UNITED STATES DEPARTMENT OF AG TURE
WASHINGTON, D. C. 674 JULY 8, 1915.
Contribution from the Bureau of Entomology, L. O. Howard, Chief.
CONTROL OF THE CITRUS THRIPS IN CALIFORNIA
By J. R. HORTON, Scientific Assistant, Tropical and Subtropical Fruit Insect Investi-
The citrus thrips,1 a minute orange-yellow insect, has in the past
few years caused extensive damage to citrus fruits in the San Joaquin
Valley of California and also occasioned considerable injury in
southern California and Arizona orange groves.
The nature and extent of the injury caused by this insect and its
life history and habits were carefully studied, and extensive experi-
ments for its control were conducted by. the writer during the period
from 1910 to 1912. It is the purpose of the present paper to give
briefly the practical control measures resulting from these studies.
The citrus thrips-is a sucking insect feeding on the plant juices
of the leaves, the fruit rind, and the bark of tender stems, in much
the same manner as the mosquito draws its food from its victims.
For this reason the insect can not be killed by stomach poisons
sprayed on the plant, but must be controlled by sprays that kill
The injury caused by the citrus thrips begins with the seedling
orange tree. The leaves are scarred and distorted, and to a certain
extent the stock is devitalized. When the seed stock is budded and
the foliage of the seedling trimmed off, the thrips attacks the bud.
Nursery buds will make a fine, luxuriant growth of 2 or 3 feet in a
1 (Euthrips) Scirtothrips citri Moulton; order Thysanoptera, family Thripidse.
NOTE.-This bulletin is of interest to the citrus growers of the Pacific coast and the Southwest.
FARMERS* BULLETIN 674.
season if properly sprayed to protect them from thrips. On the
other hand, many nursery trees have the leaves and stems so badly
scarred and twisted as to give them a blighted, unsightly appearance,
and are so retarded in growth that they must be held in the nursery
for a year or more beyond the proper time for sale in order to meet
the size requirements, thus decreasing the nurseryman's profit by the
cost of the extra care. It sometimes happens that this class of stock
is sold along with better trees, and the thrips injury continues for
FIG. 1.-Injury to young oranges by the citrus thrips (Scirtothrips citri). (Original.)
several years in the orchard. The writer knows of 5-year-old and
7-year-old groves in the foothills of Tulare County which have been
held back, principally by thrips, to such an extent that the trees are
no larger than 3 and 5 year trees in less infested situations. From
the general appearance of such trees it seems evident that they will
never attain the size and bearing capacity of trees which have escaped
severe thrips infestation in the nursery and during their early years
in the orchard.
CONTROL OF CITRUS THRIPS IN CALIFORNIA AND ARIZONA. 3
As the young fruit appears it in turn is attacked (fig. 1), and its
market value at maturity is much reduced by the enlarged feeding
scars and scabbing (fig. 2). A larger percentage of small-sized fruits
than ordinarily develop results, and there is a total loss, as the result
of early and severe scabbing, of a proportion of the fruit. To calculate
the damage caused by the insect in reducing the grade of the fruit, it is
necessary to know the system of grading and the relative market value
of the grades. Three packs are usually made in California packing
houses at the time of this writing, these packs or grades being variously
designated as "Fancy," "Choice," and "Standard"; "Extra Fancy,"
FIG. 2.-Mature oranges, showing injury by citrus thrips. (Original.)
"Fancy," and "Choice"; or "Extra Choice," "Choice," and "Stand-
ard." Whatever the terms used there is usually little difference in
the quality of fruit of corresponding grades at the different packing
houses. In other cases only two divisions are made, the first grade
generally being designated as 'Orchard Run" and the second or lower
grade as "Standard." Under the latter system the quality of the
fruit composing the first grade is about the same as would be obtained
by placing together all the fruit of the first and second grades of the
three-grade pack. Statistics upon the quantity of fruit shipped from
the entire San Joaquin Valley and the prices received for it are not
available, but from Lindsay and its tributaries 1,525 carloads of navel
FARMERS' BULLETIN 674.
oranges were shipped in 1911. The approximate average number of
boxes of fruit to the car is 390, making 594,750 boxes for the season's
shipment. From examination of thousands of oranges in the field,
throughout the district and in many groves, it was.calculated that
34 per cent of all the fruit would be classed as first grade so far as
thrips injury was concerned, 43 per cent as second grade, and 23 per
cent as third grade. Returns received by different packing houses
on a total of 358,000 boxes of navels of all grades for the season in-
dicated the following average differences in price per box between the
different grades. First-grade fruit averaged 37 cents more per box
than that of second grade; the latter 28 cents more than that of third.
