S. F. POTTS
Formerly Entomologist, Forest Service, United States Department of Agriculture
The application of pesticidal dusts, dilute sprays, concentrated sprays and aerosols, for the control of pest and diseases in agriculture and forestry, has become widespread. The method has not approached its potential of use and many areas could be treated more efficiently and effectively, and at a lower cost, if better data and information were readily available. This paper, continued from the last issue of Unasylva, describes present practices in North America and, besides dealing with insecticides and fungicides, includes information on equipment and methods for applying the new herbicides, silvicides, foliage nutrients and fertilizers. The possible harmful effects of the extended me of chemical control methods in agriculture and forestry are of much concern to FAO and many other agencies. These effects are being studied but much more data needs to be collected.
Almost everything mechanical that will fly - even airships - has been tried for crop spraying. Pest-control jobs - some spraying, some dusting - keep more than 6,000 planes and 7,000 pilots busy in the United States.
If you have a choice as to the kind of craft to hire, the main thing to consider is the job you want done. Points to think about include:
How big are your forests and fields?
How close are they to landing strips ?
What kind of country will the plane have to fly over ?
Can the plane carry a big enough load to hold down costs ?
Is it fitted with the proper spraying equipment?
In general, four types of planes are at work on forest and crop-spraying projects. All have their advantages and disadvantages. A good many of those in use today are converted surplus military training planes, which are cheap and easy to handle.
Light planes are well suited to most jobs in farming country. They can be bought and operated at relatively low cost, they maneuver well in tight quarters, and, in an emergency, they can be landed in pastures and on roads.
Spraying rangeland or a forest usually involves greater ferrying distances and larger acreages than spraying crops. For such a job, planes such as the twin-engine Douglas DS-3 have distinct advantages over light planes - longer range and bigger payload.
Helicopters also have a place in the spraying picture. They are more expensive than light, fixed-wing aircraft, but sometimes they can make up for it by their ability to take off and land straight up and down, without a runway. This saves mileage and time, and makes it easier to handle small, hemmed-in fields. Other advantages are that poor flying visibility handicaps helicopters less than it does fixed-wing aircraft, and the downwash from the rotors helps to push the spray deep into the crop.
No matter what plane you choose, it will carry spraying equipment that belongs to one of two general types. In one type the equipment includes pumps that discharge the pesticide from the plane. The other type lets gravity do the work, the way a sprinkling can does.
Most experts think that pumps do the better job. Without some pressure behind the spray, it is hard to get an even, controlled flow and good atomization.
There is a bewildering variety of spraying equipment. Dozens of different systems and devices have been developed, each with its own strong points and weaknesses. When making a choice, it is a good idea to keep in mind this basic question: Is this the equipment that will get the pesticide onto my crop in the most efficient way?
In general, good equipment should -
1. spray out the pesticide from the plane at a uniform rate;2. provide for an adjustable rate of discharge, so that crops get the right number of gallons of spray per acre;
3. spread the liquid in as wide a swath as possible beneath the plane;
4. avoid putting down too heavy a deposit in the center or at the edges of the spray swath.
A spray outfit ordinarily has -
1. a tank, to hold the pesticide.2. a pump, to move it out.
3. a piping and control system, to carry the right quantities of liquid from the tank to the boom;
4. a boom-and-nozzle assembly that atomizes the liquid that is, breaks it up into the right-sized spray particles, so that it can be spread properly.
Special jobs may call for special apparatus. Designs are changing constantly. The main thing for you, as a forester, to remember is that spraying is a technical job. If you are not sure that the man you plan to hire has the right equipment, step right up and ask some questions of the operator.
If you own a plane and want to handle the spray job yourself, you can buy spraying rigs, complete, for some of the popular makes of plane. They can be removed readily.
Many special techniques of spray-plane piloting have been developed. The trick is to lay down exactly the right dosage of pesticide with as few passes as possible. If the plane makes too many circuits, costs go up and time is wasted. If it makes too few, coverage is not even and the pests are not killed.
Standard practice in spraying fairly flat, rectangular fields is to follow a grid pattern. The pilot flies back and forth across the area in parallel lines. He holds the distance between the flight lines the same as the effective swath width of the spray plane.
Swath width depends on altitude, wind, plane, and equipment. Measuring it requires field tests. The part of a swath that receives approximately the recommended per acre dosage of pesticide is called the effective swath.
For safety's sake, pilots should fly crosswind, and should move upwind on each successive pass so that there is no chance for the poisonous spray to blow back on the plane.
When feasible, start the turnaround at the end of the spray run with a 45-degree climbing turn downwind, over the land adjoining the part of the field already treated. Then level off and reverse the turn 225 degrees. As you come around, orient yourself and line up for the next run. Then, with power reduced, let down to spraying height and start the new pass.
Be sure to fly long enough on the downwind leg of the turn to allow room to complete the 225-degree swing without crowding or arriving too far upwind to start your next run. The aim is to avoid making tricky turns to get into position.
If the 45-degree starting turn is made upwind, you may have to crowd the 225-degree turn and cross your slipstream to start the next run. This should be avoided.
When the country is rough and the area to be sprayed is irregular in shape, the grid pattern does not work so well. The rule under these conditions is to fly either along the contours or downslope. Upslope flying with a heavily loaded plan is tempting fate.
Check landmarks, hazards, and obstructions with your map before you start spraying. If adequate landmarks are lacking, flags should be put down to mark the fields or areas to be sprayed.
Equipment facts
1. Tanks. Detailed information is given below.
a) Load. Sprays differ in weight, but for computing the maximum safe load in gallons, take 7 pounds as the weight per gallon. Divide by 7 the maximum load that the plane can safely carry; the result is the maximum number of gallons in a spray load. Allow for the weight of the tank and the dispersal equipment.b) Location. The tank belongs as close as possible to the center of gravity. If it is too far from that point, the craft will be tail- or nose-heavy in flight. The tank should be fastened securely to the main structures of the fuselage. A full spray tank is heavy, and can do a lot of damage if it breaks loose.
c) Shape. Select a tank that is shaped to correspond with the space where it is to be attached. The bottom should be sloped so that the tank will drain completely, both while spraying and while the plane is on the ground.
d) Material. Of the metals, stainless steel is best, but aluminum and galvanized iron are satisfactory for most of the pesticides commonly used. Molded plastic tanks eliminate the rust and corrosion worry, and are not attacked by the usual chemical solvents. Some operators build satisfactory tanks of wood, usually plywood. They treat them inside with special paints or liquid plastics to prevent leaks, and brace them rigidly to prevent bulging. Others use removable tank liners of synthetic rubber or plastic - a different liner for each type of spray. This cuts down cleaning time and keeps one spray from contaminating another. Operators who dust as well as spray sometimes build a liquid-tight dust hopper of metal, plastic, or wood. With small changes it can also be used as a spray tank.
