justify such action. It is better to
wait until the regular season immediately following the month of
July.
FUMIGATING LEMONS.
Throughout this bulletin the recommendations are always for
citrus trees, which include the orange, the grapefruit (pomelo), and
the lemon. The acreage of grapefruit is very small. Oranges and
ons occur more or less promiscuously throughout the same districts
where lemons are grown, wile frequently an orchard will consist
y of each. The orange and grapefruit are about equally sus-
tible to injury from fumiation, ile the lemon is much more






66 HYDROCYANIC-ACID GAS FUMIGATION IN CALIFORNIA.

resistant. As all three kinds of citrus are fumigated at the same
time, regardless of kind, it is necessary to base advice on the one most
susceptible to injury. If the tenderest kinds escape injury the more
resistant very naturally will do so. Hence, in reality the recommen-
dations in this bulletin are based on the orange, partly because of its
greater acreage but mainly because any recommendations made for
it will apply equally to the grapefruit or lemon.
The lemon blossoms throughout the year in California, so that
different sized fruit may be found at all times. This is very different
from the orange, which has one regular crop. Lemons usually will
escape pitting or burning under conditions which might seriously
injure oranges. This allows a wider range of activity in fumigating
lemon trees. The work can be continued somewhat later in the
spring and commence earlier in the summer than with the orange.
In fumigating a section containing lemon and orange trees it is good
policy, where convenient, to commence on the lemons, leaving the
oranges until a later period.
EFFECTS OF FUMIGATION ON UNHEALTHY TREES.
Unhealthy citrus trees are found universally. Occasionally a
part or whole of an orchard is composed.of trees weakened by lack of
such essential treatment as proper cultivation, fertilization, or irri-
gation. Many orchards contain trees weakened from attacks of a
gum disease, of "gophers" (ground squirrels), scale insect pests,
and numerous other causes which check their normal development.
These unhealthy trees are less resistant to injury from fumigation
than perfectly healthy ones. In examining results in an orchard
recently fumigated the writer has noticed frequently that the fruit
on a few trees that had been weakened by disease was severely pitted
or burned, while that on all healthy trees was uninjured. A heavy
dropping of fruit might have taken place in the unhealthy trees, while
the others were unaffected in any way. A most striking example
of severe injury to unhealthy trees was seen in an orchard fumigated
with double tents (one tent over the other) using a dosage twice
schedule No. 1. Healthy trees in some cases were severely burned
back at the top for about a foot, accompanied by the dropping of
some leaves, while the trees weakened by gum disease usually would
)e burned back from 2 to 3 feet and drop practically all their leaves.
Severe injury to unhealthy trees has been seen even where the
three-fourths schedule was used.
Practical fumigators have always been aware of the susceptibility
of weakened trees to injury and have decreased their dosage greatly
in treating such trees. The grower should not complain if the fruit
and leaves on their unhealthy trees are slightly injured. Such fruit
is normally of the inferior grades, while the damage caused by the






FUMIGATION OF CITRUS TREES. 67

shedding of leaves is more apparent than real. These weakened
leaves normall would not be held on the trees much longer. The
fumigation merely hastens their removal and is usually followed by a
fresh invigorated growth suerior in all respects to the ohl.
GREATER SUSCEPTIBILITY TO INJURY OF SOME VARITIES
THAN OTHERS.
The lemon tree is much more rsistant to injury from fumiation
than the orange and seldom suffers any appreciable damage when
treated under normal conditions. Some varieties of oranges are
more easily injured than others. Of the varieties of commercial
mportance in California the Navel and Valencia are the least sus-
ceptible to injury from the gas treatment. The seedling is almost
equally hardy. Next comes the Mediterranean Sweet while the
Homosassa and St. Michael can seldom be treated with schedule
No. without some ijury resulting. Fortunately the Navel and
Valencia comprise the bulk of the oranges grown in this State.
THE DISTRIBUTION OF GAS ITHI A TENT
Hydrocyanic-acid gas, being lighter than air, has a tendency to
rise toward thtop of the tent. The column of gas rises straight up
fro the generatin vessel until broken up by coming in contact with
the leaves and branches of the tree. The greater density of gas
toward the top of a tent is indicated by thegreater amount of injured
fruit there than elsewhere. Only infrequently is fruit at the bottom
of the tree pitted. Dr. Morrill has given records of the difference in
destruction to the citrus white fly at different heights in a tree.'
Similar results have been observed against the scale insects in Cal-
ifornia. The insects at the top of a tree may all be destroyed while
some on branches close to the ground will escpe. Hence, in the case
of the purple scale, when the infetation is generally toward the bot-
to of the tree the necessit of a strong gas is evident.
FUIGATION FOR PHYSIOLOGICAL EFFECTS.
There are people in California who believe that citrus trees should
be treated with hydrocyanic-acid gas whether they are infested with
scale or not; that the treatment invigorates the tree, producing a
heavier crop and superior fruit than would otherwise result.
A tree infested with scale, on being relieved of its burden responds
to the treatment. Thi response is not due to the physiological action
of the gas on the tree itself but rather to the destruction of the large
number of insects which have been constantly sapping the plant
juices. The removal of this heavy drain allows the tree to resume
its normal activity, which it does by first producing invigorated
gro Bur. ept. Ar.wth
I Bul. 76, Bur. Ent., U. S. Dept. Agr.., p. 51, 1908.