Fruit shipped in only two grades gave an average difference of 51 cents
per box in favor of the first grade. It may be seen from the foregoing
data that 43 per cent, or 255,742 boxes, of the Lindsay fruit was re-
duced to second grade at a loss of 37 cents per box, or $94,624.54;
23 per cent was reduced to third grade at a loss of 65 cents per box, or
an additional $88,914.80. There was thus a total loss for the Lindsay
district alone of approximately $183,539.34 in the season of 1911
from grade reduction caused by thrips.
SUMMARY OF SEASONAL HISTORY.
In seasons such as 1911, adult citrus thrips first appear in April and
increase rapidly during April and May, during which time the insects
are congregated largely on the fruit and foliage of the orange. During
part of June, July, and August the adults leave the toughening fruit
and leaves of the orange and disperse over miscellaneous food plants,
and it is during this period of wider separation that mating and
oviposition are somewhat checked. In August and September there
is a series of flights back to the late summer growths of the orange,
where the insects concentrate in large numbers, mating and actively
depositing the eggs which produce the insects of the following spring.
The citrus thrips begins to disappear about the middle of October,
and after December practically none can be found. There are generally
a few larvae and adults in places on the trees until the early part of Jan-
uary, at which time they disappear completely. The eggs which are
deposited in the stems and leaves of the orange in the fall mostly pass
the winter successfully, hatching during the ensuing March, April, and
May. The seasonal activities of the citrus thrips, as related to orange
blossoming, growth periods, and spraying are summarized graphically
in figure 3.
SUMMARY OF LIFE HISTORY.
There is a tendency on the part of the citrus thrips to breed through-
out the year. All stages of the insect are found on the trees through-
out November and December. Larve, pupse, and adults gradually
CONTROL OF CITRUS THRIPS IN CALIFORNIA AND ARIZONA. 5
die off as the weather grows colder, until by the middle of January all
have disappeared. The winter is passed only in the egg stage. Eggs
deposited in the leaves and stems, mostly during late August, Septem-
ber, and October, hatch and the larve appear in March, April, and
The average duration of the egg stage of summer generations varies
from 10 to 18.8 days during May and June, 6.8 to 8.5 days in July and
August, and 17 to 18.8 days in September and October.
The average larval stage varies from 6.6 to 13.7 days during April
and May, 4.2 to 9 days from June to August, and 6.7 to 11.2 days in
September and October.
The average pupal stage varies from 4.7 to 13 days during April and
May, 2.8 to 5.1 days from June to August, and 5 to 19.9 days from
September to November.
Pupation takes place in crevices on the tree trunk, in dead leaves
and rubbish under the trees, and under clods and particles of trash
MARCH AP Y E dULy I -us" r s oEP.r OC N/O DEC.
15 3 15 30 15 30 15 30 15 3 15 30 15 3 15 30 15 30 15 3
7-1R/P5 VO ORANGE FOLIAGE-
FIG. 3.-Graphic illustration of the seasonal activities of the citrus thrips as related to blossoming and
later growth periods of the orange, and indicating also the spray periods. (Original.)
on the ground, but never in the ground. The pupa is naked, does not
construct a cell, and is at all times capable of locomotion.
The average duration of adult life is from 25 to 35 days, with
extreme instances running to from 46 to 49 days. Adults can live
from 2 to 6 days only without food.
The number of generations in a season will depend upon the char-
acter of the season. An early, warm spring followed by a prolonged,
hot summer may result in the production of eight or more genera-
tions. In seasons such as 1911, six full generations may be expected
between the middle of April and the first of November. For pur-
poses of control the citrus thrips must be treated as an insect having
only a single generation a season, and with an egg-laying period
extending from April to November.
FARMERS BULLETIN 674.
REMEDIES FOR THE CITRUS THRIPS.
Certain measures against the citrus thrips have been persistently
recommended in spite of abundant evidence of their inapplicability.
These are usually directed against the pupal stage and consist in the
application of insecticides to the soil, breaking the soil up fine to
destroy the insects supposedly pupating there, and burning dead
leaves and trash, which accumulate under the trees, to destroy the
pupe. These methods are worthless for the reason that the thrips
FiG. 4.-Resin-wash injury to half-grown oranges sprayed for the citrus thrips. (Original.)
do not go into the soil at all, and only a varying and often small
percentage of them pupate in the trash. Fumigation with hydro-
cyanic-acid gas will reach and kill only the larva and a small
number of adults, and is accordingly too expensive to use. Dis-
tillate-oil emulsions and proprietary emulsions containing distillate,
even when used as weak as 2 per cent, stain the ripe oranges, and
are otherwise so injurious that it is considered unsafe to use them.