e) Filler neck. Keep it big. The best necks are large enough to allow spray liquid to be poured into the tank from a 5-gallon can or bucket without a funnel. A big neck will also give convenient access to the tank interior for cleaning. Fit the neck with a removable fine-mesh screen to catch sediment. Put it far enough down into the tank, so it will not cause splashing or flooding. The filler cap should seal tight and open easily. Attach the cap to the neck with a chain. Avoid threaded caps: they are a nuisance. Paint the word "spray" on the plane's surface near the filler neck.
f) Air vent. Do not put the air vent in the filler cap. Fit a pipe into the top of the tank. Let it project straight up from the tank, then curve downward. Let it project high enough so that it won't overflow from the surge of the liquid in maneuvers or rough air. Loop it down and out through the bottom of the fuselage. A simple vent in the top of the tank may let spilled spray blow onto the windshield or into your face. Make the vent big enough to let air come in as fast as the spray goes out. Make it at least ¾ inch in diameter. If you use an emergency dumping system, you will need an even bigger air vent. Make it at least 1½ inches in diameter.
g) Outlet. A sump, or low point, that can be drained is the best spot for the tank outlet to the pump. Install a short standpipe to keep sediment and solids from getting into the pump. If you have a dump valve, put the outlet on the wall of the dump-valve tube, so that sediment will settle to the bottom of the tube.
2. Pumps. Most operators prefer rotary-gear or centrifugal pumps. Both come in many sizes and makes. Turbine-type pumps may also be used, and in some situations gravity-feed systems are satisfactory. Piston, diaphragm, and rotary-vane units, on the other hand, have not worked satisfactorily in aerial spraying.
a) Rotary-gear units. There are several kinds of rotary-gear pumps. All use some combination of gears in mesh to move the liquid. Gear pumps are self-priming, and usually operate at 1,700 to 2,000 r.p.m. They work well with solutions and emulsions. Wettable-powder suspensions wear the teeth and side wall badly; they may even make the pump "seize." Since these pumps create positive pressure (as high as 500 pounds p.s.i. in some types), you will have to install a relief valve somewhere between the pressure side of the pump and the shutoff valve. Some have an adjustable relief valve incorporated in the pump body.b) Centrifugal units. Since they are not of the positive-displacement type, centrifugal pumps require no relief valve. Their top pressure is seldom above 70 pounds p.s.i. They rotate at 3,000-4,000 r.p.m. Some centrifugal pumps have small impellers with wide vanes - to move high volume at low pressure. Others have large-diameter impellers with narrow vanes - to move a lower volume at higher pressure. The best pump for aerial spraying is one somewhere between these two extremes. The big advantage of centrifugal pumps is their ability to handle all kinds of spray chemicals with minimum wear. However, they are seldom self-priming. You have to mount them lower than the tank or set up some kind of priming arrangement. Also, whereas most gear pumps go either forward or backward, centrifugal pumps push the liquid in one direction only.
c) Turbine-type units. Turbine-type pumps, like centrifugal units, handle all sprays without undue wear. The turbine type offers somewhat higher pressures, and will pump in either direction. But it has no dry lift. You have to mount the pump lower than the tank or prime it by hand.
d) Gravity-feed systems. If you are handling herbicides, liquid fertilizers, or other agricultural chemicals best applied as coarse sprays, or if you are using special atomizing devices instead of standard nozzles, you may be able to skip pumps and use a gravity-feed system. The weight of the liquid in the spray tank creates the outlet pressure, but pressure and rate of flow will go down as you empty the tank. You can compensate for this and keep your output constant by installing a float chamber or variable orifice between the tank and the nozzles. Or even simpler - vent the tank with a tube that extends through the top of the tank down through the liquid to half an inch above the bottom. The pressure of the liquid in the tank will slow the flow of air into the vacuum area created at the top of the tank as the spray goes out. This keeps the rate of discharge practically constant until the tank is emptied to below the bottom of the tube. A half-inch tube will give enough air to handle a liquid output of up to 100 g.p.m. With this negative-pressure vent system it is important that the filler cap be air tight. If a dump valve is installed, a separate vent in the top of the tank will have to be opened in conjunction with the dump valve.
3. Material. Nonrusting metals must be used in pumps. Brass is most common, but aluminum is recommended because it is lighter. Aluminum is used more often in centrifugal and turbine pumps than in the gear types. Pump shafts should be brass or stainless steel. Get the mechanical type of packing seal; it lasts much longer, and needs less attention, than others.
The stuffing-box variety of packing gland gives fairly good service, but you have to tighten it when there is a leak. Do not tighten the packing nut too hard. If you do, the packing will bind the shaft and cause excessive wear.
4. Drives. Power sources commonly used to run a spray-plane pump include wind-driven propellers, hydraulic motors, electric motors, and the aircraft engine's accessory drive pad.
a) Wind-driven propellers. For a windmill assembly you need a wood or metal propeller with two to six blades. An automobile fan works well, but it is dangerous unless well reinforced.Mount the propeller-and-pump assembly on your plane's landing gear leg or on a sturdy bracket on the side of the fuselage, or hang it under the fuselage. With the assembly in any of these places, the slipstream from the plane's propellers gives added drive to the propeller that powers the pump. The best way to fasten the propeller to the pump is to couple the propeller shaft to the pump shaft with a universal joint. If you mount the propeller directly on the pump shaft, you will need a ball-thrust bearing to carry the thrust load from the pump shaft to the pump casing. Without it, the air pressure on the propeller may damage the pump. The propeller's blade pitch will determine how fast the pump rotates. Put a brake on the pump assembly. Using the brake helps lengthen the life of the pump and the packing glands.
b) Hydraulic motors. Hydraulic systems work well. If your plane has a hydraulic system to operate landing gear or flaps, you may be able to tie your spray pump in with it.
Generally, though, you are better off to install a separate system. Mount a hydraulic pump on an engine accessory pad, then couple a hydraulic motor to the spray pump. To complete the power system, you will need these items: hydraulic fluid reservoir (small), accumulator, pressure-relief valve, control valve, and suitable tubing.
The spray pump can be mounted entirely inside the fuselage, wherever it fits in best with the rest of the equipment.
