Ever since he had been in New Jersey, Headlee studied various phases of the relation of climatic factors to insects. He determined that the dominant climatic factor was temperature, modified to a limited extent by atmospheric moisture. Constant temperature, he showed, was a much more powerful influence than varying temperature. In fact, the temperature factor was so dominant that he found it was possible to time spraying for codling moth on the basis of temperature alone.
While studying the damage of bean weevils to beans in storage, it was found that when the atmospheric moisture was held at 30 per cent or below, the weevils were destroyed and the beans protected. It developed, however, that the material used to produce low atmospheric moisture, whether hygroscopic or not, was effective in preventing the insect from penetrating the beans. This led to study of finely divided solid materials as barriers to the penetration of seeds and grain by insects. Seeds and grain in storage, it was found, could be protected with a small amount of finely divided solid, such as a colloidal clay.
While studies of the effect of constant and variable temperatures on the development of parasitic insects and mosquito larvae continued, greater emphasis was placed on the exploration of the field of radiant energy as a practical means of insect control.
In the use of radio frequency electrostatic fields for destroying insects, a wide range of voltage and frequency combinations were employed in the search for differential heating between insect tissue and the plant or other materials with which the insects were associated. In the range between one and fifty million cycles per second, no practical differential effect was observed largely because of the complex geometry and dielectric pattern of the media and the presence of appreciable moisture. Between 1932 and 1938, when this phase of the work was discontinued, such diverse combinations of insect associations as Japanese beetle grubs in soil balls of evergreen nursery stock, cigarette beetles and larvae in baled tobacco, and stored products insects in grain and packaged cereals were treated. The limitations of equipment then available prevented investigation of the ultra-high frequency radio wave spectrum where the selective energy-absorption phenomena were more likely to occur. In most instances, the costs of generating lethal temperatures in infested stored products by this method were too high economically, and the danger of damaging living plants too great to make the procedure practical.
Investigations on the responses to visible, ultraviolet and infra-red light followed the development of light traps for mosquitoes, moths, and other nocturnal insects. Since practical applications at that time were an important objective, the known applications of radiant energy manifest in the production of thermal effects were subjected to review and attempted improvement. This resulted in a study of procedures for heating stored products and fabrics with minimum damage to the quality of those materials.
After a period of exploration of various electrical phenomena as they may affect insects in the soil and in plant and stored products, which yielded no remarkable practical results, these studies were terminated in 1939. Investigations of the effect of temperature on insect development were continued until 1943 when various explorations along these lines were conducted as part of the broader ecological field.