Blowers for Insecticides

by W. L. POPHAM FOR A LONG time there has been need for improved methods for spraying fruit trees, vegetable crops, wood lots, and ornamentals to control certain insects and plant diseases. Maybe what the public wants is a new high-velocity blower that atomizes and evenly distributes small quantities of highly toxic insecticides and fungicides. Department entomologists are now experimenting with equipment that will spray an acre of vegetable crops with 1 to 3 gallons, a fruit tree with 4 to 6 ounces, and the largest street trees with a fourth to a half pint of liquid insecticide.

The idea of using high-velocity air to distribute insecticides stems from work in 1921 and 1922, when technicians visualized the possibility of using aircraft for crop dusting and began experiments to determine its practicality. Their immediate objective was to perfect devices for releasing an even flow of dust into the slip stream of an airplane flying low over the crops, relying upon the propeller blast, the forward speed of the plane, and the turbulent air created by the wings to distribute the insecticide evenly.

Success in crop dusting with aircraft and ground blowers led to interest in the possibility of using high-velocity air for applying liquid insecticides. Many standard insecticides and fungicides are more effective when applied as sprays, and an adhesive or other conditioner may be readily added to a spray, so that the effective period is extended. As in the case of lead arsenate for the control of codling moth, it is frequently impossible to secure and retain with dusts the type of coverage necessary to insure protection. On the other hand, the common way of applying dilute sprays to orchards, field crops, street trees, and forest cover requires a lot of water and heavy equipment that is costly to operate.

The problems encountered in attempting to control the white-fringed beetle, a destructive insect of foreign origin that has become established in limited areas of the Gulf Coast States, prompted a pooling of the efforts of entomologists, chemists, and engineers assigned to the project to find a method of applying insecticide concentrates in liquid form. On this project the shortcomings of conventional methods of applying insecticides became particularly obvious.

Dusts, both calcium arsenate and cryolite, were reasonably effective in reducing the numbers of white-fringed beetles if they could be kept on the vegetation, but, unfortunately, rains day after day during the season when treatments were applied did much to neutralize the effort. Dilute sprays required heavy equipment that could not be taken off hard-surfaced roads when fields and pastures were wet. It was costly to operate and awkward to handle in small fields and in suburban areas, where treatment often was necessary. The situation demanded lightweight equipment that would distribute arsenicals or cryolite as concentrated sprays to which could be added adhesives that would withstand Gulf coast weather in June, July, and August.

Although many devices, involving several different principles, were tried and found inadequate for the purpose, a unit conceived in 1940 that involved the use of high-velocity air and low pressure on the liquid feed line began to take shape. By 1943 the unit was ready for field testing; in 1944 it became standard equipment on the white-fringed beetle control project. This tractor-mounted equipment is a combination duster-sprayer. The basic principle is high-velocity air delivered through a 3 1/2-inch flexible hose, which atomizes and projects in an even pattern either dusts or liquids, or both. The dust hopper is mounted directly back of the seat and above the fan housing, and the powdered insecticides are released through the fan. The spray tanks are mounted on either side of the engine and the liquid insecticides are injected into the air stream at the mouth of the hose, using any one of several types of nozzles, depending upon the nature or consistency of the materials used. Adding a spray that may contain an adhesive, or other conditioner if desired, to insecticidal dusts as they leave the mouth of the hose makes it possible to reduce drift substantially, and, in effect, to stick the dust to the foliage as it is applied.

When the fan is driven at 4,000 revolutions per minute, the air-is forced through the hose at about 160 miles an hour. By starting a breakup of the liquid with 30 to 50 pounds pressure on the nozzle that injects the liquid into the air stream, atomization is completed by the fast-moving air, and the size of particles is reduced to a range of 150 to 300 microns in diameter. Thus it becomes possible to treat an acre of field crops, pastures, or even forests with as little as 1 to 3 gallons of liquid.