If you pick a hydraulically driven spray-pump system, you can start and stop the hydraulic motor by using an unloader valve in the pressure line from the hydraulic pump. Run a small line from the unloader valve to an on-off valve in the cockpit, then continue the line to the hydraulic reservoir. Opening the on-off valve cuts off the hydraulic motor and stops the spray pump. You will also need a separate control valve for the spray flow. Light and powerful, this type of hydraulic system is also good for powerfeed and agitator drives in dust- or bait-hopper installations.
c) Electric motors. Electric-motor-driven pumps have been used for aerial spray work, but usually they are too heavy in relation to power produced.
d) Accessory drive pad. If you decide to use an accessory drive pad for power, you can mount the pump on the pad, or connect it up through pulleys or a flexible shaft. But these systems may have certain disadvantages. Unless you use a gear-type pump, you will probably end up with the pump mounted so high that you will have to hand-prime whenever the pump is emptied. Getting power through a flexible shaft may be difficult, and it may mean extra maintenance cost. If you have a direct-mounted or pulley-mounted system, and the pump or the lines spring a leak, there will be more fire hazard with flammable liquids than there would be if the pump were behind the firewall.
Booms, nozzles, and similar devices
A boom-and-nozzle assembly is the device most commonly used for distributing and atomizing the spray liquid. The boom is a pipe that distributes the pesticide for release from the plane. Nozzles, which atomize the liquid into spray, are mounted on the boom. The alternatives are:
1. rotating brushes or disks;
2. venturi tubes.
Whether or not you get a good spray-deposit pattern depends in large measure on the functioning of the selected device.
1. The boom and nozzle spacing. Location of the boom varies according to type and make of aircraft.
a) Biplanes. On biplanes the boom is usually mounted about a foot below the lower wing; it runs parallel to and between the spars. Or if you want to reduce drag and have a cleaner looking rig, you can install the boom in the lower wing panels, with a pipe extended down out of the wing at each nozzle location. Remember, though, that this may make it hard to repair a leaky boom (Figure 12).Tests show that if you are spraying 1 to 10 feet above the crop in a Stearman or N3N biplane, you will get good results with a boom approximately three fourths as long as the wing span. In fact, if the boom is much longer than that, too much of the spray may get into the wingtip vortices and spoil the swath pattern.
In spacing the nozzles along the boom, put them progressively closer together as you approach the boom tips. Put a cluster of two or more at each tip. Group a few nozzles close together 3 or 4 feet to the right of the plane's center line. But on the left side of the line leave a 3- or 4-foot space without any nozzles; if you put nozzles there, the propeller slipstream will distort your pattern.
b) Low-wing monoplanes. Boom installation and nozzle spacing on low-wing monoplanes should be the same as on biplanes.
c) High-wing monoplanes. On strut-braced, high-wing monoplanes the boom can be attached to the struts. It should extend outward and upward so that the outer ends are 2 feet or more below the wingtips. Some prefer to put the boom parallel to the wing, but this arrangement takes more braces. Nozzle arrangement should be similar to that described above for biplanes.
FIGURE 12. - N3N-3 biplane spraying forest with one gallon of spray concentrate per acre.
2. Nozzles. There are two points to be considered here:
a) Types. Nozzles that produce a hollow-cone spray or a flat, fan-shaped spray are most widely used for aerial spraying. The hollow-cone nozzle will produce droplets more nearly uniform in size than will the flat-spray type. It also tends to wear less. Orifice disks coated with ceramic material wear well and are desirable when you spray an abrasive mixture.b) Atomization. Because the airflow buffets the spray, the degree of atomization produced by a nozzle in flight differs somewhat from the performance of the same nozzle on ground equipment, pressure on the liquid being equal. The relative angle between airflow and nozzle output flow also is a factor. A nozzle produces smaller droplets when the orifice outlet faces forward (into the air blast) than when it faces rearward. When you set a nozzle so that it faces forward, you should direct it downward enough to keep the spray from being blown back onto the nozzle body or supporting structure. If you don't, some of the spray that collects will run off in large drops. You will be wasting spray, and the drops may injure foliage.
Load distribution
The big problem in helicopter spraying is balance. The simplest way to solve it is to install two tanks, one on each side of the fuselage directly under the rotor, and as close to the center of gravity as possible. Plan your outfit so that the spray is pumped simultaneously from each tank (Figure 13).
Atomizing apparatus
Because helicopters usually operate at such a slow flight speed, a windmill won't drive your spray pump properly. Decide on some other means of driving the pump.
A few helicopter manufacturers offer spray boom-and-nozzle assemblies for their aircraft. Some are mounted forward of the nose; others are directly in line with the rotor axis or just a few feet aft.
You can utilize the air blast from the outlet ports of the engine-cooling blower system to atomize the liquid. Direct the flow of the pesticide into the left and right outlet ports through nozzles. The effect is the same as you would get from two mist blowers mounted on the helicopter. The downwash from the rotors disperses the spray over the crop (Figures 14 and 15).
FIGURE 14. - Helicopter equipped with air-float type of spraying device.
By piping the engine exhaust into the blower outlet tubes so that the air velocity past the nozzle at the outlet ports is materially increased, you get atomization approaching fog. This system gives very fine droplet sizes without using small-orifice nozzles, which clog easily, especially when used with suspensions.
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 (Yuill et al, 1951).
Selecting a base of operations
In selecting an airfield or landing strip, consider the distance to the area to be treated; the length, width, and distance of the 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 and, in any case, 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 light biplanes the minimum length is approximately 800 feet. In 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 servicing the planes rapidly 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 extinguisher 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.
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, water courses, 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. A 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.
Communications
Whenever possible some means of communication between the airfield, 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 to 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.
If neither radio nor telephone is available, a system of signals may be used. One such signal system that has been satisfactory is to place a truck in an 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 headquarters 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 is usually 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 very difficult to obtain a uniform coverage of forests in such areas. Chances for error in spacing the flight lines are greater and, since the direction of the lines changes with the topography, it is possible that the pilot will leave some spots untreated while giving others a double treatment. The chance of this happening may be so strong in some cases that it will 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 approximately 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 conditions.
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 that:
1. weather conditions within the area are satisfactory for spraying,
2. 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.
TABLE 10. - DETAILED LISTING BY OPERATIONAL PHASE: OF A STUDY OF 178 AGRICULTURAL AVIATION ACCIDENTS, 1954
|
Operational phase |
Number of crashes |
|
Loading |
1 |
|
Starting |
3 |
|
Taxi for take-off |
0 |
|
Take-off |
14 |
|
Climb-out from take-off |
23 |
|
En route to treat crops |
4 |
|
Survey of field |
1 |
|
Starting swath run |
4 |
|
During swath run |
21 |
|
Pull-up from swath run |
28 |
|
Procedure turnaround |
18 |
|
Flare-out for swath run |
24 |
|
Clean-up swath |
6 |
|
Maneuvering to avoid obstructions |
5 |
|
Return to strip |
7 |
|
Landing phase |
23 |
|
Taxi after landing |
5 |
|
Parking |
0 |
|
Testing airplane |
0 |
|
Testing-calibrating equipment |
0 |
|
Training pilots |
3 |
|
Ferry phase |
3 |
|
Other |
1 |
If possible, additional observers should be present in each unit to check on the spray coverage. They should place clean glass plates, or dyed papers (each about 4 × 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 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.