Commercial lime-sulphur is not noticeably injurious when used at
less than 1 part to 28 parts of water. Resin wash can not be safely
used on orange trees at any strength. Where the resin mixture
comes into contact with the fruit the epidermal cells are killed and
CONTROL OF CITRUS THRIPS IN CALIFORNIA AND ARIZONA. 7
a shallow brown scab is formed. (Fig. 4.) Where the liquid col-
lects in large drops it forms a thick, amber-to-black scab which does
not slough off readily. About 20 per cent of the fruit at picking
time was thrown into the lowest grade owing to scabbing through
the use of the weakest resin wash.
There is only one cheap and effective method of citrus-thrips
control, viz, the application at high pressure of contact insecticides,
preferably mixtures containing sulphur in solution. Sulphur mix-
tures at the proper strength have given uniformly high killing
results and have thus left no doubt as to their insecticidal power
over this species. They further show a more or less marked tendency
to repel the insects and prevent rapid reinfestation of sprayed trees.
SPRAY MIXTURES AND DILUTIONS.
Of the large number of combinations of insecticides tested, the
following have given the best results, and any of the mixtures here
recommended may be relied upon to do good work:
1. Commercial lime-sulphur.-If the lime-sulphur is of a density of 36 degrees on
the Baume scale, dilute 1 gallon with 56 gallons of water; if of a density of 33 degrees
BaumB, dilute 1 gallon with 50 gallons of water.
2. Sulphur-soda solution.-Two gallons of the stock solution, prepared as described
on page 8, diluted with 25 gallons of water.
3. Commercial lime-sulphur and blackleaf tobacco extract (40 per cent nicotine sul-
phate).-Dilute 1 part of the commercial lime-sulphur, if 34 to 36 degrees Baumb,
with 86 parts of water; if 30 to 33 degrees Baum6, with 75 parts of water. Then add
1 part of the tobacco extract to 1,000 parts of the lime-sulphur diluted as above.
4. Blackleaf tobacco extract (40 per cent nicotine sulphate).-Dilute 1 part with 800
parts of water.
The commercial lime-sulphur, diluted with water and without the
addition of other chemicals, is preferred to any of the other insecti-
cides because of its cheapness and convenience in mixing. Very
good grades of lime-sulphur can be purchased in the market at a
reasonable price, and since the preparation of this product requires
care and experience, and as it must be made fresh each time or
special precautions taken to store it in air-tight containers, its home
manufacture is not advised. When necessary to carry the market
product over a season it is essential to protect it absolutely from
the air, as it rapidly loses its insecticidal power when exposed through
leaky barrels or an open bung.
Another mixture containing sulphur as the most important ingre-
dient is made by dissolving sulphur with the aid of caustic soda,
according to the directions given below. This mixture, though
FARMERS' BtULLETIN 674.
practically as effective in controlling the citrus thrips as lime-sulphur,
can not be purchased ready-made and is therefore less convenient to
handle. Furthermore, at the present writing it costs just as much
per dilute gallon as the factory-made lime-sulphur.
The sulphur-soda stock solution is prepared as follows:
Powdered sulphur ...----................--.............. 30 pounds.
Powdered caustic soda (98 per cent)..-..................... 15 pounds.
Water to make----..............--- .....-...---- ......... 30 gallons.
The sulphur is made into a paste with water, and while the mix-
ture is being constantly stirred the soda is added in sufficient quantity
to start boiling. As boiling becomes violent a little water is added
to retard it. When the sulphur has all been taken into solution
enough water.should be added to bring the stock solution up to 30 gal-
lons. If made according to the foregoing directions the final product
will be a clear, amber-colored liquid much resembling good commer-
PLAIN TOBACCO EXTRACTS.
Tests with plain tobacco extracts without the addition of lime-
sulphur or other preparations have given very good results when the
tobacco has been used at sufficient strength. Tobacco extract con-
taining 40 per cent nicotine used at the rate of 1 part to 800 parts,
liquid measure, of water is quite satisfactory; when diluted at the rate
of 1 part to 1,600 parts water, however, its efficiency is noticeably
lowered. It can not be recommended for this work in solution weaker
than 1 to 1,000, and should preferably be used at the rate of 1 to 800.
