Effective treatment with ground equipment, on the other hand, is possible with considerably higher wind velocity. By attaching an apron or trailer 25 feet long to the boom, one can get satisfactory results in winds of 10 to 12 miles an hour. Spraying without an apron can be effective in winds of like velocity.

The size and shape of individual fields and the amount of acreage needing immediate treatment also influence the method of application. The larger and more open the field the more efficient will be treatment by aircraft because of the time spent in turning at the ends and in going from field to field. Ground equipment is likely to be more efficient than aircraft in small, hilly, odd-shaped fields and in those bordered by trees, buildings, or high-power lines.

When treatment is advisable early in the growing season before peas have blossomed and while the aphid infestation is still light, ground equipment may be preferable. On the other hand, if the crop is nearing maturity, with a growth of tall, heavy plants, or if the ground is wet, aircraft application is particularly desirable.

Experiments at Forest Grove, Oreg., during 1950, revealed the principal causes for the unequal distribution of sprays from airplanes as ordinarily used for the control of the pea aphid on peas and for other insects on low-growing crops. In flights made at a height of about 2 feet above the soil surface, with the wheels touching the pea vines, the spray deposits were effective for insect control over a swath about 50 feet wide. Similar results were obtained from flights at the 10-foot level. The spray was deposited unevenly over this swath, however, and under some conditions caused plant injury in spots where the insecticide was applied too heavily and relatively inferior insect control in spots where the application was too light. In experiments designed to correct this uneven distribution of the spray, separate flights were made with all nozzles closed except in groups of three, representing a 1-foot segment of the boom. Knowledge gained during these experiments has provided criteria for determining the most efficient nozzle spacings, minimum spray boom lengths, effective swath width, and a basis for improvements in airplane sprayers and dusters.

NICOTINE in sprays, dusts, or vapor has been more widely used for years than any other insecticide.

As a spray it is always combined with a soap or an emulsifiable oil. As a dust it is generally combined with hydrated lime or other powdered carriers. A cloth apron or trailer some 50 feet long is used so that the dust will circulate and settle on the plants and little will be blown away. Under favorable weather conditions, high kills of the aphid can be obtained with both sprays and dusts.

About 1940 a machine was devised which vaporized highly concentrated nicotine by passing it through the exhaust pipe of a gasoline engine. The vapor was expelled under a gas-proof cloth trailer 100 feet long. Kills of nearly 100 percent were obtained, but the operation was slow and costly.

Nicotine has little residual value (at most only a few hours after application) and is effective only when the air temperature is rather high, preferably in the 70's or 80's.

Rotenone was first used against the pea aphid as a spray about 1922 in Maryland. The value of the finely powdered root in dusts or sprays was shown in 1935 in experiments in Wisconsin. The dusts and sprays did not kill aphids as rapidly as nicotine, but they had a residual value and continued to kill for several days if the weather was favorable. Many tests were made subsequently with different diluents and by adding oils and conditioning agents to improve the performance of the mixtures. Rarely did rotenone give a very high degree of control, such as 98 percent, but the more effective combinations often gave satisfactory commercial control, particularly in humid localities.

DDT, properly formulated, is much better than rotenone. It gives both high initial kill and has good residual properties. In the eastern and central parts of the country a 1.5 percent DDT dust dissolved in a nonvolatile solvent gave 95 percent control of the aphid. In eastern Oregon and Washington, however, it has been necessary to use a dust containing 5 percent of DDT. This higher concentration of DDT affords a longer residual action and protects the late-planted peas from a reinfestation caused by aphids that fly in from other peas or alfalfa.

Aerosols began to be used against the pea aphid in 1944 in Maryland, Wisconsin, and Virginia. Concentrated insecticides in solution are introduced into a steel cylinder of 2 to 4-gallon capacity, and some liquefied gas under high pressure is added. Released through a valve, the liquefied gas volatilizes and disperses the insecticide as a mist. Excellent kills were reported, but the method is less commonly used than dusting or spraying.

