STATE PLANT BOARD
May 1950 ET-282
United States Department of Agriculture
Agricultural Research Administration
Bureau of Entomology and Plant Quarantine
DIRECTIONS FOR APPLYING WINDBORNE AEROSOLS
FOR INSECT CONTROL OUT OF DOORS
By A. H. Yeomans
Division of Control Investigations
Studies have been made by us at the Agricultural Research Center,
Beltsville, Md., and all over the country, on the proper method of
applying windborne aerosols for the control of insects on field crops
and in orchards. Windborne aerosols are those applied as a cloud and
carried by the wind across the area to be treated. The fundamentals
of aerosol cloud behavior have also been studied in the laboratory by
use of a wind tunnel. From this background of experience the following
directions for applying aerosols for control of insects out of doors have
The insect control attained from windborne aerosols applied to field
crops and orchards is due mostly to the deposit. Therefore, the con-
siderations are directed to the factors that result in the best deposit.
It is true, however, that if the aerosol is applied while the insect to be
controlled is in flight, those individuals on the wing are killed as well
as those that come in contact with the deposit.
Machines for applying windborne aerosols should not be confused
with machines that project sprays with a powerful air blast. The air
blast machine can provide a more uniform deposit across a swath
with the larger spray particles than can be obtained with the windborne
For satisfactory performance an aerosol cloud must be released
under proper weather conditions, have uniform deposition in the
selected swath width, be composed of particles of the proper size, be
of the proper dosage and formulation, and be applied in the most
economical manner. These requirements will be discussed.
Satisfactory movement of the aerosol cloud across the area is
accomplished by making applications under the proper weather con-
ditions. A light wind is needed, steady in direction, and moving at
1/2 to 8 m.p.h. Winds slightly stronger than 8 m.p.h. can be utilized
when the cloud is drifted uphill or when an orchard or other area with
a high canopy is treated. A time of dlay should be selected when there
is a surface inversion of temperature--i. e., when the air temperature
at the ground level is cooler than at a height of 6 feet or more. Sur-
face inversion keeps the aerosol cloud close to the ground and is most
important when low-growing crops are treated and least important when
trees having a canopy of foliage are treated. Good inversion usually
occurs only at night from 1 hour after sunset until sunrise, but occa-
sionally exists all day when the ground has been cooled by rain. In
hilly terrain surface inversions usually occur only in the valleys. If
it is necessary to make treatment in the daytime without the surface
inversion, a wind of 5 to 10 m.p.h. is beneficial.
The-deposit is heaviest nearest the point of release and decreases
as the distance increases, because the larger particles settle out first.
Aerosol particles moving with the wind deposit selectively on exposed
vertical surfaces but settle uniformly on horizontal surfaces. The
amount of deposit on the vertical surface depends on the size and shape
of the object. Under similar conditions the deposit is much greater on
objects of narrow width, such as pine needles, than on ones of greater
width, such as maple leaves. The deposit on vertical surfaces becomes
important when the aerosol is applied in winds stronger than 5 m. p. h.,
and on small tender foliage it is sometimes heavy enough to cause injury.
When a large proportion of the foliage is exposed vertically, as in a
vineyard, the increased deposition is especially important.
Experience has shown that under the best conditions only 25 to 50
percent of an aerosol containing particles of less than 50 microns
mass median diameter deposits in swaths up to 2000 feet, the major
portion drifting beyond the area under treatment.
The deposits of aerosols of different particle size released in a
3-m. p.h. wind under good inversion conditions are given in table 1.
These deposits settled on an open field; the percentage would have
been higher if the aerosol had been released through dense foliage.
The swath width is chosen first by the locations in the crop through
which the machine can be taken with the least damage from the wheels.
The minimum swath width is limited by the particle size requirements.
A narrow swath requires large particles to settle out in the swath.
Large particles sometimes cause foliage injury when oil solutions are
used. The maximum swath width is limited by the dosage requirements.
A wide swath requires a heavy output from the machine. Too heavy an
output, even though the particle size is small, will cause foliage injury
due to the heavy deposit near the source. The smaller particle sizes
are less efficient in depositing within a limited area. For this reason
it is best to select a swath as narrow as possible without causing too
much damage from the wheels of the machine or foliage injury from the
large particles. Some recommended swath widths for use of DDT on
various insects are given in table 2.
It is important to select aerosols of the proper particle size.
The proper particle size depends on the swath width the aerosol is
expected to cover, the wind velocity, and the amount of foliage pene-
After the swath width has been chosen, the particle size that will
give 25 to 50 percent deposition of the aerosol at different velocities
may be obtained from table 3. These values were computed for an
aerosol cloud released at an average height of 10 feet and under
good inversion conditions. If penetration of dense foliage is required,
the particle size indicated should be reduced by one-half.