68 HYDROCYANIC-ACID GAS FUMIGATION IN CALIFORNIA.

During this investigation many acres of perfectly healthy trees free
of insect pests have been treated throughout the period of several
months preceding blossoming up until the blossoms fall. No effect
producing an increase or perfection of the coming crop of fruit has
ever resulted from these efforts. Although the acreage treated has
not been sufficiently great to justify the absolute statement that
fumigation, in itself, never produces a greater crop of fruit, never-
theless the negative results in all the orchards treated certainly prove
that if an increased crop ever results it is of infrequent occurrence and
under peculiar conditions.

EFFECTS OF METEOROLOGICAL ELEMENTS ON FUMIGATION.

MOISTURE.

The great affinity of water for hydrocyanic-acid gas is well known.
Writers on greenhouse fumigation contend that the plants should be
dry when treated, else injury might result. It has been the universal
opinion among California fumigators that if the fumigation treatment
was carried on while the trees were wet injury might follow. Gos-
sard,1 and more recently Morrill,2 assert that moisture does not pro-
duce injury to the fruit and that the destruction of insects is as great
under wet as under dry conditions. Quaintance 3 has corroborated
these results of Gossard and Morrill where the work is done on moist
fruit (apples) in cold storage.
The fumigation work which has been performed by the Govern-
ment outfit during this investigation has been carried on in all sections
of southern California, at all times of the year, and under all conditions
of weather-when the trees were entirely dry, when the trees were
wet, so wet that the moisture was falling off in drops, and when it was
raining. Mr. G. R. Pilate, a temporary assistant, was stationed with
a practical outfit during October, November, and December, 1909,
at Rivera and Downey, than which the writer believes there is no
generally damper citrus section in California. Careful records were
kept of all the climatic elements which might affect fumigation.
These records show that almost every night the trees became thor-
oughly moistened before the work was discontinued. In addition to
this, the results of the treatment of hundreds of acres by other prac-
tical outfits have been followed during the writer's residence in
southern California.
From all this experience not a single authentic instance has ~been
seen in which burning was directly attributable to absorption of gas
by the moisture on the fruit or leaves. Thus the writer has felt justi-
fied in concluding that the presence of moisture on trees can be ignored
I B1ll. 67, Fla. Agr. Exp. Sta., pp. 647-648, 1903.
2 IBul. 76, Bur. Ent., U. S. Dept. Agr., pp. 12-14, 1908.
3 Bul. 84. Bur. Ent., U. S. Dept. Agr., pp. 24,31, 1909.