Three canneries in Wisconsin used airplanes in 1942 and seven in 1943. By 1948, aircraft either airplanes or helicopters were employed by 102 out of 109 factories for applying insecticides to all or part of their canning-pea acreage. The aircraft applied 81 percent of the dusts and sprays used. Eighty-two of the factories used only DDT; 19 used a combination of DDT and rotenone.

By 1948, then, people considered DDT the nemesis of the aphids, but that hope was dispelled. DDT in minute quantities was found occasionally in the body tissues and milk of cattle fed treated pea vines before or after the vines had been made into ensilage. The possible danger to people from consuming meat or milk that might contain DDT discouraged the use of DDT against the pea aphid, and by 1951 only a small amount was used in Wisconsin, and that early in the season before peas had blossomed so sufficient time would elapse before harvest for nearly all of it to dissipate.

New phosphate compounds became available for testing in 1947 and they soon revealed a power to control the aphid just as well as DDT. They caused a high initial kill 100 percent in some plots. Parathion, the phosphate most commonly used in 1951, is highly poisonous to man and animals if it touches the skin, is inhaled, or is taken into the mouth. Applied to plants in the field as a spray or dust, however, it breaks down into relatively harmless compounds. If it is applied 10 to 14 days before harvest it is not a hazard in canned peas or on the ensilage. Consequently it has no long residual value, although tests in greenhouses indicate that it maintains its toxicity for at least a week when it is applied to peas growing in pots. Several years of work with parathion indicated that it is just as good under field conditions as any other material in preventing a build-up of the aphids that were not killed immediately after the application.

In Wisconsin in 1948 only a few growers of peas for canning used parathion but in 1949 parathion and other phosphates accounted for 63 percent of the acreage treated and in 1950 they were used on 81 percent of the treated acreage.

The extent and cost of control of aphids on canning peas alone are indicated in figures supplied by canners in Wisconsin, where an average of 37 percent of the Nation's pack is produced. Infestation was severe in Wisconsin in 1948. Aphid control was carried on by 109 of the 122 pea-canning factories, which treated 90,000 or 71 percent of the State's 126,000 harvested acres of canning peas. Dust was applied in the amount of 4,502,269 pounds. Only a few acres were sprayed. Materials cost nearly 295 thousand dollars. As the cost of application equals the cost of insecticides, the total cost was nearly 600 thousand dollars.

In 1949, the aphid was less serious, and 105 of the 121 factories growing peas treated 76,800 acres, 63 percent of the harvested acres. Dust in the amount of 2,442,990 pounds was applied and about 27,000 acres were sprayed; the estimated total cost was nearly 480,000 dollars. In 1950, 100 factories treated 63,935 acres, 54 percent of the 118,100 harvested acres. The dust used amounted to 2,031,033 pounds; 11,620 acres were sprayed. The total cost was set at 236,888 dollars.

The amount of dust applied has averaged 35 to 40 pounds an acre whether the application was made by aircraft or with ground equipment. Concentrated sprays generally are applied by aircraft at the rate of 3 to 5 gallons an acre. Dilute sprays applied with ground equipment have averaged too to 125 gallons an acre.

JOHN E. DUDLEY, JR., a graduate of the University of Wisconsin, was an entomologist in the Bureau of Entomology and Plant Quarantine from 1911 to 1951, when he retired. His first assignment was with the gypsy and brown-tail moth laboratory in Massachusetts. Since 1918 he had been in charge of the truck crop and garden insect investigations laboratory in Wisconsin.

WILLIAM C. Cook has been an entomologist in the Bureau of Entomology and Plant Quarantine since 1930. Before that time he was employed by the Montana Agricultural Experiment Station for work on cutworms. Since 1943 he has been stationed at Walla Walla, Wash., studying wire-worms and the pea aphid. He holds degrees from Cornell University and the University of Minnesota.