Airplane spraying for forest pest control


Material Information

Airplane spraying for forest pest control
Physical Description:
21 p. : ill. ; 28 cm.
Yuill, Joseph S., 1904-
Eaton, Charles B
Isler, D. A
United States -- Bureau of Entomology and Plant Quarantine
U.S. Dept. of Agriculture, Agricultural Research Administration, Bureau of Entomology and Plant Quarantine
Place of Publication:
Washington, D.C.
Publication Date:


Subjects / Keywords:
Aerial spraying and dusting in agriculture   ( lcsh )
Spraying and dusting in agriculture   ( lcsh )
Forest insects -- Control   ( lcsh )
Pesticides   ( lcsh )
federal government publication   ( marcgt )
bibliography   ( marcgt )
non-fiction   ( marcgt )


Includes bibliographical references.
Statement of Responsibility:
by J.S. Yuill and C.B. Eaton and D.A. Isler.
General Note:
Caption title.
General Note:
General Note:
"August 1951."

Record Information

Source Institution:
University of Florida
Rights Management:
All applicable rights reserved by the source institution and holding location.
Resource Identifier:
aleph - 030337499
oclc - 268908049
System ID:

Full Text
August 1951 E-823

United States Department of Agriculture
Agricultural Research Administration
Bureau of Entomology and Plant Quarantine


By J.S. Yuill and C.B. Eaton, Division of Forest Insect Investigations,
Bureau of Entomology and Plant Quarantine, and D.A. Isler, Division
of Farm Machinery, Bureau of Plant Industry, Soils, and Agricultural

Advantages and limitations of aerial spraying .......... 2
Types of aircraft..................................... 3
Spray equipm ent ..................................... 3
T ank.............................................. 4
P um p.............................................. 5
Pressure regulator, drain, and shut-off valves ...... 6
Atom izing devices.................................. 7
Calibration........................................ 8
Spray m ixtures ...................................... 10
Spraying operations................................... 11
Selecting a base of operations ...................... 11
Subdividing and marking of treatment areas .......... 12
Com m unications .................................. 13
Pattern and altitude of flight........................ 13
Weather conditions that limit spraying .............. 14
Observations during spraying ....................... 15
Evaluating results................................... 15
Precautions in the use of DDT........................ 16
Appendix............................ ................ 17
Specifications for spray materials .................. 17
Estimating deposit and atomization ................. 18

In the short period since the end of World War II airplane spraying
has been developed into an accepted means of combating forest-insect
pests. This rapid expansion has been accompanied by a series of growing
pains. Equipment, spray formulas, and operational procedures have not
been standardized, nor have the limitations of aerial application been fully
evaluated. Consequently, there is at times considerable uncertainty as
to where and how this method can be employed effectively.


The purpose of this article is to summarize current information on
the subject. However, it must be emphasized that--

(1) All statements in this article apply specifically to forest spraying
and may not be applicable to operations for controlling crop pests
or other insects.
(2) These recommendations are not final. Revisions will be necessary
as methods are improved.
(3) No one set of recommendations can be made to apply to all situa-
tions. Details must be altered to meet local conditions.

Advantages and Limitations of Aerial Spraying

Compared to ground equipment, the principal advantages of aerial
spraying for forest-pest control are speed, ability to reach otherwise
inaccessible areas, and economy. By means of aircraft large forested
areas can be treated, and they can be covered rapidly to permit critical
timing of applications. The cost of treatment, including materials, ranges
from about 70 cents to 3 dollars or more per acre, depending on the size
of the area, its location, and the dosage applied. Similar coverage with
ground equipment entails far greater costs.
The principal disadvantage of the method at the present time is that it
does not give so uniform or so thorough coverage, and therefore is not so
effective as ground equipment in the control of certain insects. Further-
more, adverse weather conditions may cause serious interruptions in
spraying operations at times, and its use may be limited by extremely
rough terrain.
In general, although aerial spraying has been most effective in con-
trolling forest insects that actively move about in the tree crowns, such
as the gypsy moth, the tussock moth, sawflies, and spittlebugs, it has
also controlled certain less active ones. For example, outbreaks of the
spruce budworm in Oregon have been controlled successfully, and experi-
mental treatments applied to lighter infestations in the Northeast have given
promising results. For the most part bark beetles and terminal feeders
have not been effectively controlled by this method, although in recent
experiments helicopter spraying appears to have given good protection
from attacks of the white pine weevil.
Aerial spraying operations have been conducted successfully over
terrain ranging from flat pine lands in Arkansas to steep, rough mountain
areas in western New York, northern Idaho, and Oregon. However, it
has been found dangerous to spray in blind canyons, on steep slopes where
down drafts prevail, and in other situations where emergency maneuvers
are restricted. This has been particularly true of operations in the moun-
taini are.i of the West. In those areas unfavorable air currents are more
*eval1n: than elsewhere aind the performance of aircraft is considerably
reduced by the increased altitude.


There are also certain legal limitations and requirements which
should be investigated when an aerial spraying operation is being planned.
For instance, the Civil Aeronautics Administration requires (1) that all
aircraft, and the spray equipment installed in them, be approved for
airworthiness, (2) that pilots operating for hire have a commercial pilot's
license, and (3) that waivers for low flight be obtained in advance. Full
details of these requirements can be obtained from the nearest CAA
regional office. In addition, some States have restrictions on the kind
and quantity of insecticide that can be applied from aircraft, and require
that operators be licensed for this type of work.

