A thermal preference method of bioassay of the toxicity of insecticidal films to house-flies.

Abstract

The method described in this paper was developed to compare the value of insecticidal deposits formed by spraying DDT or other insecticides formulated in several different ways, using house-flies, Musca domestica L., as the test species. The requirements were that the method should be sensitive enough to distinguish between deposits differing in toxicity by a factor of 1-5; that it should operate with an exposure time of about five minutes; and that the insects should be crawling over the deposit during the whole exposure period. Insects that cannot crawl on vertical glass surfaces and are reluctant to fly present no difficulty, but some way must be found of persuading such insects as house-flies to remain on the test surface.In the past, this difficulty has been overcome in one of three ways: -(a) The insects are enclosed in a space, part of the walls of which constitute the test surface. The insects have a free choice between treated and untreated surfaces when they settle.(b) All the surfaces enclosing the insects are treated with insecticide, so that the insects are always on a treated surface except when they are flying.(c) A single flat test surface is used, and the insects are in some way discouraged from crawling on the other surfaces and from flying.The methods under (a) are used when field conditions are to be simulated, so that a deposit that is repellent will appear to be less effective than one that does not repel. It is therefore not suitable for simple comparisons of toxicity.