Rate of coverage
The acreage covered per hour or per day is governed by the following:
1. load capacity;
2. swath width;
3. gallons-per-acre rate and weight per gallon;
4. speed of the aircraft;
5. distance from loading field to treated area (number of trips per hour);
6. mixing and loading facilities;
7. length of runs, and whether area is flown both ways or one way;
8. the area - its size, shape, terrain, and obstructions.
A method of estimating acres treated per hour is illustrated by the following example: the effective width of swath for a given plane is 40 feet, and the average speed of flight is 90 m.p.h.
= 436 acres 2 covered per hour
= 7.3 acres per minute1 5,280 feet = 1 mile = 1.0093 kilometers.
2 43,560 square feet = 1 acre = 0.4047 hectare.
In practice the area covered may be much less than 436 acres per hour when time is taken out for loading, fueling, turning at the ends of the area, and travelling back and forth from landing field to treated plot. Consequently, if in one hour two 100-gallon loads are delivered using 4 gallons per acre, the area covered per hour is:
At ½, 1, 2, 3, 5, and 10 gallops per acre, respectively, the acreage covered per hour would be 400, 200, 100, 66, 40, and 20 acres, respectively. At three loads per hour the above figures would increase by 50 percent, or double in the case of 4 loads per hour.
Size of plane
A number of factors contribute to spray-coverage characteristics and deposit which have a direct bearing on efficiency and effectiveness. The first of these is the size of the plane, its wing expanse and the altitude at which it flies. Small planes and low flying decreases swath width while the reverse condition increases it. For example, cubs normally disperse an effective swath of 32 to 42 feet for crops at an altitude of 2 to 10 feet; or 75 feet when flying 30 to 60 feet above the vegetation, as in spraying forests. In the case of Stearman biplanes, the normal effective swath width for the 2- to 10-foot flight level is considered to be essentially the same as the wing spread but this can be altered somewhat by the engine's power, the length of the spray boom and the nozzle arrangement. When spraying forests, Stearmans usually fly 100- to 130-foot swaths. The DC3 and B17 multimotored planes fly 500-foot swaths over forests at a flight level of 150 feet above the canopy at a speed of 150 m.p.h. (Figure 16).
Atomization
The degree of atomization is often the most important factor concerned in pest control by aircraft. It influences swath width and pattern, distribution, coverage, deposit, gallonage required per acre, and spray drift. The important factors responsible for a given degree of atomization are:
1. the nozzles (their construction, orifice size, position, and direction of discharge);
2. pressure;
3. aircraft speed;
4. air velocity and volume of the propeller slip stream and the relative proportion of spray that is released in the slip stream;
5. the composition and viscosity of the mixture.
When releasing the spray at 50 feet or more above forests for defoliators such as sawflies, tent caterpillars, gypsy moths. tussock moths, and spruce budworms a drop pattern with a mass average diameter of 150 microns seems to be about right. If the drops are too small, the deposit will be light and much of the spray will be carried up into the upper air currents and drift away from the area being treated. For crop spraying at the 2- 6- 10- and 25-foot levels a mass average drop diameter of 60, 70, 75 and 100 microns appears to be about right. At the 2- to 10-foot level there is considerable air turbulence in the wake of the plane's passage and slip stream. This turbulence is sufficient to deposit droplets of smaller size than for the 25-foot or greater level, where the effect of turbulence is lost. Also, drift is minimized at the lower flight level. Air turbulence greatly improves under surface coverage and penetration of the vegetation. Hence, with medium fine atomization and low flight it is possible to control many diseases and insects (aphids, mites, etc.) that could not be as effectively controlled with larger drops delivered at higher flight levels.
TABLE 11. - SHOWING EFFECT OF NOZZLE SIZE, NOZZLE POSITION, AND AIRCRAFT SPEED ON ATOMIZATION OF FIVE WHIRLJET NOZZLES AT 40 P.S.I. NOZZLE PRESSUREa
|
Speed of aircraft (m.p.h.) |
Nozzle number |
Diameter of orifice, in inches |
g.p.m. delivery |
Mass. Aver. Diameter in microns, with nozzle pointing |
|||
|
Conn. |
Conn. |
forward |
down |
aft |
|||
|
60 |
A1 |
B1 |
1/16 (.062) |
0.2 |
110 |
116 |
128 |
|
A2 |
B2 |
5/64 (.078) |
0.4 |
115 |
135 |
160 |
|
|
A3 |
B3 |
3/32 (.093) |
0.6 |
136 |
165 |
175 |
|
|
A5 |
B5 |
1/8 (.125) |
1.0 |
152 |
180 |
238 |
|
|
A10 |
B10 |
3/32 (.187) |
2.0 |
170 |
210 |
276 |
|
|
80 |
A1 |
B1 |
1/16 (.062) |
0.2 |
90 |
130 |
158 |
|
A2 |
B2 |
5/64 (.078) |
0.4 |
98 |
138 |
175 |
|
|
A3 |
B3 |
3/32 (.093) |
0.6 |
110 |
150 |
190 |
|
|
A5 |
B5 |
1/8 (.125) |
1.0 |
120 |
160 |
225 |
|
|
A10 |
B10 |
3/32 (.187) |
2.0 |
150 |
210 |
260 |
|
|
100 |
A1 |
A1 |
1/16 (.062) |
0.2 |
78 |
100 |
145 |
|
A2 |
A2 |
5/64 (.078) |
0.4 |
88 |
114 |
155 |
|
|
A3 |
B3 |
3/32 (.093) |
0.6 |
100 |
125 |
165 |
|
|
A5 |
B5 |
1/8 (.125) |
1.0 |
114 |
152 |
182 |
|
|
A10 |
B10 |
3/32 (.187) |
2.0 |
130 |
182 |
218 |
|
a Pump pressure is usually 5 to 10 p.s.i. greater than nozzle pressure. Check valves usually increase pressure about 5 p.s.i.
It remains to be shown precisely what degree of atomization results at given air speeds when the nozzles are placed in given positions in relation to the plane's line of travel. Table 11 gives the degree of atomization from 5 sizes of whirljet nozzles on a biplane traveling at 60, 80, and 100 m.p.h.
