Directions for applying windborne aerosols for insect control out of doors

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June 1960

DIRECTIONS FOR APPLYING WINDBORNE AEROSOLS

FOR INSECT CONTROL OUT OF DOORS

Agricultural Research Service
UNITED STATES DEPARTMENT OF AGRICULTURE

ARS-33-57

DIRECTIONS FOR APPLYING WINDBORNE AEROSOLS
FOR INSECT CONTROL OUT oF DooRSL

By A. H. Yeomans
Entomology Research Division

Windborne aerosols are those applied as a cloud and carried by the
wind across the area treated. Insect control attained from windborne
aerosols applied to field crops and orchards is due mostly to the
deposit. However, if the aerosol is applied while the insects are in
flight, individuals on the wing are killed as well as those that come
in contact with the deposit.

Machines for applying windborne aerosols are different from those
that project sprays with a powerful air blast. The air-blast machine
can provide a more nearly uniform deposit across a swath with the larger
spray particles than can be obtained with the windborne aerosol machine.

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.

Weather Requirements

Satisfactory movement of the aerosol cloud across the area is
accomplished by making applications under the proper weather conditions.
A light wind is needed, steady in direction, and moving at 1/2 to 8 mp.h.
Winds slightly stronger than 8 mpeh.e 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 day should be selected when there is a surface
inversion of temperature--i.e., when the air temperature is cooler at
the ground level than at a height of 6 feet or more. Surface 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 occasionally exists

1/ This is a revision of ET-282, issued by the former Bureau of
Entomology and Plant Quarantine in May 1950.

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all day when the ground has been cooled by rain. In hilly terrain sur-
face 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 mph. is beneficial.

Deposition

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.pehe, and on small
tender foliage it is sometimes heavy enough to cause injury. When a
large proportion of the foliage is exposed vertically, as ina 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 2,000 feet, the major portion
drifting beyond the area under treatment.

The deposits of aerosols of different particle size released in a
3-m.peh. wind under good inversion conditions are given in table l.
These deposits settled on an open field; the percentage would have been
higher if the aerosol had been released through dense foliage.

Table 1.--Effect of particle size on the percent of total insecticide
depositing on 100-foot strips across a field

Mass median diameter of particles

Distance from release
front (feet)

0-100 25.8 726 1.4
100-200 16.6 7.5 1.4
200-300 aie 6.5 Dolh
300-400 545 lest 1.4
100-500 2.8 2 yA
500-600 ah ee 1.4
600-700 0.9 ie 7 alah
700-800 05 17 1.0
800-900 25 U7 0.7

Be

Swath Width

The swath width is chosen first by the locations in the crop through
which the machine can be taken with the least damage from its 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 particles
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.

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 penetra-
tion required.

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 2. 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.

Table 2.--Optimum particle size, in microns mass median diameter, of aero-
sols applied at different swath widths and wind velocities

; Wind velocity in miles per hour
Swath width (feet)

50 oy) 98 = =
100 40 70 92 -~
200 29 50 65 till
250 26 45 60 70
300 2h 40 53 63
400 21 35 46 oy)
500 18 Bp) 41 49

aly/ Application at higher wind velocity is not recommended.

Behe

For example, 3 pounds of DDT in 3 gallons of solution with a particle
size of 40 microns released across a 100-foot front in a 3-mp.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 the 100-
foot front in the same wind and the particle size is raised to 70 microns,
the same deposit will be obtained across a 100-foot swath.

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
it is about 15 microns, and for house flies about 22 microns.

Dosage

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. 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. Ina 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 a 50-percent reduction 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 applied by
moving the aerosol generator back and forth across this front until the
material is 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 the second method,
if it is desired to release 2 gallons per 100 feet from a generator with
an output of 40 gallons per hour, the output per minute will be 2/3
gallon. Since 1 mp.h. is equivalent to 88 feet per minute, the speed
of movement will be

100 2/352 Oy eDehe
38 x 5 374 Mep

Formulations

To prevent rapid evaporation it is desirable that at least 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 1/2 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

Laie

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.

Lindane, malathion, pyrethrum, rotenone, toxaphene, chlordane, HETP,
TEPP, 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 Application

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 away from
the direction of travel. It 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.