FUMIGATION OF CITRUS TREES. 69

in so far as the effect of its diret ct ion on the hydrocyanic-acid gas
treatment is concerned.
There are other reasons of indiect and largely mechanical nature
on account of which it is necssary to consider the presence of mois-
ture, for ignorance of these will frequently result in much burned
fruit. (1) When tents become moist they become heavier. This
renders them more difficult to handle. Much fruit is torn off, while
branches and limbs are frequently broken. (2) The damp tents col-
lect much dirt and as they are pulled over the trees they sometimes
scrape the fruit with which they come in contact. Such abrased fruit
is frequently burned by the action of the gas. (3) As the trees
become damp from the dew or fog, whichever it may be, so also do
the tents get damp. The moisture affects the fiber of the cloth so that
it becomes tighter and retains the gas better than when dry. Any
person of any considerable experience in fumigation knows that more
gas is left under the tents when they are pulled off on a damp night
than on a dry one; therefore, the gas remaining in the tops of tall
peaked trees is much more concentrated than is normally the case.
This intense strength of gas soetimes causes pitting, especially in
the case of some varieties least resistant to hydrocyanic-acid gas.
ece, considering the disadvantages resulting from wet tents, it is
evident that fumigation should not be carried on at such a time.
Fumigation should be stopped after the leaves and tents become
thoroughly damp.
The author's experience has been that the presence of moisture on
trees does not reduce the efficacy of hydrocyanic-acid gas against.
scale insects. Results have been exactly as good when the trees were
wet as when dry, and observation of the work accomplished by com-
mercial outfits has corroborated it.
Experiments by Penny on plants in a closed box showed that
moisture on the leaves absorbed the gas. This would make it appear
that moisture on the foliage of orange trees absorbs some gas, and
undoubtedly such is the case. Remembering that on dry nights all
the gas within tents of the character, ordinarily used has escaped
within an hour's exposure but on damp nights frequently much gas
remains when the tents are removed, it is at once apparent that the
retardation of gas by the damp cloth easily offsets any absorption
hich may have taken place.

TEMPERATURE.

Heat.-Heat probably is the factor which is responsible for more
ury to fruit than any other cause. Throughout the experience of
c mercial fumigation instances of very severe injury have occurred
most every year in some one of the different citrus fruit producing
I 12th Ann. Rep. Del. Agr. Exp. Sta., 1900.






70 HYDROCYANIC-ACID GAS FUMIGATION IN CALIFORNIA.

regions. The cause of this injury has for the most part been an
enigma, but it has been known to occur during that character of
weather locally spoken of as "electric" or "Santa Ana," which is in
reality a dry condition of the atmosphere caused by the hot dry
winds of the desert sweeping through passes in the mountains onto
the lower lands adjacent to the ocean. The nights in southern Cali-
fornia are usually cool, but during these disturbances from the desert
they frequently become very warm.
Records of fumigation when the temperature was about 650 F.
were secured during the autumn of 1909. A part of an orange orchard
was treated on three consecutive nights during the so-called "elec-
tric weather, when temperature ranged from 660 F. to 770 F. Con-
siderable burning resulted. Work was then stopped for a week until
the nights became cooler, when the rest of the orchard was treated.
No burning at all occurred. The injury to the fruit during the hot
nights consisted of real burns covering much of the fruit-not small
pits which are the usual indications of fumigation injury. This burn-
ing was exactly of the same character as was produced in a large
orchard at Redlands fumigated during the late summer of 1908 on
similar warm nights. Records have been secured of burning in other
orchards treated under similar weather conditions.
From the data at hand it appears that injury from fumigation will
take place at high temperatures. Based on the author's experiments,
it is advised that fumigation be stopped at a temperature above 650 F.
Although in some instances work may be carried on at a slightly
higher temperature with impunity, the risk of injury appears to be too
great.
Cold.-During December, 1908, a part of an orange grove at Rivera
was fumigated by a private outfit which kept temperature records
during its work. At the time the work commenced the temperature
was above 40 F. No injury was done until the fourth or fifth set,
when the temperature had 'fallen to 370 F. This set was slightly
burned. The next set was badly burned, much fruit dropping.
Unfortunately, the temperature for this set was not taken. The fol-
lowing set, which was the last, was made while the temperature was
at 320 F. The tops of many of these trees were severely burned
back, while all the fruit on some of them dropped.
On December 3, 1909, exact records of burning from fumigation
were obtained. Three sets were made on this particular night-the
first while the temperature reached about 360 F. to 370 F.; during the
second it dropped from 360 F. to 320 F.; while.during the third set
the temperature reached 310 F. The first set was slightly injured;
the second was severely burned and much fruit dropped; while prac-
tically all fruit on the third set dropped, and some trees were so
severely burned as to lose most of their leaves.






FUMIGATION OF CITRUS TREES. 71

With these records as a basis, the writer advises that fumigation be
stopped at 380 F. or below. It is known that there are instances in
which fumigation can be carried on with impunity at a temperature
lower than 380 F.; in fact, there are records of fumigation carried on
as low as 33 F. without an considerable injury. The fact remains,
however, that there are authentic cases of severe injury at tempera-
tu below 38, ad any person carrying on work at these lower
tmperatures runs the risk of inflicting injury, and with products of
the commercial value of citrus fruits one can not well afford to asume
such risks.
LIGHT.