In general, most other types of cone nozzles with the same included angle and output give about the same degree of atomization. Flat or fanshaped spray nozzles do not atomize quite as finely as the cone types.
The finest atomization is produced with the nozzles pointing forward but this may cause some of the spray to collect on the structures of the airplane. If nozzles point to the rear the atomization is coarse. Therefore, it is usually best to point the nozzles down or slightly forward. Small nozzle orifices, including oil burner nozzle tips, can be used for fine atomization when applying solutions and emulsions but not for mixtures containing wettable powders which require an orifice of 3/32 inch or greater to avoid clogging. The use of small orifices requires a large number of nozzle tips or nozzle clusters to deliver the desired output. When spraying through finely atomizing nozzles it is important to have a good 30- to 40-mesh screen between the pump and the nozzles.
Nozzles that deliver the coarsest spray and the most uniform drop size have consisted simply of short hollow tubes pointing aft of the boom.
Increasing or decreasing the pressure increases or decreases the atomization to a lesser degree than a corresponding change in size of orifice or speed of aircraft.
Gear pumps can develop considerable pressure but high pressure requires considerable power which may limit the use of air-driven pumps where a high spray output is required.
Increasing or decreasing the speed of the aircraft has a great effect on degree of atomization like increasing or decreasing the air velocity of a mist blower. Some increase or decrease in atomization can be made by changing the viscosity and composition of the spray mixture.
Swath widths should be conservatively estimated and generally lapped in order that sufficient material be deposited over the entire sprayed area. The number of nozzles and their size will be governed by the desired application rate, swath width, and speed of flight. Some spray delivery rates in gallons per minute for various application rates for three swath rates and three Bight speeds are listed in Table 13. Table 14 on page 148 gives the same sort of information for larger aircraft.
TABLE 12. - NUMBER OF NOZZLES REQUIRED FOR GIVEN SPRAY DELIVERY RATES a
a Pump capacity should be 20 percent greater than the maximum total nozzle output required.
TABLE 13. - SPRAY DELIVERY REQUIREMENTS IN GALLONS PER MINUTE: FOR VARIOUS APPLICATION RATES, SPEEDS, AND SWATH WIDTHS
|
Flight speed, in miles per hour |
Gallons, per acre |
Required spray delivery in gallons per minute |
||
|
30 ft. swath |
40 ft. swath |
50 ft. swath |
||
|
60 |
½ |
1.8 |
2.4 |
3.0 |
|
60 |
1 |
3.6 |
4.8 |
6.1 |
|
60 |
2 |
7.3 |
9.7 |
12.1 |
|
60 |
3 |
10.9 |
14.5 |
18.2 |
|
60 |
4 |
14.5 |
19.4 |
24.2 |
|
70 |
½ |
2.1 |
2.8 |
3.5 |
|
70 |
1 |
4.2 |
5.7 |
7.1 |
|
70 |
2 |
8.5 |
11.3 |
14.1 |
|
70 |
3 |
12.7 |
17.0 |
21.2 |
|
70 |
4 |
17.0 |
22.6 |
28.3 |
|
80 |
½ |
2.4 |
3.2 |
4.0 |
|
80 |
1 |
4.8 |
6.5 |
8.1 |
|
80 |
2 |
9.7 |
12.9 |
16.2 |
|
80 |
3 |
14.5 |
19.4 |
24.2 |
|
80 |
4 |
19.4 |
25.9 |
32.3 |
Calibration
The calibration problem has been well considered by Yuill, Eaton, and Isler (1951) as follows: "The importance of proper performance in spray equipment cannot be 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 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 to 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.
4. Compute the flow rate in gallons per minute.
An alternate method is as follows:
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 exactly the same location used when filling tank and measure the amount required to refill to exactly the same level.
4. Compute the flow rate as above.
Two or three replicate flights should be made, and the results 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 gallons per minute;
S = speed 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.
Any delivery rates in gallons per minute may also be computed as follows:
when
D = delivery required, in gallons per minute;
R = application rate, in gallons per acre;
S = flight speed, in miles per hour;
W = swath width treated, in feet.
The swath width of planes equipped with a full-wingspan-boom can be estimated roughly by multiplying the wingspan by 3.5 to 4. However, this is only an approximation. It applies only to planes in which spray is released over the full wingspan, and from a height above the trees greater than the wingspan."
Selective chemicals enable foresters to thin, release, and favor one species over another. The concentrated spray method of application, from the ground or from the air, will enable the forester to cover both large and small areas quickly and cheaply. For example, as little as 1 to 3 quarts of 2, 4, 5-T ester can be used in 2 to 3 gallons of final spray per acre effectively to release conifers from aggressive, competing hardwoods of any size.
The use of silvicides in forestry is only in its infancy. Nevertheless, the following uses have been demonstrated.
1. the control of undesirable plant and tree species, as in the preparation of area for planting;2. releasing coniferous seedlings and trees (both natural and planted) that are being crowded out by undergrowth, hardwood sprouts, or overstory;
3. control of brush along power lines, trails, and roadways through the forests.
For any efficient silvicidal job it is necessary to evaluate the requirements including that for application equipment, methods, and cost. From this evaluation must come the answers to such questions as: Do we want all of the growth cleared or are we satisfied with thinning it out a bit? Is it composed of dense, tall brush and trees or is there a clump here and there? What species are involved or predominant? Is the area readily accessible to spray equipment or will it require some special effort to get through and make the application? What are the hazards - stumps, rocks, terrain, drift, etc.? Size of area is important too, because it costs more per unit area to treat small areas than large ones.
Cost is the essential factor of any silvicidal program. To reduce cost and increase speed and ease of coverage we must look to four categories as follows:
1. cheaper and more phytotoxic, selective chemicals;
2. effective methods for using less total chemical per acre;
3. methods for using less diluent and total spray volume per acre;
4. cheaper, lighter, and more mobile spray equipment.
In all of these categories, except the first one, we must depend on equipment and application methods. An example of selecting the cheapest effective chemical for control of willows is the use of 2, 4-D amine (which costs U.S.$2.70 per gallon) instead of 2, 4, 5-T (which costs U.S.$9.00 to U.S.$10.00 per gallon).
Foliage applications of one to two quarts 2, 4, 5-T per acre costs about U.S.$2.25 to U.S.$5.00 per acre for the chemicals alone. Cost of labor and equipment will vary according to conditions referred to later.
Application of silvicides may be classified as to foliage, basal stem, and stump or stubble. Foliage application is the quickest and best adapted way to treat for the first time over to convert growth to a relatively nonwoody plant area. This method is limited to seasonal operation.