During the 1945 season, units of this type were used to dispense both DDT and cryolite for white-fringed beetle control, almost entirely replacing other types of sprayers and dusters previously used on the project. Both chemicals were applied as dust and as sprays to vegetation along roadsides and railroad rights-of-way and to field crops and uncultivated pasture lands. The rate of application with DDT was 8 pounds of 10 percent dust or 1 pound of technical DDT in 2 gallons of solution per acre. Cryolite was applied in concentrations of 4 to 6 pounds in a gallon of water with an emulsifier and adhesive added.

At normal working speeds for a tractor, these units gave an effective distribution of insecticide over a swath 30 to 50 feet wide, depending upon the density of the vegetation receiving treatment, terrain, and the directional adjustment of the hose. One man with a tractor treated approximately 8 to 10 acres an hour, using 1 to 3 gallons of concentrated spray to the acre; with dilute sprays the same area required much heavier equipment, 3 to 5 men for satisfactory operation, and the application of 300 to 500 gallons of liquid.

The tractor equipment established beyond a doubt that high-velocity air could be used to atomize and distribute liquid insecticides. But as developmental work progressed, it became evident that a blower that would deliver a larger volume of air with some increase in velocity would extend the swath of effective treatment, give better atomization of liquids, and better penetration of dense vegetation. This led to an investigation of fans or blowers of various designs that might better meet requirements and resulted in securing the interest and collaboration of engineers trained and experienced in air dynamics, who cooperated with the Bureau of Entomology and Plant Quarantine in developing an axial-flow-type blower for the purpose. The unit was designed to provide maximum volume, velocity, and pressure in relation to size, weight, and power.

The specific objective of the original design was a simple, relatively inexpensive device that would project finely atomized liquids in an even pattern for at least 100 feet from the nozzle, which would be adequate for treating street trees, ornamental plantings, wood lots, and any types of field crops. The pilot model was given exhaustive tests in connection with experimental work for the control of forest and shade tree insects in 1945, using DDT in various formulations. At a speed of 3,800 to 4,000 revolutions per minute it gave good coverage for a distance of 125 feet from the nozzles. The air stream containing the insecticide feathered Out to a pattern about 18 to 20 feet wide at a distance of 100 feet from the nozzle. At this fan speed the spray particles ranged in size from about 50 to 150 microns. Some were smaller and some larger, but the bulk of them fell within this range.

When operated under a wide variety of conditions in the field, no difficulty was encountered in reaching the tops of the largest trees. When spraying along roadsides, coverage adequate for control of many of the forest insects that cause defoliation in the New England area was obtained for a distance of 200 to 300 feet from the nozzle. As in the case of the tractor equipment, this experimental unit may be used either as a duster or sprayer, or the dust may be conditioned with moisture as it leaves the nozzle. Power is furnished by a 22- to 25-horsepower gasoline motor and the total weight of the unit with the insecticide tank full is somewhat less than 1,000 pounds.

Both manufacturers of spray equipment and chemical companies engaged in the formulation of insecticides are showing great interest in the possibilities that exist for applying many of our more commonly used insecticides as liquid concentrates or conditioned dusts. Manufacturers made available to entomologists and chemists in 1945 several types of portable blowers for experimental work. These were used to treat mosquito and fly breeding grounds, street trees, wood lots, recreational areas, orchards, and row crops with good results.

Only a limited number of insecticides have been applied experimentally with blower equipment of this general character. These include DDT in various formulations, nicotine in concentrations up to 1 quart of alkaloid in 1 gallon of solution, benzene hexachloride, and cryolite in concentrations of 4 to 6 pounds in a gallon of water with an emulsifier and an adhesive added. During 1946 experimental work was continued and expanded to include other insecticides and additional crops. Soon entomologists, chemists, and engineers expect to be much closer to an answer to the question : To what extent will finely atomized insecticide concentrates and conditioned dusts replace dilute sprays and conventional dusting procedures?

THE AUTHOR W. L. Popham since 1941 has been an assistant chief of the Bureau of Entomology and Plant Quarantine. A native of Montana and a graduate of Montana State College, Mr. Popham joined the Department in 1924, as State leader for Montana in black stem rust control; in 1930 he became a field supervisor for 13 North Central States, and in 1934, in Washington, he was placed in charge of barberry eradication work.