The best time to apply insecticides from the air is usually from daylight to 9 or 10 o'clock in the morning. Air movement and temperature then are at their lowest and humidity at the highest. In some localities the short time just before sunset also is satisfactory.

The fundamental factors interact closely. No one can be isolated and studied by itself. Furthermore, when the control of an insect is being studied the effects of all factors must be considered in respect to its habits and environment, and the insecticide to be used.

Many of the new organic insecticides, primarily the chlorinated hydrocarbons and hydrocarbons containing phosphorus, are highly effective in small amounts. Their development has advanced all methods, air and ground, that employ low volumes of concentrated insecticides. The discovery of each has made it necessary to work out spray or dust formulas for each nest by testing the new compound in combination with other compounds, with different solvents or dust carriers, and at different concentrations.

Some research has been directed toward finding how the physical properties of spray liquids affect dispersal and the efficiency in killing insects. An example is the finding that liquids of high viscosity are more coarsely atomized than less viscous ones and therefore give a narrower spray swath. The more volatile the spray, however, the more rapidly it evaporates; therefore a smaller amount reaches the insects.

Compared to the amount of research on distributing apparatus and insecticides for use in aircraft, the research on developing aircraft especially for spraying and dusting has been very limited. Most of the need for planes immediately after the war was met by adapting war-surplus biplane trainers because they were cheap and sturdy and could carry up to 1,200 pounds of insecticide. Many an operator, however, has preferred small, two-place, high-wing monoplanes, particularly for treating small acreages. Their performance has been improved. Some spray and dust equipment has been designed so it can be removed easily and the plane can be used for other purposes.

One builder of aircraft dispensing equipment has designed a spray tank in the shape of a seat, the back and bottom having a capacity of about 3o gallons. Such tanks need not be removed.

THE HELICOPTER was designed for general-purpose uses, but its ability to fly low and slow has been of particular advantage in spraying and dusting. It is well suited for treating small, inaccessible areas. It can be landed near or in the field being treated, and considerable ferry time is saved. The helicopter is said to be more effective than fixed-wing aircraft in giving thorough coverage to plants, but its first cost and operating costs are quite high.

In order to fill the need for a fixed-wing aircraft which would be more suitable for agricultural purposes than war-surplus trainers, the National Flying Farmers Association, Texas Agricultural and Mechanical College, the Civil Aeronautics Administration, and the United States Department of Agriculture sponsored the development of such a plane. The prototype model was constructed at the Personal Aircraft Research Center of the Texas Agricultural and Mechanical College. The design was based on findings in a survey among commercial operators and research organizations to determine the essential characteristics that it should have.

SAFETY was given special consideration. The cockpit is located so as to give the pilot excellent visibility. For protection in a forced landing, all loads and heavy masses are located in the wings or forward of the cockpit, and the pilot has a special seat, safety belt, and shoulder harness. The leading edge of the landing gear is sharp so it can cut wires it might accidentally touch in flight. Two structural members and a crash tripod over the cockpit give additional protection. The all-metal, low-wing monoplane can carry 1,200 pounds of insecticide. It operates at speeds up to 100 miles an hour. Specially designed flaps and ailerons give excellent slow-flight characteristics and slow landing speeds, less than 40 miles an hour.

SPECIAL TYPES of distributing apparatus were developed. Space for the equipment is in the fuselage and the wing, which was made extra thick for the purpose. The sprayer has tanks and a boom in the wing and an engine-driven pump in the fuselage. The fuselage has a dust hopper with a conventional spreader underneath. On the drawing boards were plans for other types of distributing apparatus, particularly duster units mounted in the wing to give a wider and more uniform swath and equipment for distributing seeds and fertilizers. The plane was flown successfully in 1950.

J. S. YUILL has been an entomologist in the division of forest insect investigations, Bureau of Entomology and Plant Quarantine, since 1935. Since 1946 he has been engaged in the development of aerial spraying for forest-insect control. He attended the University of Arizona and the University of California.

D. A. ISLER is a senior agricultural engineer in the division of farm machinery, Bureau of Plant Industry, Soils, and Agricultural Engineering. A graduate of Ohio State University, he joined the Department of Agriculture in 1927 and has worked on the development of equipment for control of various insect pests. Since 1945 he has worked at the Agricultural Research Center at Beltsville, Md., on the development of aerial spraying equipment for control of forest pests.

GEORGE D. CHILDRESS is chief of the aviation extension division in the Office of Aviation Development, Civil Aeronautics Administration. A native of Virginia, he engaged in commercial airport and flying school operations in and around Roanoke from 1927 to 1939, when he joined CAA as an assistant aeronautical inspector.