Types of Aircraft

No existing model of aircraft is ideal for forest spraying. However,
several have given reasonably satisfactory performance. Most of the
work at present is being done with biplane trainers, Stearman and N3N.
These planes are sturdy and dependable. They operate at about 80 miles
per hour and carry approximately 70 to 100 gallons of spray when equipped
with standard engines of 220 or 235 horsepower. Performance is satis-
factory up to an altitude of about 5,000 feet. With engines developing
300 or 450 horsepower the spray load and performance are greater. Some
of the larger single-engine, high-wing monoplanes, such as the Stinson
SM7A, when provided with adequate power, have been quite satisfactory.
Small two-passanger monoplanes have not been used extensively for this
purpose, because of their limited spray load and range. Multi-engine
transports and converted bombers (Ford Trimotor, DC-3, B-18) have
given good performance for large-scale operations. They carry loads
of 400 to 1,000 gallons, or more, and have been particularly suitable in
treating extensive areas at considerable distances from airfields. They
are not employed, however, where the terrain is extremely rough, and
in all operations they are flown at a greater height above the trees than
are the more maneuverable light planes.
Helicopters have been used to a limited extent in forest spraying.
Best results have been obtained in treating plantations and other small
areas where the high maneuverability and the ability to land and take off
from adjacent open ground have been advantageous. The chief limitations
for this type of aircraft are high initial and operating costs and small
spray loads.
Spray Equipment

At the present time various types of spray equipment are used by
commercial operators. However, not all types are satisfactory for
forest spraying. Satisfactory performance of any given type for crop-
pest or mosquito control, moreover, does not necessarily guarantee its


suitability for forest spraying. In general, equipment should be so con-
structced as to provide the following:

(1) Dispersal of the spray liquid from the plane at a uniform rate.
(2) Proper discharge rate to give the required dosage (gallons per
(3) Adequate agitation in the spray tank (necessary when suspensions
or unstable emulsions are used).
(4) Correct atomization of the spray liquid for its maximum lateral
distribution (swath width) beneath the plane.
(5) Proper location of nozzles or other outlets to avoid excessively
heavy deposits in the center of the spray swath.
(6) Metal parts that are resistant to corrosion.
(7) Hose and flexible connections that are resistant to naphthenic
solvents commonly used in DDT sprays. (Standard aircraft
gasoline-and oil-resistant hose has been satisfactory.)

A relatively simple boom-type sprayer has proved quite satisfactory
on biplanes in forest spraying. A diagram of this apparatus is shown in
figure 1.1-/ With modifications it has also given good performance on
other single- and multi-engine planes.









Figure 1. --Diagramn of spray
ap; ;aratus.

A discussion of the selection
or construction of the component
parts of an aerial spray system,
its installation, and calibration


The spray tank is usually
mounted in the front cockpit in
biplanes, behind the pilot's seat
in light cabin-type planes, and in
the cargo space of larger ships.
Its capacity depends on the space
available and the load permitted
by CAA regulations for each plane.
If space permits, it should have
a sloping bottom to provide positive
drainage of the liquid and to facil-
itate cleaning when changing from
one type of spray mixture to
another. A removable screen

I/ For detailed drawings see Aerial-Spray Equipment for a Stearman

N23 Airplane, by D. A. Isler,
Enrin., Inform. Ser. No. 87,

U.S. Bur. Plant Indus., Soils, and Agr.
13 pp. 1948.


strainer, not less than 40 mesh, should be fitted into the tank filler
neck to remove any sediment in the spray liquid. Thereshouldbe a dump
valve to permit the pilot to reduce the spray load quickly in case of a
partial power failure or other emergency.
The tank must also be provided with an air vent to prevent the devel-
opment of negative pressure. Such a vent should be attached to the top
of the tank or the filler neck. The vent opening should be outside the
cockpit or cabin so that fumes or spray liquid cannot reach the pilot.
The size of the vent required will depend on the discharge rate of the spray
apparatus. On planes equipped with dump valves it must be large enough
to permit free discharge of the anticipated flow rate from the valve.
For planes discharging up to 25 g.p.m. (gallons per minute) the vent
should be at least 3/4 inch i.d. (inside diameter), and for 50 g.p.m. the
vent should be at least 1 inch i.d. For higher discharge rates the internal
area of the vent should be increased in direct'proportion to the increase
in discharge rate.


Discharge of the spray at a uniform rate is essential. In general
systems in which pressure is maintained by means of a pump have given
the most satisfactory results. Gravity-flow systems do not discharge
uniformly unless some means of compensating for the decreasing static
head of liquid as the tank empties is provided.
Positive-displacement (gear and other rotary) and centrifugal pumps
are used -n spray planes. The gear and other rotary purmps are suitable
for spraying solutions or emulsions. However, they are subject to exces-
sive wear if used for spraying suspensions or any liquid containing abrasive
material, or-if allowed to rotate after the tank is empty. With any pump
of this type there must be a bypass line or some other means of relieving
the excessive pressure that builds up in the liquid lines when the control
valve is closed. Centrifugal pumps, on the other hand, are satisf-ctory
for all three types of spray--solutions, emulsions, and suspensions--
and they develop adequate but not excessive pressure.
The size of pump to be selected depends on the capacity required of
it in relation to the swath width, plane speed, and application rate. The
capacity should be about 20 percent greater than the maximum flow rate
required, to insure adequate agitation and to provide a factor of safety
to compensate for losses due to friction and decreased efficiency due to
wear of the pump. For biplane trainers the 1-inch gear and centrifugal
pumps are adequate. For estimating pump capacities for other planes
see the section on calibration, p. 8.
On light planes to insure positive flow of liquid from the tank to the
pump, particularly when a centrifugal pump is used, the pump is
generally mounted on the landing-gear members, or in some other con-
venient location below the level of the bottom of the tank. When so


