August 1951 E-822
United States Department of Agriculture
Agricultural Research Administration
Bureau of Entomology and Plant Quarantine
STUDIES ON IMPROVING THE DISTRIBUTION OF HYDROCYANIC
ACID IN CITRUS FUMIGATION
By Harold R. Yust, /
Division of Fruit Insect Investigations
In field fumigations of citrus trees for control of the California
red scale (Aonidiella aurantii (Mask.)) over a period of years, members
of the Whittier, Calif., laboratory have observed that the concentration
of hydrocyanic acid varies at different locations in tent-covered trees.
To control this scale most effectively with fumigation, good distribution
of the gas is necessary. Poor distribution means locations of high sur-
vival, which may become heavily infested in a short time. This is most
likely to be the case in groves with strains that are resistant to this
fumigant. The importance of uniform distribution was recognized by Gray
and Kirkpatrick (1) and Pratt et al. (4). Extensive studies were made
to determine whether the vaporizer applicators in general use could be
improved or a better applicator developed. This paper reports the re-
sults of these studies.
Materials and Methods
In special preliminary tests the vaporizer nozzle was properly
located under the edge of the tent with the outlet directed toward the
center of the tree, and other controllable factors were carefully reg-
ulated. Subsequent tests were made during the course of regular commer-
cial fumigations. The two makes of vaporizers in general commercial
use 7 were used. When they were both used in the same test, the pumps
were carefully adjusted to deliver equal amounts of liquid hydrocyanic
acid. Unless otherwise stated, a 20-cc. schedule was used, that is,
20 cc. of liquid hydrocyanic acid per unit. A unit is about 100 cubic
feet for trees of 700 to 900 cubic feet, less for smaller trees, and
more for larger ones, so that the average concentrations are roughly
uniform for trees of all sizes. The tents were usually made of 8-ounce
I/ Miss B. M. Broadbent, F. Munger, H. D. Nelson, and I. A.
Welborn, Jr., assisted in phases of this study.
2/ Developed by the American Cyanamid Co. and E. I. du Pont de
Nemours and Co., Inc.
The gas distribution was determined by sampling selected locations
under the tents. In some tests gas samples were taken from eight lo-
cations-about 6 feet from the ground 2 to 3 feet inside the tent at
each of the four cardinal directions, 2 to 3 feet from the ground (low)
midway between the tree trunk and the tent on the east and west quadrants,
and 2 to 3 feet from the top of the tent to the north and south of the
tree center. In other tests gas samples were obtained from only four
locations-about 6 feet from the ground 2 to 3 feet inside the tent on
the east and west quadrants, 2 to 3 feet from the ground (low) midway be-
tween the tree trunk and the tent on the south, and 2 to 3 feet from the
top north of center. The samples were drawn as described by Fulton et
al. (1), except that Tygon tubing was used.
In the preliminary tests the exposures were usually for about 5
minutes and the same trees were frequently fumigated successively with
different equipment. The coefficient of variation in concentration
(standard deviation x 100/average concentration) was usually calculated
as a measure of the variability of the gas distribution. A small co-
efficient of variation indicates good distribution and a large one poor
distribution. Mortality counts, when made, were usually based on 100
scales from each of four locations in each tree.
Application With Standard Commercial Vaporizers
To determine the distribution of hydrocyanic acid with the vapor-
izers in general use, 20 relatively open lemon trees were fumigated on
5 nights. Gas samples were taken from 8 locations under each tree
i, 2 and 5 minutes after release of the gas. The trees varied in size
from small to moderately large (10 units to 24 units) and had average
foliation. The nozzle of the applicator was properly placed under the
west side of each tree. The average concentration at each sampling lo-
cation is given in table 1.