Coquillett, in the coure of his early investigations in the use of
hydrocyanic-acid gas, determined that daylight fumigation was more
injurious than fumigation at night. This he attributed principally
to the fact that the actinic rays of light decomposed the gas into other
gases of a nore injurious nature. Commercial fumigation work
since then ha been carried on exclusively at night. Occasionally a
person of questioning mind has attempted some experiments hoping
to dispel the old idea of daylight injury, but current information has
it that these attempts have never been successful.
Some records of daylight work have been taken during this investi-
gation. In one jnstance about 50 large orange and lemon trees were
treated, partly while cloudy and partly in the sunshine. These trees
had the whole upper half burned back-branches as well as leaves
and fruit. It was the severest injury to itrus trees the writer has
ever seen.
One very cloudy afternoon about 25 trees (orange and lemon) were
fumigated for red scale with a three-fourths schedule. The tempera-
ture ranged between 700 F. and 800 F. The lemon trees were only
slightly afeted, but the orange trees were very severely damaged, a
large part of their tops being burned back.
The first row of trees fumigated at night and the last in the morn-
ing are frequently more or less injured. The cause of this injury in
the former case is that the work is sometimes commenced before the
sun has set and while it is still warm. In the latter case the sun has
come up before the tents were removed from the last row of trees.
The injury under such conditions is most apparent from fumigation
carried on in the late summer and early autumn when the weather is
warm. On the cool days late in autumn the treatment can be and
is carried on with impunity at a degree of light at which injury would
result on the warm das earlier in the season. Judging from the
periments and many observations which the author has made
pecting the effect of fumigation during the daytime, the contention
that daylight work is generall' unsafe is entirely correct. Although
7330-Bull. 90-12-6






72 HYDROCYANIC-ACID GAS FUMIGATION IN CALIFORNIA.

the rays of light appear to have a direct effect in producing injury,
these investigations have shown that this injury is intensified by the
presence of heat, and the writer contends that burning during the
daytime is directly attributable to heat as well as to the light itself.
Fumigation should never be attempted in the daytime, even on
cloudy days. Carry on the work at night and do not commence the
operations until the sun has disappeared and the shades of darkness
are approaching. Remove the last row of tents in the morning before
the sun rises.
WINDS.,

Fumigation should never be attempted during a heavy wind, for
two reasons: First, the gas escapes out of the tent so that poor work
results; second, injury to the trees might result. Dr. Morrill has
called attention to the variability of results on trees fumigated during
a stiff breeze, stating that on different parts of the same tree he found
the killing to vary from 30 to 100 per cent.1 Observations made dur-
ing this investigation cover instances in which the gas was driven
from tents by winds in a very few minutes. The trees, of course,
required a second treatment. The burning of trees fumigated during
winds has frequently been observed. In light winds the injury
appears to be more prevalent on that side of the tree from which the
wind comes. Heavy winds appear to produce the burning fully as
much on the opposite side, or may affect the entire tree. A sudden
change in temperature accompanying a wind appears to be especially
severe in its results. Undoubtedly some of the cases of severe burn-
ing during "electric" weather are due partly to the wind as well as
to the heat.
The author's experience has led to the conclusion that fumigation
should never be carried on during a windstorm. As soon as a breeze
arises sufficiently strong to "flap" the tents, it is well to discontinue
work until calmer weather.

INJURY TO SPRAYED TREES.

Distillate oils are still used by a few orchardists in combating scale
pests. As these men become discouraged with the oil treatment they
adopt fumigation. In this connection it appears desirable to state
a recent experience in the fumigation of trees previously sprayed with
an oil combination spray.
A lemon orchard of 40 acres was sprayed early in the autumn with
a combination of Bordeaux mixture and distillate oil. Two months
later about 25 acres of this orchard were fumigated partly with a No.
1 and partly with a three-fourths schedule. This fumigation con-
I Bul. 76, Bur. Ent., U. S. Dept. Agr., p. 12 1,908.