mounted it is driven by a small wood propeller or a truck radiator fan
fitted on an extension of the pump shaft. If a metal fan is used the hub
or spider should be reinforced with aircraft steel and high-shear rivets
to prevent failure due to fatigue or shearing of rivets. If the pump is
not made with an outboard radial and thrust bearing, one must be pro-
vided to prevent excessive wear of the other pump parts. A small drum
and brake band, with a control to the cockpit, may be installed so that
the pump can be stopped when the plane is not spraying. In some of the
light planes the pump is driven by power take-off from the airplane
engine, or by either an electric or hydraulic motor. In larger planes
one or more pumps may be used. They are ordinarily mounted in the
cargo space and are driven either by separate gasoline engines or by
an electric or hydraulic motor.
Whatever type of pump installation is used, it is essential that all
fittings, valves, and liquid lines be large enough to permit unrestricted
flow of the spray from the tank through the pump to the nozzles.
Otherwise the delivery of spray from all outlets will not be uniform
and the proper atomization will not be obtained. In the biplane trainers
1 1/4-inch lines are preferable; no lines should be less than 1 inch in
diameter. In larger planes, where higher flow rates are required, the
size of lines should be increased accordingly.

Pressure Regulator, Drain, and Shut-off Valves

Pressure regulators are not found on all spray planes. Their use is
recommended, however, for the following reasons: (1) They prevent
excessive pressure in the liquid lines when positive displacement pumps
are used, (2) they insure a uniform flow rate at all times by holding a
constant pressure, (3) within limits, the flow can be changed as needed,
without altering the degree of atomization, by increasing or decreasing
the number of nozzles and resetting the regulator to the original pressure,
and (4) the liquid returned to the tank through the bypass line provides
some agitation to the liquid in the tank. For effective agitation the
bypass line should discharge near the bottom of the tank rather than at
the top. This arrangement will also reduce foaming when emulsions or
suspensions are being applied. The pressure regulators used on N3N and
Stearman planes usually are of the standard hydraulic-relief valve type.
They can be purchased pre-set for various pressures and can be adjusted
through a range of about 20 percent above or below the normal pressure.
Some models are rather heavy, but the weight can be reduced by turning
down the valve body in a lathe. Diaphragm-type pressure regulators
and flow-regulating gate valves also may be used to adjust the spray
pressure, provided a relief valve is installed to prevent an excessive
pressure on the inlet side of the regulator. Regardless of the type of
regulator, it should be installed near the pilot so that he can adjust the


pressure during flight. A pressure gage should be mounted in the cockpit
so that the pilot can see that the proper pressure is being maintained.
Drain valves are desirable but not essential. They are particularly
convenient when it is necessary to flush out the tank and pump or to
determine the flow rate.
Shut-off valves usually are of the cam-action, quick-opening gate
types. Such valves have straight-through, full-area openings. The
opening should be of the same size as the pump discharge line in order
to permit unrestricted liquid flow. Sliding gate valves and three-way
valves are also used in some installations, and solenoid-operated valves
on some planes that have suitable electrical systems. Globe-type valves
are not recommended.

Atomizing Devices

The types of atomizing devices used on spray planes are too numerous
to describe here. For general forest spraying nozzles in combination
with the pressure system described in the preceding section have been
the most satisfactory. They have the following advantages over many
other devices:

(1) The flow rate can be changed without affecting the atomization
by merely adding or removing nozzles. (In adding nozzles the
total output must not exceed the capacity of the pump.)
(2) For each condition of flow rate and atomization the location of
the outlets can be adjusted to obtain the maximum swath and
to reduce excessive deposit within the swath.
(3) They are comparatively cheap.

Nozzles delivering a hollow-cone or flat spray, and made without cores,
vanes, or disks, produce a narrow range of atomization (fewer excessively
large or exceedingly small drops) for a given capacity and have less
tendency to clog. Nozzles of this type with 1/8-inch-diameter orifice and
a rated output for water of 0.8 g.p.m. per nozzle at 25 p.s.i. (pounds per
square inch) have given good performance on biplanes. The orifice can
be directed to the rear to produce a moderately coarse spray or forward
to obtain a medium spray.
For simplicity of installation the nozzles are usually mounted on a
tubular boom, hung on brackets beneath the wing (beneath the lower wing
on biplanes); but the boom may be installed inside the wing and the
nozzles attached to pipe nipples extending beneath the wing surface.
Biplanes equipped with a cluster of nozzles beneath and inboard from
the wing tips and a cluster at the tail section have been used for some
specialized applications. These internal boom installations reduce drag
considerably, but are not so readily adaptable to the varying needs of
most commercial operators as are the external-boom installations.


