The susceptibility of tsetse flies to topical applications of insecticides. VI.—Data on more chlorinated hydrocarbons and organophosphates, and a general discussion

Extract

Five insecticides (Thiodah, Bromodan, dimethoate, Dipterex (trichlorphon) and Sumithion) were applied topically in solution to young adults of the tsetse fly Glossina morsitans Westw. The toxicity of Thiodan (LD50 for males, 0·0012 µg.; for females, 0·0014 µg.) lay between that of Telodrin and dieldrin, and that of Bromodan (LD50 for both sexes combined, 0·0032 µg.) between that of dieldrin and γ BHC. Dimethoate, Dipterex and Sumithion were all less toxic than Baytex, but were not fully evaluated.

Results recorded in this paper and earlier ones in this series, and those of workers using Musca domestica L., are discussed in an attempt to rationalise the search for new toxicants for tsetse flies. Chlorinated hydrocarbons fall into two classes, in both of which there is an approximate positive correlation between toxicity to Musca and to Glossina. One class, including Telodrin, Thiodan and Bromodan, is exceptionally toxic to tsetse and more than 40 times as toxic to Glossina as to Musca. These have a molecule with a particular type of nucleus (hexachloro-l,4-methano-cyclohexene) with a single bridged unsaturated ring. The more common chlorinated hydrocarbons (γ BHC, DDT and aldrin) are two to five times, and dieldrin 12 times, as toxic to G. morsitans as to M. domestica, and all are very toxic to both species.

Among organophosphorus compounds there is very little correlation between toxicity to M. domestica and that to G. morsitans, but it appears probable that no compound of low toxicity to Musca (i.e., LD50 of the order of 0·5 µg.) would be likely to compete with those compounds already known to be most toxic to Glossina. Replacement of the nitro group on the benzene ring of Sumithion by a thiomethyl group, to form Baytex, increases toxicity to Glossina fivefold, but that to Musca by only one-fourth. There is less information on organocarbamates, and it may be a coincidence that 2-isopropoxyphenyl N-methylcarbamate, the most toxic of those tested on G. morsitans, was also most toxic to Musca.