Table 1.-Average concentrations (milligrams per liter) of hydrocyanic
acid at selected locations under medium-foliated lemon trees at desig-
nated intervals after release of the gas from the west
Location j minute 21 minutes 5 minutes
6 feet above ground:
North 4.60 3.44 2.70
East 3.83 3.30 2.56
West 1.45 2.04 1.90
South 2.65 2.49 2.24
East 4.26 3.30 2.49
West 1.21 1.94 1.82
North 3.51 3.19 2.48
South 2.76 2.56 2.24
The concentrations were low on the side of release (west) near the
ground and at the 6-foot level. On the side opposite the nozzle the
concentrations were comparatively high at the same levels,
Gas-distribution studies of commercial fumigations were also made
in densely foliated lemon trees in connection with studies of scale mor-
tality. Gas samples were taken from 4 locations in 12 trees and scale
samples from 4 locations in 16 trees. The fumigation schedules used
were 22 and 24 cc. per unit. The gas was released from the east side of
the tree. The average concentrations for 45 minutes at the 6-foot-east,
6-foot-west, low-south, and top-north sampling positions were O086, 1.28,
0.87, and 1.14 ag. per liter, respectively. In general the concen-
trations on the east were low at the beginning and throughout an appre-
ciable part of the exposure period, whereas those on the west, opposite
the nozzle, were comparatively high during most of the exposure period.
The low concentrations on the south were probably due to a tendency of
the operator to go beyond the mid-point and put the nozzle under the
southeast quadrant. The kill of scales in the second-molt stage at the
6-foot-east, 6-foot-west, low-south, and top-north positions was 58.3,
77.1,l 51.2, and 76.6 percent, respectively. These data show the
correlation between gas concentration and scale mortality. Similar re-
sults have been obtained in the fumigation of orange trees with differ-
ing amounts of foliation.
In the fumigation of the grove in which the foregoing experiment
was carried out, it is the practice to treat the trees from the east side
while moving down the rows from north to south. Population counts made
in the grove corroborated the results obtained in the experimental fum-
igations. The scale population was heaviest on the east aspect of the
trees, where the gas concentrations were lowest, and lightest on the west
side, where they were highest. The data suggest the desirability of re-
leasing the gas from opposite sides of the trees in alternate fumigations.
Tests were made with the vaporizer to determine the effect of fol-
iage density on distribution of hydrocyanic acid. Fourteen dense orange
trees and an equal number of open lemon trees were fumigated in special
tests. To get information on the distribution in the absence of a tree,
14 fumigations were also made in 2 tree forms. The tree forms were
equivalent in size to a medium-sized tree and approximately the same size
as the orange and lemon trees used. Gas samples were taken j minute and
21 minutes after release of the gas. The average coefficients of var-
iation in the concentrations are given in table 2. It is evident that
foliage density influences the distribution of hydrocyanic acid. In a
tree form containing no tree good distribution was almost immediate.
The data indicate that pruning out of interior growth from the trees would
aid in getting better distribution of hydrocyanic acid.
Table 2.-Effect of tree density on the distribution of hydrocyanic acid
Coefficient of variation in concentration (percent)
at indicated interval after release
minute 2J minutes
Dense 92.1 46.0
Open 52.2 23.0
Form (no tree) 7.5 5.4
Release of Gas From Two Locations
In special tests in w'Ich hel? the hydrocyanic acil was released
with a standard vaporizer from one point and half from the opposite point
under the trees, the gas distribution was much more nearly uniform than
when it was released from one place. Tests were made in three groves, in
cooperation with commercial fumigators, to determine whether in practical
operation the use of this procedure would improve the distribution of the
gas. Gas samples were taken at 4 locations from each of 16 trees in each
category. The average coefficient of variation determined from the samples
taken after 1/2 minute was 91.9 percent when the gas was released at one
point and 64.4 percent when it was released from two points. For the
samples taken 2 1/2 minutes after release the respective coefficients
were 33.1 and 36.9 percent.
These data indicate that release of liquid hydrocyanic acid from
two points beneath the tents instead of one would not improve gas dis-
tribution sufficiently to justify the extra labor and time required to
reset the nozzle. Furthermore, protective stupefaction might increase
with consequent decreased kill.
Standard Vaporizer Nozzle With a Baffle Added
In several fumigations lifting the nozzle of the vaporizer to direct
the gas upward greatly improved the distribution. This experience led to
the development of a mechanical device, or baffle, for changing the di-
rection of the discharge of gas. The device consisted of a flat piece
of sheet metal 2 by 2 inches attached to the vaporizer nozzle on a hinge.