FUMIGATION OF CITRUS TREES. 73

tinued over a period of fully two weeks, much of which was ideal
fumigation weather. The resultant injury was very severe, being
marked chiefly by a dropping of leaves. In many trees the leaves and
fruit were also burned. The old leaves on the tree at the time of
spraying especially were affected, the number of these shed being
sometimes so great as to form a thick blanket underneath the tree,
entirely covering the ground. In these very same trees the young
tender growth at the top of the tree, which had appeared since the
spraying and which normally is the first to be injured by the gas
treatment, escaped uninjured.
The trees were healthy and well cared for, which, coupled with the
fact that only the sprayed portion of the trees was affected and not
the younger and tenderer growth, proves that the cause of the injury
was the spraying. It is well know that distillate oil weakens a tree,
and possibly the unnatural addition of Bordeaux mixture makes this
weakening even more severe. As further proof of this situation,
trees in a neighboring orchard similarly sprayed at about the same
time were fumigated. Injury resulted. Unlprayed trees under the
same conditions and treated with the same dosages at the same time
were uninjured.
These results show that it is unsafe to fumigate trees which have
been recently treated with a Bordeaux-distillate emulsion. Although
it does not also prove that trees recently sprayed with distillate alone
would be injured, it would seem good policy not to attempt such
fuigation until proof of its harmlessness has been secured.
THE APPEARANCE OF FUMIGATED TREES.

Orange trees containing young growth usually will have the tender
tips of this growth burned back with the ordinary fumigation dosage.
The wilting of this affected portion is visible the following day, espe-
cially if sunshiny. During cloudy weather the effects are not marked
until fully a day afterwards. The tender growth in lemons is burned
back even more severely than in the orange. Where the new growth
in the tops of the tree is very long it may be affected for 6 inches or
even a foot.
Some weakened old leaves might be shed a few days following the
treatment. Healthy leaves are seldom shed and seldom burn, unless
some abnormal condition is present. Even in such conditions it is
the frit that is first injured.
The burning back of the tender growth does not injure the tree in
any way. With such vigorous plants as citrs trees all indications of
jury have disappeared within a few weeks following the treatment.
gators and many growers look upon the burning of young growth
an indication that the proper dosage has been given the tree for
d results. Of course such assumptions are correct only in part.






74 HYDROCYANIC-ACID GAS FUMIGATION IN CALIFORNIA.
THE PRESENCE OF OLD SCALES ON FUMIGATED TREES.
Scales do not fall from the different parts of a tree as soon as
destroyed. On young and growing fruit they are easily shed, but they
may cling to the old fruit, leaves, and branches of trees until removed
by mechanical means. Dead purple scales probably cling to the
different parts of a tree much more generally than any of the other
common citrus pests. Leaves, branches, or old fruit severely infested
will normally remain so as long as they continue to be a part of the
tree. This condition leads many growers to condemn a treatment as
unsuccessful because on examining a tree long after the operation
scales are found present. The mere presence of scales may incorrectly
indicate to them that they have not been destroyed.
The prevalence with which successive generations of scale insects
exist simultaneously on citrus trees renders it impracticable and really
impossible to draw conclusions in exact percentages as to net results
of experimental work whenever the results fall short of eradication.
At the time of the treatment the scale on some leaves might all be
alive, while on other leaves the majority may be dead, and with all
gradation between to be found elsewhere.

A DEVICE FOR COVERING FUMIGATION GENERATORS.
During the course of this investigation much attention has been given
to perfecting a device for attachment to the top of the commonly used
open-style fumigation generator that will serve to interrupt the direct
rise of the hydrocyanic-acid gas. The result of these efforts, in which
the writer was greatly aided by Mr. Frederick Maskew, is shown in
figure 12. The device itself consists of a copper cover of such size as
to make it available for use with any of the regular-pattern generators
now employed by the fumigators of southern California. It is stamped
in a concave form from a sheet of copper, with corrugations to per-
mit the escape of gas. The shape is such as to conform to the size of
the opening of generators of different capacities and also to direct the
course of the escaping gas downward and distribute it uniformly
through the lower part of the tent. It is attached to the generator
by hinges of stout copper wire secured by a key bolt passing
through the handle. The cover is raised by a slight pressure of the
thumb on a projecting piece which is curved in such a manner that
the cover will remain in an upright position when so required. When
the generator is emptied of its contents, the cover swings clear by its
own weight. A glance at the illustration will satisfy the practical
fumigator that it is adapted to all the requirements of rapid work in
the dark, while its use has demonstrated that it is simple, strong, and
durable. It is very possible that if the copper cover were lined with
a thin covering of lead its durability would be increased. A common