-0';a* s of wh theirr anr inPternal or external boom is used, slightly over
,*If bt. n t ,ver two- h rds, of the nozzles Thould be evenly spaced in
*' .utr half ofthe w 2 s ;an tc provide for maximum lateral distri-
[ution (f the s 'ray.
There are several devices for preventing the dribbling of spray liquid
from the nozzles alter h s ray valve has been shut off. A shut-off
valve at each nozzle or gro ji of nozzles is probably the most effective,
but a check valve at each nozzle is the most common. Special selector
valves which vent the boomn back to the tank or permit application of
negative pressure to the boom are also used.


The importance of proper performance in spray equipment cannot
b overemphasized. Unfortunately the methods required for precise
evaluations of equipment are too involved for use in the field. However,
the tests described below will give a rough estimate of performance and
and should bring out any serious inadequacies.
The flow rate, or output, can be determined as follows: (1) Put a
measured amount of spray liquid in the tank, (2) have the pilot turn the
spray on for a timed interval (30 or 60 seconds) while in straight and
level flight at the air speed to be used in the spraying operation, (3) when
the plane lands, drain and measure the liquid remaining in the tank, and
(4) compute the flow rate in gallons per minute. An alternate method
is to (1) fill the tank to a definite level such as a specific point in the
filler neck, (2) fly the plane as described above, (3) after landing, spot
the plane in exactl-i the sp,..e location used when filling the tank and
measure the amount required to refill to exactly the same level, and
(4) conipute the flow rate as above. Two or three replicate flights should
be made, and the resul.- for any one spray mixture should not vary by
more than 3 percent. Fuel oil alone can be used for determining the flow
rates of DDT-fuel oil solutions. For emulsions and suspensions the
mixed sprays should be used. The flow rate required can be computed
by the formula
when F = flow rate (output) in gallon per minute
S = sp..d of the plane in miles per hour
W -width of effective swath (not total swath) in feet
D = dosage to be applied in gallons of liquid spray
per acre

The swath width of planes equipped with a full-wing-span boom can
Se-ti,,ated roughly by mrultiplying the wing s-an by 4. However, this
s only an approximation. It 'p.-i s only to planes in which spray is
leas "d over the full wing span, and from a height above the tr.ccs
-:eater than the 'viig span.


More reliable estimates can be obtained from actual test flights
over open ground. These tests should be made just after dawn in winds
of not over 4 miles per hour. Glass plates 2 to 4 inches square are
used to collect the spray deposits. A day or so before the tests they
are cleaned and then coated with a thin film of zinc stearate to prevent
the spray drops from spreading into a film on striking the surface. Zinc
stearate can be obtained from any drugstore or chemical supply firm.
A small amount of this material is placed on each plate (about the size
of a match head for a plate 2 inches square). It is then melted by warm-
ing over a hot plate, and spread evenly with the finger. Paper or uncoated
glass cannot be used, since the drops spread excessively and unevenly on
these materials. The plates are placed on wood blocks or other supports
to keep them above grass or other vegetation, at 10-foot intervals on a
line 400 to 1,000 feet long, and at right angles to the wind direction.
With fuel oil as the spray (water or emulsions evaporate too rapidly) the
plane is flown directly into the wind and over the center of the line of
plates. The flight altitude should be the same as the height above the
trees and the air speed of the plane should be the same as that at which
control applications will be made.
In tests of this kind the spray should be turned on when the plane is
about 500 feet downwind from the test line and left on as the plane
continues in straight and level flight for at least 1,000 feet upwind from
the test line. The plates are left in place for about 5 minutes after the
plane passes to allow the spray cloud to settle to the ground. Then a
visual estimate of the quantity and size of spray drops is made by
comparing them with photographs of known sizes and quantities, of drops.
(See Appendix, p. 17.) The estimates should be made within 20 minutes after
the spray flight; otherwise there may be considerable evaporation of the
deposited drops. Test flights should be repeated once or twice to avoid
erroneous estimates that may result from drifting of the spray during the
time the cloud is settling.
When the plates are examined, it will be found that the quantity of
spray deposited decreases from the center to the margins of the swath.
For a Stearman plane equipped with a boom-type sprayer adjusted for a
calculated discharge rate of 1 gallon per acre, the rate of deposit on the
plates should not exceed 1 gallon and not be less than 1/2 gallon per acre
at the center of the swath. At a distance of 60 feet from the center, the
rate of deposit should not be less than 1/4 gallon per acre, and at 100 feet
not less than 1/10 gallon. Where larger planes with similar equipment
are used, deposits in the center of the swath should be approximately the
same as for a Stearman, but the total swath should be greater depending
on the size of the plane. For control of most defoliators the effective
swath may be considered as the distance over which the rate of the deposit
is not less than 1/4 gallon per acre. The effective swath for a Stearman
with boom sprayer should be about 2 chains (132 feet) wide.

- 10 -

It has been found that the heaviest deposits (near the center of the
swath) usually are made up of drops having a wide range in size. Moving
oitward from th center the range becomes narrower until at the margins
there is only a sparse scattering of very small drops. Since the center
portion contains the full range of drops from the smallest to the largest,
only that portion need be examined in making estimates of drop size.
The largest drops should be at least 300 but not greater than 600 microns
in diameter (1 micron = 0.001 mm. or about 0.0004 inch).

