Activation of Malathion by the Locust Bodywall

Abstract

WE reported earlier¹ that dilution of malathion in the mineral oil KEB (kerosene extract bottoms) not only increases its toxicity to mature adults of the desert locust, Schistocerca gregaria, but also the speed of its action, when applied topically on the hind wing base. We later discovered² that the killing power of a 10 µg/g dose of malathion 2.5 per cent solution in KEB varies considerably with the site of application on the locust body. Thus it yielded a 100 per cent kill when applied topically to the neck membrane, no deaths at all on the hind femur and intermediate values for other sites of application. Injections into the body cavity were always less toxic than topical application in the same general area of the body. There was, however, no significant difference in the killing power of malaoxon (2.5 per cent solution in KEB) topically applied to the neck membrane when compared with its injection through the same membrane³. This led us to believe that topically applied malathion is converted to its P = O analogue, malaoxon (which is more toxic and more rapid in action than malathion) by the locust bodywall and that the KEB only helps its penetration and conversion by spreading the insecticide on to a wider area of the insect bodywall. The experiments reported here were designed to show this activation by the bodywall as well as penetration differences and to explain the differences in toxicity and speed of action reported earlier¹.