The baffle position could be changed manually by means of a cable ex-
tending through a conduit along the gas hose. By pulling the cable the
operator drew the baffle in front of the nozzle opening, causing the gas
to go upward and outward instead of across to the opposite side of the
tree. In operation the baffle was pulled into place after the hydro-
cyanic acid was pumped into the vaporizer. It was estimated that a
little over half the hydrocyanic acid had escaped from the hose before
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the baffle position was changed.
Eleven medium-sized, dense orange trees were used in special tests
to compare the baffle nozzle with the standard nozzle from the stand-
point of gas distribution. The same trees were treated with each nozzle.
Tests were made on three nights, and gas samples were taken from four
locations. The average coefficients of variation determined from the
samples taken I minute after release of the gas were 84.8 percent for
the standard nozzle and 44.8 for the baffle nozzle; for the samples taken
2j minutes after release the respective coefficients were 38.0 and 24.4
The distribution was greatly improved by changing the direction of
the stream of hydrocyanic acid during part of the release period, so that
the lowest concentration was not always close to the nozzle and the high-
est was not always on the opposite side of the tree. However, in the
field the moving parts of the baffle nozzle were found to catch on veg-
etation or the tents and retard operations. It is probable that a stream-
lined unit could be constructed that would overcome this disadvantage.
Nozzle With Outlets 2 Feet Apart
Standard nozzles were modified in many ways in an attempt to in-
crease the concentration of gas near the nozzle and decrease it on the
opposite side of the tree. Many nozzles with more than one outlet re-
leasing the gas in different directions were devised and tested before
one was found that improved distribution. Multiple-outlet nozzles with
the outlets not more than 3 inches apart failed to improve distribution.
With two or more outlets not more than 3 inches apart the gas tended to
be drawn into a single stream. Further tests showed that the outlc
should be at least 2 feet apart.
Special field tests were made to compare the gas distribution by
a special nozzle having outlets 2 feet apart with that by a stadr
nozzle. The special nozzle differed from the standard only in avig n
extra outlet 2 feet back of the regular one. The extra outlet was i ply
a crosswise slot 3/32 inch wide halfway through a 3/4-inch pipe coupling.
The gas escaped at a right angle to the coupling through 180 . In the
tests 16 orange trees were treated with each nozzle arrangement. ..
trees were dense with foliage and medium in size. Gas samples were taken
from four locations and the tests were made on five nights. The average
coefficients of variation determined from the samples taken j minute
after release of the gas were 84.7 percent for the standard nozzle and
54.0 percent for the special nozzle. For the samples taken 2j minutes
after release the respective coefficients were 47.9 and 32.3 percent.
The distribution of hydrocyanic acid was better when release was
made through a nozzle with outlets 2 feet apart than when it was made
with the standard nozzle, but was probably not so good as when it was
made with the baffle nozzle. However, the nozzle with outlets 2 feet
apart has several advantages over the baffle nozzle. It is simFle, has
no moving parts, and can be moved in and out from under a tree without
catching on the tree or on the tent.
The nozzle with outlets 2 feet apart was compared with the standard
nozzle on the basis of the kill of the California red scale in four commer-
cial fumigations. The comparisons were made with medium to large trees
in three orange groves and one lemon grove. Ten or more trees were treated
with each nozzle in each grove. Trees of about the same size were treated
with each nozzle in the same throw of tents. The same vaporizer was used
for both nozzles. A schedule of 20 cc. per unit was used in one grove,
22 cc, in another, and 24 cc. in the other two groves. Mortality counts
were made on scale-infested fruits from each cardinal direction of the
trees. The kills were more uniform on the trees to which the gas was
applied with the special nozzle, but the average kill was no different
from that obtained with the standard nozzle.
The multivane blower applicator described by Fulton and Nelson (2)
and the propeller-blower applicator described by Yust (5) were developed
to give improved distribution of hydrocyanic acid in tent-covered citrus
trees. After a long period of changes and tests, the distribution of
gas with the propeller-blower applicator was studied in a series of
special tests conducted on seven nights. On each night two lemon trees
were treated with the propeller-blower applicator and with a standard
vaporizer. Since short exposures were used, each tree was fumigated with
each apparatus. Gas samples were taken from eight locations j minute and
2j minutes after each release. The trees were relatively open and varied
from small to large.
