The control of mosquitoes, tsetse flies and stableflies by insecticides usually requires chemicals which can last for weeks or months in spray deposits. Natural pyrethrins do not have this property and cannot compete in this field of application with, for instance, the organo-chlorine compounds such as DDT or dieldrin. However the development of resistance to the established insecticides and the increasing concern about their polluting tendencies has resulted in a search for replacements which while having an adequate persistence where they are applied are likely to decompose in the outside environment. Some of these have come from among the newer synthetic pyrethroids. Earlier pyrethroids were not really toxic enough to these insects but the discovery of tetramethrin and resmethrin has shown that related chemicals can be even more toxic than the best insecticides from other groups. The effects on toxicity, measured by topical application, of variations in the alcohol and acid components in relatives of these two insecticides are described. Stereoisomerism is an important factor and the cis isomers are always more toxic than the trans. Replacement of the side-chain methyl groups in the acid moiety by halogens such as chlorine and bromine also increases toxicity. Alterations of the alcohol component can increase or decrease activity and one compound, NRDC 161, is the most toxic chemical that we have ever tested. Since persistence at the site of spraying is the essential attribute of possible replacement chemicals measurements of contact toxicities, with the mosquito A. stephensi as the test insect, were also made. Resmethrin, an ester of 5-benzyl-3-furylmethyl alcohol and chrysanthemic acid, is not persistent because it is decomposed by light. Substitution with 3-phenoxybenzyl, as in phenothrin, improved light stability but not to the extent required under field conditions. Phenothrin still contains the light-sensitive chrysanthemic acid and replacement of this by the 2,2-dichlorovinyl analogue has given an ester which is as long-lasting as the best compounds from other chemical groups as well as possessing a greater intrinsic toxicity. Despite these improvements the newly-developed compounds have some disadvantages for the control of these biting insects. They are still too expensive at the present time and some which are the best insecticides are also losing the low vertebrate toxicity which was one of the advantages of the earlier pyrethroids. Also like the natural esters, the synthetic compounds tend to irritate insects which are resting on spray deposits. Consequently some leave the source of toxicant before they have received a toxic dose. This is especially likely to happen when lower, more economical dosages are employed.
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Barlow et al. (1975) studied this question.