For example, 3 pounds of DDT in 3 gallons of solution released
with a particle size of 40 microns across a 100-foot front in a 3-m.p. h.
wind will leave a deposit of about 1 pound per acre across a 300-foot
swath. If 1 pound of DDT in 1 gallon of solution is released across a
100-foot front and the particle size is raised to 70 microns, the deposit
will be 1 pound per acre across a 100-foot swath with the same wind.
When the proper particle size has been selected, the aerosol
machine should be set to produce this particle size according to the
directions of the manufacturer.
Sometimes temporary control of flying insects is desired. The
optimum particle size for this type of treatment depends upon the
kind of insect, and not upon the deposit. For adult yellow-fever
mosquitoes the optimum particle size has been found to be about 15
microns, and for house flies about 22 microns.
The dosage depends on the deposit per acre of insecticide required
to control the insect. It is measured by the amount applied per 100
feet of front, and varies with the swath width. Some recommended
dosages of DDT against various insects are given in table 2. These
values are based on the premise that 25 percent of the insecticide
deposits in the first swath. Application to successive swaths results
in overdrift, which increases the deposit by 10 to 20 percent in each
swath. In a large field the wastage due to overdrift is thus reduced to
that from the last few swaths. For swaths less than 300 feet wide the
dosage can be reduced by about 20 percent, and for swaths 300 to 500
feet wide by about 10 percent in each successive swath until the 50-
percent point is reached. This reduced dosage should then be repeated
to the end of the plot.
The amount of insecticide required per 100 feet can be released
by two methods. The total amount required on a front can be measured,
then applied by moving the aerosol generator back and forth across this
front until the entire amount has been exhausted. The second method
is to calibrate the output of the generator and then calculate the proper
speed to move across the front to give the desired dosage. As an
example, of second method only, if it is desired to release 2 gallons
of solution per 100 feet from a generator with an output of 40 gallons
per hour, the output per minute would be 2/3 gallon. Since 1 m.p.h.
is equivalent to 88 feet per minute, the speed of movement would be
100 x 2 =3.4 m.p.h.
To prevent rapid evaporation it is desirable that at lease one-fourth
of the aerosol solution be a nonvolatile liquid. Best results have been
obtained with a very concentrated solution. A much used formula is
5 to 7.5 pounds of DDT dissolved in 2 gallons of benzene or xylene plus
3 gallons of SAE 10W motor oil or an agricultural oil. An agricultural
oil is used where tender foliage is present. The amount of DDT that
can be dissolved in the solvent depends on the temperature. Benzene
is preferable to xylene.
Benzene hexachloride, dinitro-orthocresol, pyrethrum, rotenone,
toxaphene, chlordane, hexaethyl tetraphosphate, tetraethyl pyrophosphate,
and nicotine have been similarly formulated. The last three insecticides
are particularly noxious in aerosol form; an operator should therefore
wear a proper gas mask when releasing them.
Method of Applying Aerosol
The initial impetus of the aerosol cloud, as it is emitted from the
nozzle of the machine, should only place the cloud in the wind, and
not deposit it on the foliage. The initial impetus is usually expended
within 15 feet or less. It is preferable to point the nozzle low and
back of the line of travel. The nozzle should never be pointed at
foliage within the range of the initial impetus, because heavy deposit
might cause burning. For treating low-growing crops the aerosol
should be directed below the top of the surface inversion. When
aerosols are applied in towns, the nozzle should be pointed over the
tops of parked cars.
Table 1. --Effect of particle size on the percent of total insecticide
depositing on 100-foot strips across a field.
release front (feet)
Mass median diameter of particles
75 microns 40 microns 25 microns
Table 2. --Recommended swath width and dosage of DDT for control of insects
in the field with an aerosol under good conditions. 'Numbers in parenthesis
refer to Literature Cited7
A rmyworm 1/
Spotted cucumber beele I/
Japanese beetle (6)
False chinch bugl/
Tarnished plant bug (8)
Mosquito (2) (3)
Sand fly (4)
Horn fly l
Black fly (5)
Potato flea beetle-
Gypsy moth (7)
Adult and nymph
Adult and nymph
Adult and nymph
Adult and nymph
Adult and nymph
Tests with these insects were made by
Tests with these insects were made by
H. A. Jaynes
Table 3. --Optimum particle size, in microns mass median diameter,
of aerosols for application at different swath widths
Wind velocity in miles per hour
1/ Application at higher wind velocity is not recommended.
UNIVERSITY OF FLORIDA
3 1262 09242 9124
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1946. Salt marsh and anopheline mosquito control by ground
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