Several other field tests were made on orange and lemon trees in
cooperation with commercial fumigators. The orange trees were large and
moderately dense and the lemon trees were medium in size and dense with
brush and foliage. The lower branches of the orange trees cleared the
ground and the blower could be easily pushed under the tent, but the
lemon trees had large low branches so that it was usually impossible to
push the applicator through or under them. Gas samples were taken from
four locations under each of four lemon trees and three orange trees
treated with each applicator.
Table 3 shows that the propeller-blower applicator gave a much better
initial distribution of the hydrocyanic acid than the vaporizer in both
series of tests. However, distribution in the commercial tests was not
so uniform as in the special tests, for which more open trees were used.
Mortality counts and population studies of the California red scale
were made in groves treated by commercial fumigators with the two types
of applicators. About the same degree of scale control was obtained
with both applicators. Apparently the improvement in gas distribution
was not sufficient to offset disadvantages introduced by the blower.
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Table 3.--istribution of hydrocyanic acid obtained with the standard
vaporizer and the propeller-blower applicator
Coefficient of variation in concentration (percent)
at indicated interval after release
Special tests Commercial tests
minute 21 minutes i minute 2J minutes
Vaporizer 56,7 23.6 111.8 60.1
Propeller-blower 25.4 12.0 41.9 16,1
The commercial fumigations brought out the following disadvantages
in the propeller-blower applicator that were not apparent in prior ex-
periments. (1) More strenuous work for the operator, (2) slower rate of
fumigation, (3) more drift gas, (4) pulling the blower backwards, (5)
difficulty in maneuvering the blower in rough ground, (6) tendency of the
blower to catch in the foliage and tent, (7) injury to foliage in front
of the machine by the hydrocyanic acid and the blower, and (8) tendency
of gas concentrations to be lower with the blower. Fumigation with the
blower was more strenuous and slower than with the vaporizer, because the
machine was pushed under the tent and pulled backwards from tree to tree.
The blower was pulled backwards to reduce the hazard presented by the
drift gas being blown forward in the path of the operator. There was a
tendency for the foliage and the tent to catch on the blower when it was
shoved under the tree, although the machine was thoroughly screened. Con-
centrations were probably lower in trees with dense foliage near the
ground, because the atmmized hydrocyanic acid condensed on the foliage.
Field fumigations of citrus trees for control of the California red
scale (Aonidiella aurantii (Mask.)) were made to determine whether more
uniform distribution of hydrocyanic acid could be obtained by improving
the standard vaporizers or their operation, or by developing an improved
applicator. With the standard vaporizers the gas concentration was low
on the side of the tree on which it was released and high on the opposite
side, and was more variable when the foliage was dense than when it was
Distribution of hydrocyanic acid with the vaporizer was improved
(1) when half the gas was released from one side and half from the opposite
side under the tree, (2) when a baffle was placed on the standard vapor-
N IVERSITY OF FLOR DA
il I I I I I I I I!1',1II
3 1262 09239 6042
izer nozzle to change the direction of the stream of hydrocyanic acid
during part of the release period, and (3) when outlets 2 feet apart were
provided on the vaporizer nozzle. A propeller-blower applicator gave
much better distribution of hydrocyanic acid than a standard vaporizer.
Unfortunately, improved distribution of hydrocyanic acid did not
result in improved kills of the California red scale under conditions of
commercial operation, and the baffle nozzle and blower applicator were
found to have serious operational disadvantages.
(1) Fulton, Robert A., Busbey, R. L. and Yust, Harold R.
1941. The behavior of hydrocyanic acid gas under a fumigation
tent. Jour. Econ. Eit. 34: 777-783.
(2) Fulton, R. A., and Nelson, H. D.
1946. Use of the blower applicator in fumigation. Calif. Citrog.
31: 154, 166-167.
(3) Gray, Geo. P., and Kirkpatrick, A. F.
1929. Resistant scale investigations. Calif. Citrog. 14: 364.
(4) Pratt, F. S., Swain, A. F., and Eldred, D. N.
1931,. A study of fumigation problems: "Protective stupefaction"
its application and limitations. Jour. Econ. Ent. 24:
(5) Yust, Harold R.
1950. Propeller-blower applicator for fumigation with hydrocyanic
acid. U. S. Bur. Ent. and Plant Quar. ET-286, 3 PP.