Particle size of insecticidal suspensions and their contact toxicity vi. Effect of temperature on relative toxicity

Two aqueous suspensions, one containing crystals of uniform size and the other containing colloidal particles, were made with each of seven solid contact poisons (two DDT-analogues, DDT, rotenone, 2-bromomercurithiophen, dieldrin and endrin). The relative toxicity of each pair of suspensions was found in dipping or measured-drop tests on as many as possible of the species Oryzaephilus surinamennsis L., Tribolium castaneum Herbst and Tenebrio molitor L. In addition, some tests were made by injection of colloidal suspensions. In each test, insects were kept after treatment at two temperatures: 28 degrees C. and 11, 17 or 20 degrees C.; counts of kill were made after 24 hr. The tests measured three different kinds of temperature coefficient of insecticidal action. In the contact tests, the colloid was nearly always more toxic than the crystals. In all the tests, except those with dieldrin, the relative toxicity (colloid : crystals) was greater at the lower after-treatment temperature, i.e. the temperature coeficient of the relative toxicity was negative. But in the case of dieldrin, the coefficient was positive. The DDT-analogues, DDT, rotenone and endrin were more toxic at the lower after-treatment temperature, i.e. they had negative temperature coefficients of kill by contact action. In all these cases the temperature coefficient of kill by contact action was greater for colloid than for crystals. Dieldrin and 2-bromomercurithiophen had positive temperature coefficients of kill by contact action. With dieldrin, the temperature coefficient was greater for colloid than for crystals; but with 2-bromomercurithiophen, the reverse was true. The temperature coeficient of kill by injection was negative for DDT, but positive for dieldrin and endrin; the other poisons were not tested by injection. A possible explanation for the results of the contact tests is given in an Appendix. The explanation is based on a number of assumptions about the penetration of insect cuticle by solid poisons. These assumptions lead to the conclusion that the effect of temperature on the relative toxicity depends on the temperature coefficient of kill by internal action of the poison on the insect. This can be measured by injection tests. If it is negative or zero, the ratio of toxicities (colloid: crystals) by contact action, measured quite soon after treatment of the insects, will be greater at a low temperature after treatment than at a high temperature after treatment; but if the coefficient is positive, the effect of temperature on relative toxicity cannot be foretold. The experimental results seem to confirm the assumptions.