FUMIGATION OF CITRUS TREES. 75

rest of the use of hea dosages of fine fragments of yanid s the
burning and ultimate dropping of many of the leaves drectly above
the generator in the pathway of the rapidly rising gas. This result is
usually spoken of as the "chimney" effect. The generator cover
eliminates this "chimney" burning.
A second and highly important point is the effect of open generators
on the tent. The outer part, or skirt, as it is sometimes called, of
fuigating tents is constantly being perforated with small holes, even
when used by the most careful of workes. This effect has been
noticed to some extent in the outfit of this investigation, which is
believed to have been as carefully handled s any fumigation outfit
could be. Thee holes are
know to be acid burns. A
few simple tests have demon-
strated conclusively that
many of thee acid holes are
due to acid carried along
with the escaping gas and
reaching that part of the tent
narest the generator. By
placing large pieces of canvas
in the path of gas escaping
from open generators in
which dosages similar to
those often used in field work
re employed it was found
that drops of acid reached
the canvas as high as 5 feet
from theground. The writer
has frequently seen generat-
ing vessels placed not more Fio. 12.-A cover device attached toafumigaton generator.
than 2 feet inside the tent. rgatn in cover allow gas to escape. (Author's
At such a distance one can
readily see that drops of acid might reach the tent. The cover described
above so deflects the gas, arid incidentally such acid as is carried with
it, that the drops are thrown to the ground, thus saving the tents.
The decreased cost in mending of tents will doubtless pay for the cost
Ssuch a cover device several times over in a season of fumigation.
A third advantage, which has not as yet been demonstrated, but
hic there is reason to believe will develop, is a better distribution
o gas through the tent. Heretofore the most difficult part of the tree
Swhich to destroy insects has been the lower part. This is also the
Sof the tree in which the purple scale is largely to be found.
th the open generator the gas rises straight up in a narrow column
r several feet, being broken up and distributed through the top of
tree first. As the gas is lihter than air, it is not to be expected






76 HYDROCYANIC-ACID GAS FUMIGATION IN CALIFORNIA.

that it will quickly become uniformly distributed throughout the bot-
tom of the tent even if at any time it becomes as concentrated here as
at the top. The greater burning effect and better killing effect in the
top of the tree would tend to substantiate this assumption. Field
observations in fumigating large trees show that the gas is of no great
strength at the lower part of the tent for several minutes after the
charge is set off. With this new cover the gas is broken up and dis-
tributed through the bottom of the tent first. By the time it reaches
the top it is pretty generally distributed throughout the tent. As
the bottom of the tree is among the first to receive the full benefit of
the gas, a more complete killing of scale at the bottom of the tent may
be expected than with an open generator.

THE EFFECT OF CLIMATIC CONDITIONS ON SCALE INSECTS.
Climatic conditions exercise a marked effect on the different insects
affecting citrus trees. The purple scale and black scale thrive best in
the more moist country adjacent to the ocean. The red scale thrives
well in the drier interior regions of southern California as well as near
the coast, while the yellow scale is more of a heat-withstanding form
than any of the others. This is demonstrated by its prevalence in
citrus trees in the hot interior valleys of northern California where the
purple scale and to a large extent the black scale appear unable to
survive.
The direct effect of heat on scale insects may be evidenced by data
on the black scale collected during the summer of 1907. Com-
mencing at Pasadena, which is at the opening or gateway of the San
Gabriel Valley, the writer proceeded to Duarte, Pomona, Ontario,
Riverside, Orange, and Santa Ana, Cal., respectively. The San
Gabriel is one of the interior valleys and Pasadena is situated near
that end which opens toward the ocean. As one approaches River-
side from Pasadena the climate becomes generally hotter. Orange
and Santa Ana are nearer the ocean and much cooler than any of the
other places examined.
It might be mentioned that about a month previous to this special
investigation there had been a very hot spell of a few days' duration.
At Pasadena examination showed that about one-fourth of the eggs
under the old scales were dried up and brown, this condition showing
the effects of heat. At Duarte the destruction was somewhat
greater. At Pomona and Ontario, which were much hotter even
than the two preceding places, more than three-fourths of the eggs
and young insects were dead. At Riverside, where the heat was
most intense, a very small percentage of healthy eggs or live insects
was found. In the cooler sections of Orange and Santa Ana very
much less than a fourth of the eggs and young were destroyed, while
insects in all stages of development were in evidence.






FUMIGATION OF CITRUS TRES. 77

The orchards in Riverside and San Bernardino Counties are seldo
Sseverely infested as elsewhere. These observations explain why
scale-insects are less destructive in these two counties than in region
nearer the coast. The hot weather appears to be almost as efficient
as some insecticide treatments.
THE EFFECT OF FMIGATION ON LADYBIRDS (COCCINELLIDm)
AND SCUTELLISTA CYANEA MOTSCH.