Spray Mixtures

Of the many types of insecticidal spray materials available on the
market today, only oil solutions of technical DDT have been used commonly
for forest spraying by airplanes. DDT emulsions have been used, but
only to a limited extent. Suspensions of DDT, either in the wettable-
powder form or as colloidal dispersions, likewise have not been commonly
used. The value of some of the newer insecticides for this type of spray-
ing has not yet been adequately determined. Research is being continued
in an effort to find the most effective and economical chemical and formu-
lation for these purposes. The present recommendations are offered
merely as being the most satisfactory ones yet tested.
In the preparation of oil solutions of DDT the usual procedure is first
to dissolve the DDT in a naphthenic solvent and then dilute this mixture
to the required volume with No. 2 fuel oil. In warm weather 1 quart of
the solvent per pound of DDT should be adequate, but if the spray is to be
exposed to subfreezing temperatures the amount of solvent should be
increased to 1 1/4 or 1 1/2 quarts per pound.
Around reservoirs or other places where this solution might impart
an objectionable taste to domestic water supplies, xylene and kerosene
can be substituted for the naphthenic solvent and the fuel oil. However,
except in just such special cases, xylene and others of the lighter solvents
are not recommended. They do not retain the DDT in solution at low
temperatures, and tend to evaporate rapidly in the air; furthermore,
low flash-point products present a possible fire hazard. In large-scale
operations these DDT solutions may be purchased, ready for use,
accordirn.g to specifications.2/
In small operations it may be more economical to purchase con; nmercial
brands of solvent concentrates, or even to prepare the spray on the job.
Commercial concentrates should contain about 30 percent of DDT and
should be made with a suitable solvent. These concentrates are diluted
with a fuel oil prior to application. Where the spray is to be prepared

2/ A general guide for use in preparing specifications is presented
in the Appendix.


on the job, drum lots can be mixed with the apparatus shown in figure 2.
It is prepared as follows: Warm the solvent to about 70 F. by placing
the container in the sun, or by means of a hot-water coil, and then pour
it into the drum. CAUTION: Solvents are inflammable. Use proper
precautions. Place the DDT in the wire basket, add the solvent, and
circulate it until the DDT is dissolved. Add fuel oil until the required
volume of finished spray is obtained and then circulate the liquid again
for thorough mixing. Where it is desirable to mix larger batches, an
orchard-type power sprayer with a steel tank can be used in the same
manner. Running the agitators during mixing will speed the operation.
Oil solutions are usually VALVE
applied at the rate of 1 pound
of DDT in 1 gallon of solution ROMSAT
oa osQvr aAROMATI
per acre. Against very active RESISTANT
defoliators in the outer and _--WIRE BASKET HOSE
upper portions of the tree
crown, however, 1/2 pound
of DDT in 1 gallon has given
good results. Although there ,
has been some spot burning \ P
on broadleaved trees, and OPEN END DRUM PUMP
occasionally some dropping
of old needles on pine, fir, Figure 2. --Diagram of apparatus for
and spruce at these dosages, mixing small quantities of spray.
and spruce at these dosages,
in no case has there been serious injury.

Spraying Operations

When an extensive area of several thousand acres is to be treated,
the application of spray usually is contracted to the commercial operator
offering the lowest bid for the work. For a smaller area it may be more
satisfactory to employ a reliable local operator without the formality of
a contract. In either case, however, the success of the operation depends
largely on careful planning. In forest spraying there are innumerable
details that vary with different conditions, all of which must be anticipated
for efficient execution of the program. A few of the more important ones
are described below; others will become apparent as the work is planned.

Selecting a Base of Operations

In selecting an airfield or landing strip for forest spraying, one must
consider the distance to the area to be treated, the length and condition
of runways, and the servicing facilities.
The maximum practical ferry distance between the landing strip and
the area to be treated will depend on the operating range of the aircraft.
However, regardless of the type of plane the shorter the distance the better.


Much valuable time can be lost and the cost of the operation increased
by long ferry trips. In some cases it may even prove practical to
construct temporary landing strips within or near such areas to reduce
ferry time.
The length of runways required will depend on the type of plane to
be used. For the light biplanes the minimum length is approximately
800 feet. In the higher altitudes, during hot, humid weather or on a
soft surface, however, longer runways are necessary. Runway surfaces
should be smooth enough to permit driving an automobile over them at
40 miles per hour.
Facilities for rapidly servicing the planes should be provided at the
airfield. If not already available, aviation gasoline of the proper octane
rating should be brought in. It may be dispensed from drums or a tank
mounted on a truck. A suitable hand-operated or small engine-driven
pump will greatly simplify loading the gasoline in the airplane. When a
gasoline engine-driven pump is used, however, care must be taken never
to permit the exhaust to be directed toward open drums. A fire extin-
guisher should be handy at all times. If gasoline drums or tanks contain
sediment or water, the gasoline can be strained through a chamois-lined
funnel while it is being pumped into the plane. The spray liquid may
also be carried in drums or tanks on a truck and pumped directly into
the plane. All pump parts and hose should be oil-resistant, because DDT
solvents are destructive to natural rubber.

