Comparative Toxicity of Insecticides Administered in Various Ways to Several Species of Insects1

Abstract

Insecticidal activity of various materials was tested against the American cockroach, Periplaneta americana, in three ways: by blood stream injn., by contact, and by oral feeding. These methods evaluated materials in terms of blood stream toxicity, contact toxicity and stomach poison activity. The materials tested generally required smaller dosages by injection than by contact or by oral feeding. Contact toxicity usually required about twice the dosage of insecticide needed for blood stream toxicity. Pyrethrum was the only material that was equally toxic by contact or by injn. There was a striking similarity in the reaction of the cockroach to contact and stomach poisons. The materials not toxic by contact were not toxic when fed and those toxic by contact were effective insecticides when fed. Toxic dosages of insecticides by feeding were usually considerably higher than dosages of the same materials applied by contact. Sex differences in resistance to insecticides were exhibited in varying degree by the cockroach, depending on the insecticide used and the method of testing. With the exception of dichloroethyl ether, the female was always more resistant than the male. Sex difference was smallest in the blood stream test where the female averaged 1.6 times the resistance of the male. With both contact and oral feeding tests the female averaged 3-4 times the resistance of the male. The application of predetermined dosages to each individual insect increased the accuracy of the evaluation of insecticidal efficiency. Due to large sex differences it is important either to test the sexes separately or, in mixed populations, to have both sexes equally represented. Other insects used for blood stream and contact tests varied * greatly in their susceptibility to insecticides. In order of increasing resistance they were as follows: silkworm (Bombyx mori), catalpa worm (Ceratomia catalpae), cockroach (Periplaneta americana), squash bug, (Anasa tristis), and Japanese beetle (Popillia japonica). The mirkweed bug, Oncopeltus fasciatus, used in contact tests appeared to be more susceptible to insecticides than the cockroach while the mealworm, Tenebrio molitor, exhibited as high a degree of resistance as the Japanese beetle.