Several writers on fumigation have mentioned that ladybirds are
less easily killed by hydrocyanic-acid gas than the scale-insects of
the citrus. In order to prove this contention, specimens of the two
common ladybirds (Coccinella californica Mann. and Ilippodamia con-
vergen Gur.) were suspended under a fuigation tent in an open
cage 6 to 7 feet above the round, while others were placed right on
the ground. Schedule No. 1 was used. The tent was left on the
tree one hour. The temperature was 60' F. The insects were
examined abut ten minutes after the tents had been removed, but
exhibited no sins of life whatever; all appeared to be dead. The
following morning a second examination showed that many had
evived from their previous stupefied condition. A count made of
those suspended from 6 to 7 feet above the ground showed that out
of a total of 64, 32, or just 50 per cent, were killed. Of 85 on the
ground 33, or about 39 per cent, were destroyed. Both species
appeared equally resistant to the gas.
It thus appears that the insects on the ground survive a gas treat-
ment somewhat more readily than those toward the top of a tree.
When it is considered that the strength of gas was as great as is ever
used in commercial work against the common scale insects of citrus
trees, in fact soewhat stronger than is used by most fuigators, and
also that in regular operations many of the ladybirds after becoming
stupefied fall off onto the ground, where they are less affected by the
gas, it would seem safe to presume that the larger number of these
sects on a tree at the time of fumigation will survive the treatment.
Scullista cyanea Motsch. is a small hymenopterous parasite of the
black scale which was introduced into California from South Africa
several yea ago. The larve feed on the eggs of the black scale,
usually one in each scale, although there may be 2, 3, or even 4
present. Frequently as high as from 50 to 75 per cent, or even more,
of the black scale on a tree have been seen attacked by this parasite.
The work of this insect is remarkably good, yet not sufficiently perfect
to llow the fruit to come into the packing house in a clean condition.
This necessitates fumigation as though the parasites were not present.
This is not meant to depreciate te usefulness of this parasite, even
though it fails to keep citrus trees entirely free of the black scale.
By destroying, as it does a large percentage of eggs it confers a





78 HYDROCYANIC-ACID GAS FUMIGATION IN CALIFORNIA.

decided benefit. The prevalence of the black scale on many other
hosts offers a wide range of activity for the parasite, and it is on these
noncitrus plants that our little friend does some of its very best work.
If it did nothing else, its work against the black scale on the pepper
tree (Schinus molle) makes it especially worthy. of praise, and the
question has frequently come up whether or not fumigation destroys
the Scutellista. Numerous observations and experiments respecting
this point have been made during this investigation indicating that
fumigation destroys many of the Scutellista in its adult and pupal
stages. The majority of the parasites, however, are unaffected, even
when schedule No. 1 is used. Parasitized scales have been removed
the day following such treatment and placed in vials, with the result
that some adults would immediately issue and others continue to
issue for many days afterwards. The adults of Scutellista in the open
are destroyed by a weak dosage. The reason they escape beneath
the scales is that such parasitized scales are tightly sealed to the
leaf or branch, apparently by some secretion produced by the larvae,
and the gas does not penetrate such scales as easily as it does those
nonparasitized.
One of the greatest benefits of the Scutellista is its work in trees
which have been fumigated. The eggs of the black scale to a large
extent survive the gas treatment. This leaves abundant oppor-
tunity for a future infestation on trees treated when eggs are present.
If Scutellista occurs in the orchard, these undestroyed eggs are
devoured, thus completing a treatment for which fumigation itself
is only partially successful.
THE COST OF FUMIGATION.
The cost of fumigating an orchard depends principally on the size
of the trees and the dosage-rate used. The average California citrus
orchard requires an average expenditure of from $25 to $40 per acre
for one fumigation treatment. Large seedling trees are much more
expensive, while young trees cost considerably less.
The directors of fumigating outfits base their estimates on two
distinct considerations: The chemicals and covering the trees. Con-
tract fumigators usually furnish the cyanid at 30 cents per pound,
which also includes the sulphuric acid necessary for generation of the
gas. The price of covering trees varies with their size, number,
location, topography of land, etc. The fumigator will charge more
per tree where the orchard consists of a half acre than if it has 50
acres. Trees that require a 45-foot tent usually will cost more to
cover than those requiring a 36-foot tent. The average price of
covering in commercial work where nothing larger than 45-foot tents
is used is from 10 to 12 cents per tree. Large seedling trees whose
covering requires derricks may cost from 40 to 50 cents, or even more