The commercial tobacco extract containing a high percentage of nico-
tine sulphate is very convenient to handle and costs approximately
$0.016 for each gallon of the diluted spray, when used at the rate of
1 part to 800 parts of water.
TIME AND NUMBER OF SPRAY APPLICATIONS.
Unfortunately no specific dates, which will hold for every season,
can be fixed for the applications of the spray. The investigations of
the seasons of 1910 and 1911 have shown that the date on which the
thrips first become numerous and injurious and the navel-orange
blossoms lose their petals varies as much as 30 days in certain seasons,
dye to the nature of the spring weather, and, further, that it varies in
different orchards in the same season. The greatest injury to the
fruit is done between the time the petals fall and the fruit is half
grown. It has been demonstrated that three applications of the
insecticide are necessary during this period to prevent marking of
the fruit. The first spring growth has usually hardened by the time
CONTROL OF CITRUS THRIPS IN CALIFORNIA AND ARIZONA. 9
the petals have all fallen, and the thrips then seek the young fruit.
The petals do not all fall at once, but come down gradually, and the
transfer of thrips is therefore gradual.
The first application should be made as soon as four-fifths or more
of the petals have fallen. This checks the insect at a time when the
orange is most susceptible of deep injury and when the blossoms have
passed the period at which pollination might be interfered with by the
After the first application more larve will issue from eggs deposited
in the very young fruit, and additional adults will appear from the
specimens pupating at the time of the application. The second
application must therefore be timed to prevent this renewed attack,
which may be expected to reach the danger point in from 10 to 14
days after the first spraying. This second spraying should not be
too long delayed, as a comparatively few larvae may, by their persist-
ent habit of feeding in a circle about the base of the fruit, cause con-
siderable injury. Special effort should be made to drench all the
fruit as well as the few remaining tender leaves thoroughly, as it is
here only that the insects occur.
The third application may be longer delayed if the first two have
been thorough and well timed. It generally takes the insects from
two to three or four weeks to become dangerously numerous again,
as they reinfest the sprayed trees very much more slowly after the
After the third application the fruit rapidly loses its attractiveness,
and the insects then find it necessary, in order to secure food, to
spread out over the few remaining tender orange leaves and certain
miscellaneous food plants. During the latter part of August and in
early September there is usually another abundant growth of shoots
upon which the thrips congregate in great numbers. A fourth appli-
cation in late August or more probably in September should be timed
to catch the insects as soon as they become numerous and before any
great amount of leaf injury appears.
The importance of protecting this growth is evident to those
familiar with the stunted condition of orange trees in certain orchards
of Tulare County as the result of continuous feeding of large numbers
of thrips during the first five or six years of growth. The writer has
in mind an orchard in which trees five years from the nursery are
no larger than the average 2-year-old trees in localities more favor-
ably situated with regard to thrips, and which each year have a
very large percentage of the leaves so severely injured that they roll
up into tight curls.
FARMERS' BULLETIN 674..
SPRAY APPLICATIONS TO NURSERY STOCK.
While definite dates can not be given for the application of sprays
to nursery stock, it follows in the case of trees budded in the fall,
where the original stock is allowed to put forth a good growth in the
spring, that it is sometimes advisable to spray during April, but only
when thrips have become quite numerous and for the purpose of
ridding the trees of them before the scion has grown sufficiently to
attract them. Preferably the stock should be largely cut back as
soon as the bud is well under way,
and this is generally done in Tulare
County before May 1. The prunr
.-e ings should be burned in every case
to destroy eggs and larvae which may
be present. The growing scions
must then be watched carefully, and
as soon as thrips appear in numbers
spraying should begin. They should
be further watched with the same
care throughout the remainder of
the growing season and sprayed as
often as the abundance of thrips
: makes them liable to severe injury.
Nursery stock will .usually require
from three to five applications a
season, depending largely on the
amount of growth it produces.
6 Once the scion has completed its
first growth and become distasteful
Sto thrips the next most important
growth will usually occur late in
July or in August.
FIG. 5.-Correct spray rod and nozzle connec- To summarize, the first applica-
tions: a, Two nozzles fitted on "Y" branch; tion should be made when thrips
b, shut-off at base of spray rod. (Original.)
begin to get numerous on the spring
growth, usually between April 15 and May 15, after which from two
to four further applications will be necessary, according to the con-
ditions of infestation.
SUGGESTIONS ABOUT SPRAYING.