Subdividing and Marking of Treatment Areas

In large forested areas the portion to be sprayed is usually divided
into units that can be treated in 1 to 3 days. If the area to be treated is
broken by uninfested forest types or cultivated areas, it is divided into
still smaller units. Sometimes these units can be laid out in rectangular
shape. However, it is generally more practical to use ridges, roads,
and other features of the terrain as the unit boundaries. Large-scale
topographic maps, aerial photos, and aerial mosaics are particularly
helpful in this connection. If possible the pilot or chief pilot in charge
of flying operations should be consulted when the treating units are
marked out on the maps.
On many spraying operations markers are used to aid the pilot in
locating the various treating units and in maintaining an accurate flight
pattern. These markers may consist of white or orange flags, small
wind socks, or light-colored feed sacks stuffed with brush; and they
may be placed in the tops of trees or raised on sectional magnesium
poles. In stands of relatively short timber the usual practice has been
to climb the trees and rope or wire the markers in place. \ recent
interesting innovation in the West, where the height of the trees made
climbing impractical, was the employment of a line-throwing shoulder
gun to pass a light cord over the top of a tall tree to be used as a marker.
A flag or paint bomb was then hauled up on the cord to mark the location.


On large units the markers are spaced at predetermined intervals
along two opposite sides of a unit, and the pilot is instructed to apply the
required number of swaths between them. Where roads or trails occur
along one or two sides of a unit, a small captive meteorological balloon
is used to mark each spray flight line. Here, the balloon, inflated with
helium or hydrogen and attached to a light cord, is allowed to rise about
40 feet above the canopy. The pilot flies directly over the balloon, which
is then moved one swath width along the boundary and another spray run
is made over it. This procedure minimizes application errors, but can
be employed only where continuous openings in the canopy allow the
ground crew to move the balloon quickly along the boundary. For spraying
plantations colored cloth panels on bamboo poles can be substituted for
the balloons. Smoke flares have also been tried, but they are not
recommended where they might create fire hazards.


Whenever possible some means of communication between the air-
field, the treating area, and the pilot should be provided during the
spraying. This should be done because, even with the most careful
planning, unexpected situations that require changes in procedures
always arise during actual spray operations. A radio telephone is
probably the most satisfactory solution of this problem. Although most
spray planes do not have radio equipment, with a proper ground set at
the treating area and another at the airfield, information or instructions
can be relayed as necessary. When these ground sets are not available,
local telephone service may be such that a field telephone can be installed
in or near the treating area and at the airfield.
If neither radio or telephone is available, a system of signals may be
used. One such signal system that has been satisfactory is to place a
truck in a open spot visible from the air, and adjacent to or within the
area being treated. When a white cloth panel is placed on the truck cab,
the pilot will apply the spray as previously planned. When an orange
panel is displayed, he will return to the airfield for instructions. If
neither panel is shown, he will circle the area until one or the other is
displayed. This system can be varied at will, but to avoid confusion
the number of signals should be kept at a minimum.
On certain large spray operations in the West it has not been possible
to have ground personnel in each unit or block being treated. Under such
conditions there has been little need for ground-to-air communication.
However, even then telephone or radio communications between head-
quarters and outlying landing strips have been helpful.

Pattern and Altitude of Flight

The flight pattern to employ in applying the spray is governed primarily
by the shape and topography of the unit being treated. Where the unit is
approximately rectangular and the topography is flat, the grid type of


pattern usually is the most satisfactory. This pattern requires the pilot
to fly in parallel lines back and forth across the unit from one side to the
other, the distance between the flight lines being the same as the effective
swath width of the spray plane.
Where the spray unit is irregular in shape and the terrain is steep,
flights should be made along the contours or down slope. It is not safe
to fly up slope, especially with a heavily loaded plane at high altitudes.
It is ver'" difficult to obtain a uniform coverage of forests in such areas.
S'.ances for error in spacing the flight lines are greater and, since the
irectio of the lines changes with the topography, it is possible that the
pilot will ,-ave some spots untreated while giving others a double treat-
mcnt. The chance of this happening may be so strong in some cases that
it wil be advisable to increase the rate of application by reducing the
distance between flight lines.
With light planes it is desirable to apply the spray from an altitude of
.tproximately 50 feet above the treetops in order to obtain the maximum
swath width without excessive loss of spray by drift. For safety, however,
the altitude must be increased when there are obstructions such as snags,
where the terrain is rough, or when larger planes are used. With fixed-
wing aircraft spraying should never be done at less than 50 feet above the
treetops. Generally the altitude and other flight procedures should be
determined by a pilot, or chief pilot, who is well experienced in forest
spraying, in consultation with the supervisor of the control operation.
A qualified pilot will know the performance characteristics of his plane
and the limitations imposed by topography, elevation, and associated air

Weather Conditions that Limit Spraying

One of the greatest limitations in airplane spraying is the weather--
specifically, air movement and rain. Wind may cause excessive drift of
the falling spray, and convection currents (thermals) may prevent it from
desceidri, into the trees. Forest spraying, therefore, should not be
..te,,.ted when the wind velocity above the trees exceeds 8 miles per hour,
r when there is enough convection to make the air bumpy. Air movement
is usually at the minimum about dawn. On good days spraying can be
star1 ted as soon as there is sufficient light and continued for 3 to 4 hours.
In some: localities a short period in the evening may also be favorable.
S c prebs.nce of water on foliage may reduce the effectiveness of
ra A moderate amount of moisture is not serious, but when it
in,- fom the leaves an appreciable amount of the falling spray may be
i n d o Similarly, when a heavy rain falls before the spray deposit
hals dIried on the folizgt', enough insecticide may be lost to necessitate
resprayin Spray deposits usually dry in 1/2 to 4 hours, depending on the
temperature. Deposits of DDT-oil sprays, once they have thoroughly
dried on the foliage, will remain effective for 1 to 2 weeks under average