FUMIGATION OF CITRUS TREES. 79

per tree. After the treatment of an orchard, knowledge
of the number of trees covered and the amount of cyanid used fur-
nishes immediate means of caculating the cost.
The estimates of association, count, and private work will vary
soewhat from the above figures, for in these cases the work is sup-
posed to be performed at tual cost. The following figures enter
here and are the averae of field conditions. "anid is purchased at
practically 25 cents per pound in large quantities (tons) and 25 cents
in smaller lots. Sulphuric aid costs about 1 cents per pound.
Five men are required to run an outfit. The foreman receives about
50 cents per hour, while the other 4 men receive about 35 cents each
per hour. This makes a total cost of about $1.90 per hour for labor.
By adding to the cost of labor the cost of cyanid and aci, as well as
allowing a certain amount for transporting the chemicals to the field,
and including the cost of, as well as wea and tear on the tents and
other equipment necessary in fumigation work, we have a basis for
estimating the real cost of the operation.
Most trees fumigated require between 5 and 18 ounces of cyanid.
An averae dosa would be about 10 ounces. A supply cart (pp.
22-23) can be equipped complete for about $35.
Generating pots cost as follows: One and one-hal gallon, 35 cents;
2-gallon, 45 cents; 3-gallon, 65 cents.
Below are given the prices of different sized octagonal sheet tents
ready for use, as furnished by one of the largest dealer in fumigating
tents in southern California. These prices are based on the assump-
tion that an entire outfit of 30 tens is to be purchased. If only a
single tent is purchased, the ct will be slightly greater than these
figures. Fluctuations in the cotton market will cause a variation in
the price of tents.

S(7 200 (8- -ounce
Size. ounce) ounce) s ial
ipa scial 8rMy


17-foot.............................................. $6.97 $61

............................... ..28.50 30.75 27.00
41- t.... .. ........ ........................ . .. ....... ... 36.10 38.75 34.20
43-foo t ............................................................ 43.70 47.15 41.40
........ ..................................................... 45.60 49.20 43.20
............................................................ 50.35 54.32 47.70
52-foot.............................................................. 62.70 67.65 59.40
5-foot ..................................................69.35 74.82 65.70
6 o .......... ....................... ........................ 91.20 98.40 8640
........................................................... 114.76 123.82 108.72

I The 7 and 8 ounce special drills are those recommended by this investigation (p. 12). The inferior grades
of dril ordinarily employed are about 20 per cent heaper than the -ounce special.

The cost of thirty 45-foot tents of special 7-ounce drill togeer with
the other equipment necessary to complete he outfit will approxi-
mate $1,400.






80 HYDROCYANIC-ACID GAS FUMIGATION IN CALIFORNIA.
GENERAL CAUTIONS.
Hydrocyanic-acid gas is one of the most deadly of gases, so that
considerable care is necessary in its use. Such exaggerated cautions
have been written, in view of its poisonous properties, that the public
at large have come to believe that a single whiff of this gas will
produce the immediate death of an individual. This erroneous idea
should be corrected. A whiff of the gas will not cause immediate
death; neither will two or three whiffs. If subjected to a strong gas
for a minute or two, undoubtedly a person would be overcome. The
writer has never yet had a record of a person killed by hydrocyanic-
acid gas while fumigating. In California, men work around tents
every night for weeks at a time without any ill effects. During these
operations they are breathing the gas in a diluted form much of the
time. Repeatedly field men are seen sitting, either to rest or eat
their lunch, on the edge of a tent covering a tree which had been
dosed a few minutes previously. The writer has personally stood
within a foot of a generator for an hour at a time, taking tempera-
tures of the escaping gas as different dosages were tried out, some
of which would be in excess of a pound. Frequently the wind would
waft the fumes into his face. Outside of an occasional dizziness and
headache, no serious results were experienced. Scores of similar
cases could be cited.
These experiences have been mentioned, not with the idea of
tempting people to be careless in the use of this gas, but merely to
correct the erroneous conception that a whiff of the gas will cause
instant death. This gas is most dangerous, and the writer has seen
men who were subjected to a great strength of it for several minutes
at a time overcome by its effects, although they revived later. If the
proper precautions are taken the careful operator will run no risk
whatever. Place the charge in the generating vessel with extended
arm so that the head of the operator is away from the escaping gas.
Being lighter than air, the gas rises straight up in a narrow column
until several feet above the ground. As soon as the dosage has been
set off, quickly leave the tent or room, whichever it may be. If this
procedure is followed there is no danger of ill effects to the operator.