Basal stem treatment
Basal stem treatment is particularly useful to control sparse infestations inaccessible to other spray equipment; and for treating stems that survive foliage applications. Treatment can be made any time, preferably during the dormant period without chance of injury to sensitive crops. This permits use of manpower during the off-season. Relatively inexpensive equipment is required. However, it is not as economical or as widely applicable as foliar sprays and stems larger than 2 inches in diameter are difficult to kill. A high labor cost is involved because each stem must be thoroughly wet on all sides at the ground line. To obtain desired results it is important to apply sufficient liquid to assure physical run-down outside the stem to the root crown, bud, or transition zone of the plant. Knapsack or round back compressed air sprayers are more often used to apply the chemicals. However, a useful light power unit consists of a 4 h.p. gas engine, ½ inch gear pump, and two lines of light hose with a suitable nozzle, gun and shut-off valve attached to each line of hose. The chemicals are used at 5 times the concentration of dilute sprays i.e., 16 pounds 2, 4, 5-T acid or more per 100 gallons of No. 2 fuel or diesel oil, or 1 pint in 2½ gallons of oil. To aid in marking, up to 60 percent of the oil in the mixture can be supplied from strained black carbon waste oils of filling stations and garages. White strings may also be used to lay out spraying lanes of 40 to 60 feet in width to help guide the spray men.
Stumps should be thoroughly wetted at time of cutting to reduce sprouting. This practice is relatively inexpensive from a labor standpoint, especially when stump diameter is greater than two inches. Usually one man treats the stumps behind a regular cutting crew. Ammate may be applied as crystals at the rate of 2 ounces per 0 inches of diameter, or thoroughly wetting with concentrate solution of 4 to 6 pounds per gallon of water. The 2, 4, 5-T esters are used at 16 to 20 pounds per 100 gallons of oil (or 1 pint per 2.5 gallons of diesel oil) to wet the stump thoroughly as for the basal stem application.
Foliage sprays
The 2, 4-D weed killers will control certain species as willow, alder, sumac, elderberry, brambles, raspberry, blackberry, and other easier-to-control woody plants. However, it is not sufficiently effective against most dominant hardwoods like oak, ash, hickory, persimmon, and maple. For these and most other resistant woody species like osage orange, 2, 4, 5-T is more effective than 2, 4-D. A common mixture is one containing 2 lbs. of 2, 4, 5-T and 2 lbs. of 2, 4-D acid equivalent per gallon. Water is a cheaper diluent for it than oil. For the dilute or full dilution sprays, 3 to 6 quarts of 2, 4-D - 2, 4, 5-T mixture is added to each 100 gallons of water for foliage application.
" Silvex" or "Kuron"3 is more effective against a few species like scrub oak than 2, 4, 5-T and Amino triazole is more effective than other materials for thistles and poison ivy. "Ammate" is applied to foliage at the rate of 75 pounds per 100 gallons of water. Thorough wetting with a spreader-sticker is important. Compared to most other silvicides it is more expensive and is rougher on equipment, particularly copper and brass gears and pipes. For semi-concentrate application, concentrations up to 8X, or 6 pounds per gallon, can be prepared. Ammate is more nearly a "one shot" treatment, making retreatment unnecessary for several years because it effects an 80 percent or better root kill of mixed species. Being nonvolatile, there is less danger from drift.
³ "Silvex" and "Kuron" are trade names for 2 (2, 4, 5-Trichlorophenoxy) propionic acid.
Conventional power equipment
The conventional equipment ordinarily consists of a conventional pistol type hydraulic sprayer (as Hardie, Bean, Iron Age, Myer, etc.) with ¾ inch high pressure hoze. Nozzle sizes fit the pump capacity. The 15 g.p.m. pumps can use the 3/16 inch orifice but the ¼, 15/64, and 5/16 inch orifice will require the 35 and 60 g.p.m. pumps. This necessitates a big, heavy outfit.
Knapsack sprayers
The 2, 4, 5-T, "Silvex", Amino triazole, Dalapon, and other hormone types are very effective when applied in the form of concentrated sprays. Knapsack sprayers or compressed-air back sprayers may be effectively used to apply 4 to 10 gallons of finished spray concentrate per acre. This gives no spray drip and run-off. The low gallonage used enables each man to cover 3 to 6 acres per day. This system is also very economical for spot spraying and for retreatment of old sprayed areas. It is limited by the fact that it cannot reach growth higher than 12 to 15 feet. A 6-to 8-foot aluminum extension rod is required for high growth. A most important item is a suitable low gallonage atomizing nozzle to provide suitable atomization without delivering too much spray material. The nozzles should be 2.5 to 6 g.p.h. cone type, oil burner nozzles, such as Monarch F80 series4, Nos. 6.00, 7.00, and 8.00, or the equivalent in other brands. The round tank, 2 to 3 gallon-compressed air back sprayers costing about U.S.$8.00 each are most economical.
4 Monarch Works Mfg. CO., con Salmon and Westmoreland Sts., Phila., Pa.
Mist blowers
For applying herbicides and silvicides the mist blower is the most fantastic plant killer known. For most brush and forest management operations mist blowers in the 5 to 12 h.p. sizes are most applicable. These units are mounted on a four wheel drive power wagon, jeep, regular tractor, or a crawler type tractor with bulldozer blade mounted in front. The relative abundance of stumps, trees, rocks and ditches determine the kind of carriage vehicle. The caterpillar or crawler type tractor is best suited for extremely difficult conditions.
On tractors, the mist sprayer is mounted on a sturdy rack at the rear of the tractor so that the air outlet points at right angles to the line of travel. The outlet should point slightly above the top of the growth to be treated. The shut-off valve should be close enough to the driver's seat for him easily to turn it on and off. For most of our work we used 12 to 20 pounds pressure and a No. 1 or No. 2, 1/8 inch Conn. Whirljet nozzle5. If the air velocity was greater than 115 m.p.h. the nozzle was pointed in the same direction as the air blast. In swath spraying to cover an entire area the mist blower treated 30-foot swaths from two opposite sides by spraying while going down on one side of the strip and coming back on the other side. About 30 to 40 acres could be treated per day on favorable terrain. None of the spray should be allowed to drift onto useful plants and crops.
5 Spraying Systems Co., Bellwood (Chicago), Illinois.
Knapsack mist blower
Where drift hazards can be avoided, the 2 h.p. knapsack mist blower can be a useful tool for spraying brush up to 30 feet in height using 2 to 4 gallons of finished spray concentrate per acre of solid growth. Less material is needed for spot treatments. A drop size range of 50 to 75 microns mass average diameter is best for most purposes. A good knapsack mist blower can treat 10 to 15 acres of brush per day. It is also useful for spot spraying and retreatments. A good spray team consists of two men with two blowers to do the spraying and one man to refill and service the two sprayers.