The gasoline-power outfit, by reason of its large nozzle capacity,
simplicity, reliability, and comparatively low cost of operation, is the
only class of sprayer here recommended for spraying bearing orchards,
young orchards in excess of 10 acres, and large nurseries. Hand-
power outfits, when of the right type and capable of maintaining a
pressure of not less than 125 pounds, are suitable and even preferable
CONTROL OF CITRUS THRIPS IN CALIFORNIA AND ARIZONA. 11
for spraying seed-bed and nursery stock, and they may also be used
in young orchards of small acreage.
The spraying outfit should be on hand, set up, and in perfect
running condition not later than April 1, and the insecticide materials
at hand and conveniently located near the water supply, and as
close as possible to the orchard or nursery to be sprayed. It is neces-
sary to order supplies not later than the January or February pre-
ceding the spraying operations in order to insure having the material
at hand when wanted.
HOW TO SPRAY BEARING ORCHARDS.
It is best to use only two 50-foot leads of hose on a power outfit, with
10-foot rods each fitted with a" Y (fig. 5) which is angled to handle two
nozzles. The latter should be of the large chamber type, with disks
bored to one-sixteenth
inch, and should throw a
double cone of spray which
breaks into a fine mist at
about 4 feet (fig. 6). The
first application should
usually be started just be-
fore all the petals are down.
While the sprayer is being
driven between the rows
each rodman should begin
work at about the middle
of his tree on the side
away from the sprayer
and work around the tree
until he meets the starting
point; he should then
switch to the same point
on the next tree without
shutting off the nozzles
and with as much economy
of movement as possible.
(See figure 7, which shows
easy and correct position
The nozzles should be
FIG. 6.-Mist spray from twin nozzles. (Original.)
held about 2 feet from the
tree so that the broad portion of the stream plays upon fruit and
leaves. The trees should be swept from tip to base, special attention
being given to the fruit and the tender growth, where the insects
congregate. The pressure, if maintained at 150 pounds or more,
will turn the leaves over so that both sides will be sprayed. No
FARMERS' BULLETIN 674.
attention need be paid to the inner portions of the tree, as thrips do
not occur there.
One should not attempt to spray too many trees with a single
outfit, and an application once commenced should be finished within
10 days. It has been
found after much experi-
: .-. ence that only about 25
S acres of from 12 to 18
year old trees or 50 acres
of from 5 to 7 year old
trees can be successfully
handled with one gasoline-
power sprayer. This is
calculated on the basis of
ten 200-gallon tanks of
spray per day, allowing 8
gallons per tree for trees
from 12 to 18 years old or
4 gallons for trees from
5 to 7 years old, allowing
100 trees to the acre. It
is a common mistake to
use the wash too spar-
ingly and to try to get
over the ground too fast.
Table I, published also in
a former report,' was pre-
pared to show approxi-
mately the correct amounts
to apply to trees of dif-
FIGo 7.-Correetpositionof operatorinspraying. (Original.) ferent ages, and from it
ferent ages, and from it
the quantity of spray material required for the season may be
TABLE I.--Quantities of liquid required in spraying for the citrus thrips.
treesof One application.
Years. dilute per acre
spray of 100
per tree. trees.
2 to 3... 2 200
5 to 7... 4 400
8to10.. 5 500
12to18. 8 800
'Jones, P. R., and Horton, J. R. The Orange Thrips: A Report of Progress for the Years 1909 and
1910. U. S. Dept. Agr., Bur. Ent., Bul. 99, pt. 1, iv+16 p., 2 flg., 3 pl., Mar. 6, 1911. See p. 15.
CONTROL OF CITRUS THRIPS IN CALIFORNIA AND ARIZONA. 13
The spraying must be very thorough, and to be effective the insects
must actually be hit by the spray. It will very much improve the
results if the rodmen are shown the insect they are to spray*for and
just where it will be found in greatest numbers. In this way the object
of spraying is made definite. By keeping the thrips reduced to a
minimum during the period between the dropping of the petals and
the time when the fruit is half grown, most of the fruit scarring and
the leaf curl of the early summer growths of foliage can be prevented.
An application at the proper time in late August or in September will
prevent the severe leaf curling which usually occurs to all late summer
HOW TO SPRAY NURSERIES AND YOUNG TREES.
