Since 1925, Clyde C. Hamilton has been concerned with studies of the many insects which attack ornamental plants, and nursery and shade trees. DuringWorld War II, when Hamilton served with the War Production Board in Washington, D. C., Filmer handled the essential work on this project.
Nicotine and pyrethrum have proved to be especially efficient insecticides when used against plant lice, but cost has always been a limiting factor in their use. Plant lice were so important as pests and the insecticides so efficient in combating the pests that Headlee set out in 1927 to find a method of decreasing the size of the requisite toxic dose. Headlee, Filmer, and Hamilton worked on the premise that successful use of the insecticide depended on its being brought into contact with the inner tissues of the insect. When they found that sodium oleate soap would lower surface tension and permit wetting of the insect, it became possible to greatly decrease the dosages of pyrethrum and nicotine.
In the early '30s, studies of control of spider mites on greenhouse plants, especially roses, led to the development of derris oil emulsion. But though this material controlled mites, after three or four treatments rose foliage suffered damage and leaves dropped off. Thiocyanates and rotenone products which were used to replace the derris oil emulsion, were not found to be completely satisfactory. Then, starting in 1940, investigations of azobenzene showed that if this material was applied by burning in piles or as an aerosol from a pressure fumigator, excellent mite control resulted, although sometimes rose blooms were thereby injured. Following World War II, TEPP and other phosphate insecticides were developed and tested as aerosols, and since 1950, a number of new acaricides using mist blowers for application, have been tested against mites attacking nursery stock.
In the '30s, a nicotine-bentonite paste was found to give excellent control of plant borers. About the same time, so-called colloidal lead arsenate was found effective against brown garden beetle because in this form it penetrated the soil better.
In the late '30s pyrethrum sprays were in general use for control of leaf roller on roses and other greenhouse insects, but conventional spraying was very laborious and time-consuming. Pressure fumigators on the other hand were found to save time and give effective insect control. These fumigators also gave control of orchid scale and were used until DDT was introduced after the war to control both the scale and orchid weevil. The necessity for reducing cost of pest treatment also led to the study of electric dusters and a search for lighter dust diluents which would be easy to apply, give better coverage of plants, and consequently better insect control.
Since World War II, DDT has been found effective against many plant borers. On root feeders, chlordane, the new insecticide, was found to give outstanding control of taxus weevil or black vine weevil, a serious pest of Taxus, azaleas, and rhododendrons. A heavy dosage of chlordane was also found to give control of the oriental earthworm in golf greens.
Use of mist blowers (which deliver a small amount of spray into an air blast) for insecticide application to shade trees and nursery stock has been developed recently, and has resulted in great reductions in amount of insecticide needed and cost of application. Simultaneously with the development of mist blowers has come the development of new insecticides which can be applied either by mist blowers or conventional spraying and dusting equipment. Another new and outstanding development is the use of BHC, lindane, or DDT for control of leaf miners.
One of Hamilton's many valuable contributions to the whole field of entomology was his sponsorship of Entoma, a manual of sources of insecticides, equipment, and entomological services. He not only suggested the manual but served as its editor for the first five editions. Entoma, now published by the Entomological Society of America, is an invaluable source of pest control information for entomologists and those in related fields.