Helicopters
The helicopter is being used successfully to treat many miles of rights-of-ways along power lines using 3 to 6 quarts of 2, 4, 5-T or 2, 4, 5-T propionic acid per acre in 3 to 5 gallons of No. 2 fuel oil per acre. Its use in many areas is prevented by the hazards involved, such as drift and flying risk. The helicopter should not be flown directly over power lines. Therefore, one objective is to fly parallel to the line and throw a coarse spray of about 200 microns drop diameter in from the edge of the right-of-way towards the center.
Where drift is not a serious risk the helicopter is ,an excellent tool for applying silvicides to release conifers from competing hardwoods or to kill the growth in areas that are to be planted to conifers or other trees. Large, tall trees are killed just as easily as low-growing trees, brush and sprouts.
Airplanes
Airplanes may be used in much the same way as helicopters except that larger areas are required and the drift hazard is far greater.
There has been considerable confusion in defining particle size and in the use of terms relating to concentrated and dilute spray, aerosol, smoke, and dust application. Incorrect interpretation and lack of understanding of the fundamentals involved have resulted in much needless waste of time, money, and lack of maximum effective results. It seems timely to suggest definitions for some of the terminology in this particular field of disperse systems and to indicate their proper use.
Particle size
Thorough understanding of the meaning of particle or drop size and proper methods of recovering and measuring particle samples would help remove the confusion associated with some of the terms that follow. The term particle size has had several meanings, often misconstrued, depending on whether one refers to radius, diameter, volume (4.2R³), surface area (3.14d²), or density. In stating actual size of round droplets one should make clear whether radius or diameter is meant, because a droplet with radius of 50 microns6, is 8 times larger in terms of volume than one with a diameter of 50 microns and 64 times larger than one of 25 microns diameter. A particle of given diameter or volume with a density of 5 is 2½ times as heavy as one having a density of 2, and particles of the same size or density may expose very different surface areas. The term average diameter (or average radius) should be qualified, since average diameter can be considered as either a mass average diameter, numerical average diameter, or mass median diameter. If all droplets were of the same size their average numerical, mass, and median diameters would be identical.
6 A micron is 1/1,000 millimeter.
The mass median diameter is the drop diameter that satisfies the condition that half the spray volume is of drops larger, and half is of drops smaller, than it.
Mass average diameter is the diameter of the drop of average volume. A typical spray pattern having a numerical average drop diameter of 40 microns, also gave a mass average drop diameter of 52 microns and a mass median drop diameter of 56 microns. Over half of the drops were less than 31 microns in diameter.
Mass average diameter, sometimes incorrectly called mass median diameter, is the diameter of the drop whose volume is obtained by dividing the volume of spray in the sample by the total number of drops. The diameter of a sphere of this volume is then found, by the equation:
Numerical average diameter is found by adding all the drop diameters and dividing by the number of drops. It is always less than the mass average diameter, and most of the volume of the spray pattern is in drops having diameters larger than the numerical average. The volume of a sphere of this diameter multiplied by the total number of drops does not equal the total volume of spray.
In any discussion of particle size the method of obtaining particle samples of spray or dust should be adequately stated or understood by both writer and reader. The importance of indicating the sampling method is illustrated by the fact that when the mass average diameter is greater than approximately 75 microns, nearly complete droplet patterns can be recovered out of-doors on foliage and microscope slides; but when the mass average drop diameter is 10 microns or less, over 96 percent of the drops are too small to deposit on or impinge upon foliage, insects, and slides out-of-doors, thus making it necessary to trap drop samples in the air.
Size of dust particles. The size of dust particles quoted on a package or container refers to the diameter of individual particles, and is not related to the size of dust particles deposited on plants and other objects. This deposit consists mostly of large particle "groups", or "glomerates" of many particles, since most extremely fine individual dust particles do not ordinarily deposit satisfactorily on plants and insects out-of-doors.
Deposit as related to contact effect and stomach poison effect. A great deal of confusion has arisen because of a lack of understanding of the relationship of particle size to contact and stomach poison effect. Many assume that an aerosol or spray that is too fine to leave a deposit can be an effective contact poison. Excepting fumigants, they could not be an effective contact toxicant because no particles can kill insects without being deposited on them or on the plants. Whether contact or stomach poison effect is considered, effectiveness is directly dependent on the quantity and distribution of the deposit. Oil-coated dust and air-mixed-oil-coated dust are terms that should convey two quite different meanings. An oil-coated dust is made before application in a mixing device, such as a ball mill. It may contain from 2 to 12 percent oil by weight, and is applied in the usual manner by conventional dusters. Most materials containing more than 8 to 12 percent oil or other liquid cannot be dusted. Air-mixed-oil-coated dust is a new application method in which special spray-dust equipment simultaneously applies and mixes atomized dust and finely atomized oil. For dust to deposit and adhere most effectively it should be coated with 20 percent or more of oil by weight. Greatest deposit and adherence seems to occur when all of the dust is coated with about 60 percent oil by weight. Spray-dust, and "wet-dust" have the same meaning as the above term, except that it also includes similar application of water and sprays other than oils.
Definition of spray methods
The terms dilute, ordinary, or conventional spray method refer to ground applications of 75 to 1,000 gallons per acre of mixtures of low insecticidal concentration. In the so-called mist-spraying of dilute spray with air carrier, most of the spray volume is in drops of 150 to 1,500 microns diameter. In solid stream spraying the drops are 1,000 to 3,000 microns diameter, and water is the carrier when spraying living plants. The volume of spray is sufficient to wet foliage very noticeably, often to the extent of considerable drippage and run-off. Pressure is applied directly to the liquid, as compressed air in the tank of a knapsack sprayer, or hydraulic pressure on the liquid between pump and nozzle in a high pressure power sprayer.
Concentrate application has these necessary characteristics:
1. a low volume (¼ to 15 gallons) per acre, or per unit area;
2. insecticide concentration many times that of any dilute spray mixture of the same insecticide;
3. fine atomization compared to that for dilute sprays; with most of the spray volume in drops less than 300 microns diameter.
The spray can be a solution, emulsion, or suspension. Concentrated sprays can be applied with several types of equipment. Atomization can be accomplished:
1. mechanically, with direct pressure through special conventional nozzles, compressed air, air velocity (as from a blower or the forward speed or propeller blast of aircraft), centrifugal devices;
2. liquefied gas;
3. steam or thermal generated smokes, etc.