- 15-

Observations during Spraying

While the spray is being applied, one or more observers should be
stationed at vantage points in or near each unit being treated. It should
be their job to see (1) that weather conditions within the area are satis-
factory for spraying and (2) that the pilots are maintaining the proper
flight pattern and altitude. When they find that the wind velocity (preferably
measured by field anemometers) exceeds the maximum allowable for
adequate control, or that the spray is not descending into the trees
properly, they should either inform the man in charge of the operation or
signal the pilot to stop spraying.
If possible additional observers should be present in each unit to check on
the spray coverage. They should place clean glass plates (each about 4 by 4
inches), preferably in openings, at 50 to 300 foot intervals across the
unit, or in as many parts of the unit as possible. They should examine
the plates after the morning spraying. If the application has been uniform,
they will find that all plates will carry at least a light deposit of small
drops. The number of plates to use will vary with the density of the forest
canopy. Where solvent-fuel oil solutions of DDT are applied, a very thin
oily film or sheen will be visible on the foliage. This too, may be used
as an index of coverage, for if most of the leaves on the lower branches
and on the understory plants have a visible film or spots of the spray, it
is reasonably certain that the upper tree crowns have been adequately
treated. The observers should locate all skips or misses in the deposit
and spot them on a map for retreatment.

Evaluating Results

The success of any spray job is measured by how well it controls the
pest being treated. Determining the degree of control is therefore an
essential part of the job. Even under what appear to be excellent con-
ditions of weather and application, there is always the possibility of
something going wrong so that the final results will not be satisfactory.
Therefore, a preliminary estimate should be made soon after the appli-
cation in order that any parts of the area in which control is inadequate
can be re-treated immediately.
Usually the effect of DDT spray on insects will be apparent in 24 to
48 hours, although in cool weather it may be delayed 3 to 4 days. At the
proper time trees should be examined at random points throughout the
area to estimate the proportion of the pests killed. It is sometimes
helpful to place cloth trays beneath trees at various locations, to catch
the poisoned insects that drop or spin down on threads. If the spraying
has been successful, normal survivors will be hard to find and at least
95 percent of the pests will have been eliminated.

- 16 -

The preliminary estimates alone, particularly in the case of defoli-
ating insects, will often show without question that adequate control has
been obtained; in which case no further inspection will be needed. If
doubt exists, a final survey may be necessary. The extent of the survey
arid the methods to be employed will be governed by the particular insect
and the forest conditions. The details should be worked out with the help
of an entomologist or forester who is acquainted with the insect problem
in question.


DDT is highly poisonous to many insects, including the beneficial
ones. It is also poisonous to other forms of animal life and to man.
Common-sense precautions should be observed in handling and applying
DDT sprays. When planning a large-scale operation, consult the Federal
Fish and Wildlife Service, Public Health Service, Department of Agri-
culture, or your State agricultural experiment station. Some of the more
important precautions are listed below.

(1) When handling spray avoid spilling it on the skin. Don't
wear spray-soaked clothing.
(2) Bathe and change clothing after each day's work.
(3) Don't leave spray or DDT where unauthorized persons or
domestic animals can have access to them.
(4) Where cultivated fields, orchards, or pastures are adjacent
to infested forest, the pilot should make proper allowance
for wind to avoid spray drift onto agricultural land.
(5) Notify beekeepers in advance when sprays are to be applied
in areas where their bees may be working, so the hives may
be closed during application. Don't spray near hives.
(6) Whenever possible avoid spraying lakes and streams. Appli-
cation rates above 1 pound of DDT per acre kill some fish.
(7) Don't overdose on the theory that if enough is good more is
better. Application rates above 2 pounds of DDT per acre
have been injurious to nestling birds; higher rates may be
harmful to other wildlife.



Specificiations for Spray Materials-3/

These specifications and requirements for the purchase of spray
materials are intended primarily as a general guide. For any specific
spray operation some changes may be dictated by the quantities required,
availability of materials, and transportation costs.

(1) DDT, technical grade.

Setting point, 192.2 F. minimum.
Organic chlorine, 48 to 51 percent by weight.
Ash content, 0.5 percent by weight, maximum.
Chloral hydrate, 0.025 percent by weight, maximum.
pH by extraction, 5.0 to 8.0.
Water-soluble material, 0.25 percent by weight, maximum.

Methods for determining these characteristics are given in Federal
Specification O-D-370, May 13, 1949. The material should be a fine to
medium granular powder with a white to cream color.

(2) Auxiliary solvent for DDT.

The auxiliary solvent shall consist principally of hydrocarbons and
shall have the following physical characteristics:

Flash point (Cleveland open cup) not less than 160 F.
Distillation range:
Initial boiling point not less than 360 F.
Final boiling point not greater than 750 F.
Viscosity (Saybolt Universal) at 100 F., 30 to 55 seconds.
DDT solubility, at 32 F., not less than 30 percent by weight.
Relatively nontoxic to hardwood and coniferous foliage when
the spray is applied at the rate of 1 to 2 gallons per acre.
At present there is no standard laboratory test for this

(3) DDT solution--finished spray.