For large nurseries, where the gas-engine outfit can be advan-
tageously used, it is preferable to the hand outfit. Two 25-foot or
even 15-foot leads of hose and 12-foot spray rods are generally most
convenient for this work. However, when an outfit has already been
fitted with 50-foot hose and 10-foot rods, with the intention of spray-
ing older trees as well as nursery stock, this equipment may be made
to serve very well for the latter. In such case the excess hose length
should be coiled over a peg or bracket fastened to the spray tank or
engine hood, so that the young trees will not be injured by the drag-
ging hose. It is preferable in setting out a nursery to leave driveways
wide enough to accommodate a sprayer and team at intervals through-
out the length of the bed. Where it is desired to have the nursery
rows 4 feet apart, which is the usual practice, it is convenient to have
wagon room between the fourth and fifth rows from one side, and
again between the 12th and 13th, 20th and 21st, etc. With this
arrangement eight rows of trees, four either side of the driveway,
may be reached each trip, using 12-foot spray rods; eight more rows
may be taken on the return trip, etc.
The large chamber-type or single Bordeaux nozzles may be used
to good advantage, but the rapidity of delivery of the spray need not
be so great as that necessary for orchard work. It is better to pro-
gress more slowly, covering all portions of the little trees, without
undue waste of liquid. The trees will need attention only when the
growth is tender.
FARMERS' BULLETIN 674.
COST OF SPRAYING AS COMPARED WITH RETURNS.'
Thrips injury to citrus fruits is confined to the rind and does not
appreciably affect the eating quality of the fruit. Except in seasons of
unusually gross infestation no great amount of fruit is lost entirely
by reason of thrips injury. The argument has been advanced that
Where the fruit is separated into but two commercial grades, which
embrace everything fit to ship, as is now largely the case, thrips
injury will have but little effect on the price. The damage thrips do
to the trees by interfering with the functions of the leaves throughout
the early years of growth, however, is generally overlooked. The
following statement takes no account of this indirect injury to the
trees, which is difficult to estimate, but merely gives the profit realized
from producing a better grade of fruit by spraying.
SCost of spraying one acre of 18-year-old navel orange trees.
2 rodmen at $2.50 each per day, cost per acre .......................... $1. 22
Driver and team at $5 per day, cost per acre........................... 1.23
Cost of labor per acre, one application.. ----........-... ............. 2.45
14 gallons lime-sulphur at 14 cents, cost per acre, one application........ 1. 96
Fuel, oil, and miscellaneous:
Gasoline, 1 gallons at 25 cents, per acre, 3 applications .................. .625
Oil at $1 per gallon, per acre, 3 applications ................. ........ .025
Repairs and batteries, per acre, 3 applications ......................... .21
Estimated cost of fuel, oil, etc., per acre, 3 applications ................. .86
Cost of labor per acre, 3 applications ................................... 7.35
Cost of insecticide per acre, 3 applications. ............-............... 5.88
Cost of fuel and miscellaneous per acre, 3 applications .................. .86
Total cost of treating 1 acre of 18-year-old navel orange trees........... 14.09
Returns from sale of fruit.
Number of packed boxes fruit produced per acre .........-...-........-.. 324
Per cent of fruit raised from second grade to first grade by spraying ............. 18
Boxes raised from second grade to first grade by spraying ...................... 58
Difference in price received per box for first-grade over second-grade fruit...... $0.51
Amount saved per acre by spraying----.............--.................... $29.58
Profit from sale of fruit.
Amount saved per acre by spraying ................................... $29.58
Cost of spraying per acre ........--.-....------ ..-.....-....--............. 14.09
Clear gain per acre from the treatment -.........---- --- ......-----......... 15.49
1 The figures given upon cost of spraying are based on the Bureau of Entomology's own spraying work
in the season of 1911. The number of boxes of fruit given, 324 per acre, was the actual production of the
portion of grove under experiment, and as these trees were not at their best and since 18-year-old trees
usually produce more than 324 boxes per acre, the saving effected by spraying would tend to be greater in
most cases. The difference of $0.51 per box between first and second grade fruit was that which was
actually shown by packing-house returns, and practically all the grade reduction was caused by thrips
CONTROL OF CITRUS THRIPS IN CALIFORNIA AND ARIZONA. 15
In the above calculation the cost of spraying an acre of 18-year-old
trees is higher than will usually be the case, since, as a rule, the
grower is obliged to have a team on hand all the time and may there-
fore reduce the item of team hire; he may also be able to reduce the
cost of labor somewhat in many cases. In seasons of gross infesta-
tion, and in certain orchards every season, the returns will be greatly
increased over the figures given because of the excessive infestation in
such seasons and orchards.
WASHINGTON: GOVERNMENT PITNTING OFFICE: 1915