Atomized spray, atomized oil, vapor-spray and vapor-dust are poorly adapted terms used to denote concentrated spray application. Actually, any spray mixture is atomized upon application whether dilute or concentrated. Vapor dust and vapor spray are improper hybrid expressions.
Semi-concentrate sprays are those having an insecticidal concentration of 2 to 7 times that of regular conventional sprays. This often requires an application of 15 to 35 gallons per acre for crops, and 30 to 70 gallons per acre for fruits.
X-concentration means the number of times the concentration of the mixture is greater than the regular conventional spray. For example, the conventional spray concentration is X1. Mixtures that are 5, 10, 20, etc., times its concentration are known, as X5, X10, X20, etc., concentration.
Aerosols
The word "aerosol" is one of the most incorrectly used terms. Relatively few people have used it correctly or understood its meaning. Physical chemists prefer to designate aerosols as air suspensions of extremely fine, dry, or liquid particles of almost colloidal fineness that are considerably less than 4 microns in diameter and which may remain suspended in the air for hours or even days. Clouds of extremely fine individual dust or smoke particles are typical aerosols. They are not vapors or gases and ordinarily do not pass through the breathing pores of insects. Aerosols will eventually settle out in calm air of a convection-free room or container, but do not deposit out-of-doors to any appreciable extent on plants, insects, and other objects. Therefore, under ordinary conditions true aerosols are not effective field insecticides. Highly successful field control with "liquefied gas" aerosols are reported, but these are not true aerosols. Most of the insect mortality is due to that portion of the atomized concentrate which is in droplet sizes exceeding 15 microns diameter. Aerosols can be made from liquids by mechanical atomization, condensation of thermally generated vapors into "smokes", and by liquefied gas propulsion. In discussions of dusts and atomized sprays, specifications should be made of the particle size or degree of fineness. Small droplets originally above aerosol size sometimes become aerosols when allowed to drift in the air long enough for volatile fractions to evaporate.
Smokes are true aerosols whether the particles are liquid or dry. The particles or droplets are separated. Most of them are one micron or less in diameter in the case of screening smokes, but may be larger in the case of certain other less dense smoke clouds. Under certain conditions the size of smoke particles can be greatly increased but as soon as the particles grow beyond aerosol size they are no longer smokes or aerosols and therefore do not appear or behave like smokes.
Vapor is a gasified liquid or solid mixed with air and may be capable of condensation into drops or larger particles, often producing a smoke-like appearance, depending on fineness.
Ground equipment
Many sizes and types of equipment may be used for applying concentrates when equipped with a suitable atomizing nozzle. These include hand atomizers, knapsack sprayers, liquefied gas spray dispersing apparatus, thermal generators, mist blowers, and power driven non-blower type, low gallonage sprayers.
A liquefied gas sprayer is an apparatus which disperses a mixture of liquid gas (as freon, methyl cloride, CO2 etc.) and pesticide, or some other chemical to be sprayed as a finely atomized concentrate. At ordinary room or spraying temperatures the liquid gas in the mixture volatilizes and thus generates its own pressure for atomizing and dispersing the spray. The gas output that carries the mist is more nearly comparable in volume with that from small orifice compressed air nozzles and therefore it cannot project the mist great distances like the air blast of mist blowers.
Thermal generators (mist, fog, or aerosol). Thermal generators are applicators that utilize heat to atomize liquid concentrate sprays. They may disperse a fine mist an or aerosol fog. They may or may not have a blower (fan) to drive the droplets to some distance from it. With a blower, they may operate with a breeze, against a contrary wind, or in the absence of any noticeable air movement. When the unit operates with a fan it is essentially a mist blower that uses heat to assist in atomizing the liquid. Either steam, hot air, hot volatized oil, or hot metal rings or hot exhaust pipes are employed to atomize the spray liquid.
Mist blowers. A mist blower is a power driven ground machine that disperses highly concentrated sprays in finely atomized (10 to 150 microns) form at low gallonages per acre (or per unit area) with air as the principal carrier instead of water or other liquid carrier.
Low gallonage power concentrate sprayers may be operated without a blower for applying insecticides and herbicides to low growing crops, weeds, and brush. The spray liquid is pumped through low gallonage conventional type (as oil burner) cone or flat spray nozzles at 20 to 100 pounds pressure. This type of apparatus is cheaper, lighter, and more simple than blower type apparatus, but has the disadvantage of producing coarser drops, lack of driving power to carry the mist, and its effectiveness is limited to low-growing plants.
Aerial equipment
Compressed air power atomizers use a compressor to develop air pressure for delivering both air and liquid through atomizing nozzles at 10 to 50 pounds pressure for applying concentrates to low-growing plants.
A helicopter mist sprayer is an airborne mist blower for applying concentrated sprays in which a large volume of low velocity air from large propellers or rotary wings mounted above the aircraft tends to drive the spray mist downward. The spray may be atomized by oil-burner type nozzles with pressures above 25 pounds per square inch (p. s. i.) developed by a pump driven by the helicopter's motor; or it may be atomized by high air velocity from a fan.
An airplane mist sprayer is a fixed winged, airborne concentrate applicator, which utilizes the speed of the aircraft to help atomize the spray, and which may or may not utilize pump pressure or the air blast from the propeller of the aircraft's engine to atomize the spray. Power for driving the pump is usually derived from a small airdriven propeller.
MILLER, C. W. Introduction to Physical Science. 2nd edn. J. Wiley & Son. New York and London, 1935.
POTTS, S. F. Concentrated Spray Mixtures and their Application by Ground and Aerial Equipment as Compared with Standard Spraying and Dusting Methods. U. S. Department of Agriculture, Bureau of Entomology and Plant Quarantine, E-508, 20 pp., illus. Washington, D. C. 1940.
POTTS, S. F. Particle size of insecticides and its relation to application, distribution, and deposit. Jour. Econ. Ent. 39 (6): 716-720, 1946.
SELL, WILLY Staubausscheidung an einfachen Körpern und in Luftfiltern. (ATOMIZATION OF SINGLE ELEMENTS AND OF AIR FILTERS), Forschungsarbeiten Verlin deutschen Ingenieuren V.I.D. (RESEARCH ASSOCIATION OF GERMAN ENGINEERS), Verlag No. 347, 1931.
TIRRELL, GEORGE E. Depositing of spray chemicals: hydraulic method vs. air method. Trees Magazine, 14 (3):6. Oakland, Calif. 1954.
YUILL, J. S., EATON, C. B., AND ISLER, D. A. Airplane Spraying for Forest Pest Control. U. S. Department of Agriculture, Bureau of Entomology and Plant Quarantine, E-823, 21 pp., Washington, D. C. 1951.