The DDT solution shall be composed of 1 pound of technical DDT in
sufficient mixed solvent to make 1 gallon. The mixed solvent shall con-
sist of No. 2 fuel oil and sufficient auxiliary hydrocarbon solvent, meeting
requirements under (2) above, to prevent DDT crystallization at 32 F.

3/ Prepared by R. H. Nagel, Division of Forest Insect Investigations,
and E. E. Fleck, Division of Insecticide Investigations.

- 18-

On occasion low-priced hydrocarbon byproducts which are good
solvents for DDT appear on the market. If such a product meets the
specifications set up for auxiliary DDT solvents under (2), and prevents
DDT crystallization at 32 F., it may be used without fuel oil dilution
and may result in an appreciable saving in cost. The insecticide solution
shall be clear, homogeneous, and free from particles of undissolved DDT
or foreign matter. Containers shall be free of scale or other foreign

(4) DDT solution--concentrate.

The DDT-solution concentrate shall contain at least 2 1/2 pounds of
technical DDT per gallon of a concentrate made with a solvent meeting
the requirements under (2).

(5) Contract purchase of spray.

For large-scale operations it is customary to purchase the spray,
either in finished form or as a concentrate, by a bid contract. The
following terms are usually included in the contract:

Specifications for the finished spray or the concentrate are
Each bidder is required to furnish samples of the materials
that he proposes to use.
Each bidder must submit data on his plant capacity and other
information as evidence that he can insure delivery within
the time limits specified.
The approximate amount of spray required is given, and the
provisions are made that (a) within limits, additional
quantities may be purchased at the same rate and (b) the
purchaser may terminate the contract at any time should
insect diseases or other circumstances make it necessary
to reduce the area to be treated.

Estimating Deposit and Atomization

Sometime in the course of nearly every forest spraying operation, it
is necessary to estimate the quantity of spray deposited and the size of
the spray drops, either to evaluate the performance of spray equipment
or to determine whether adequate coverage has been obtained. In either
case actual measurement of deposit or atomization is impractical because
of the timc and special equipment required. On the other hand, a visual
, estimate of quantity of spray or drop size based only on the observer's
judgment is almost valueless, because even among the most experienced
people errors may be as preat as 300 percent. Figure 3 shows samples

- 19-

of the spray deposit as it appears to the unaided eye when collected on
glass plates coated with zinc stearate as described on page 9. It will be
seen (1) that these deposits vary in density and are composed of drops
having a wide range in size, and (2) that the range in size of the drops
and the quantity of spray deposit decrease with the distance outward
from the flight line. Since it is impractical to prepare standards for
illustrating all the combinations of drop size and deposit rates that occur,
photographs of known quantities of deposits and of known drop sizes have
been prepared (fig. 4).
These photographs show uniform drops as they appear to the unaided
eye when deposited on glass plates coated with zinc stearate. The range
of size given--50, 100, 200, and 500 microns in diameter--includes nearly
all the drops that reach the ground from a spray plane. For each size
there is also shown the density of drops required to give definite quantities
of deposit normally encountered. Although these standards bear little
resemblance to actual spray deposits, reasonably satisfactory estimates
of the drop size and the quantity of spray can be made by using the stand-
ards as a basis for comparison with actual spray deposits collected in
the field.
The drop size is determined by comparing the diameter of the two
or three largest drops in the deposit sample with the drops in the standard
photographs (fig. 4). For example, in the spray sample shown in figure 3,
A, the largest drops are considerably larger than the 200-micron stand-
ards but not so large as those in the 500-micron standards; the estimate
would therefore be about 400 microns.
Ihe quantity of spray deposit in the sample can be estimated by the
following procedure:

(1) Estimate the average size of the larger, more prominent drops.
Those in figure 3, A, would be about 300 microns.
(2) Compare the density or number of these drops with the density
of similar drops in the standards. The larger drops in
figure 3, A, are somewhat more numerous than those in the
500-micron standard at 3/4 gallon per acre, but less numerous
than the 200-micron drops at 3/4 gallon per acre. Since the
size and the number of the larger drops in the sample lie
between the two standards, the estimated quantity would be
about 3/4 gallon per acre.
(3) Now in the same manner as for(l), estimate the average size of
the smaller drops in the sample. In figure 3, A, this would be
approximately 100 microns.
(4) Next estimate the density of the smaller drops, as in (2). In
figure 3, A, they compare with the standard showing 100-micron
drops at 1/4 gallon per acre.


(5) The estimate of the total deposit in the sample will then be the
sum of the two previous estimates. In the example this is
3/4 plus 1/4 gallon per acre, a total of 1 gallon per acre.

At first a comparison of deposits containing a range in drop sizes may
seem rather difficult, but with a little practice there should be few
extreme errors. It should be remembered, however, that this method
is suited only for rough and rapid determinations. Although the results
are much more reliable than estimates made by even the most
experienced observers without the aid of reference standards, this
method will not give the accuracy of the more complicated laboratory


*0 *



Figure 3. --Example of aerial spray-deposit pattern: A, Center of swath,
deposit rate of 1 gallon per acre, spray drops up to 400 microns;
B, 50 feet from center, deposit rate of 0.5 gallon per acre, spray
drops up to 300 microns; C, 100 feet from center, deposit rate of
0.2 gallon per acre, spray drops up to 150 microns; D,150 feet from
center, deposit rate of 0.05 gallon per acre, spray drops up to 75 microns.

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