The protection of sheep against blowfly strike. IV. An evaluation of certain organic phosphorus compounds

Abstract

An evaluation of the phosphorus insecticides by means of the bio-assay method has revealed that certain of these compounds combine very good larvicidal action with outstanding diffusing power in the fleece of living sheep. They compare favourably in this respect with the insecticides previously tested (Du Toit and Fiedler, 1953), the best of which was Aldrin with an average period of protection of 39 weeks. Dieldrin gave a protection of 37 weeks, and gamma B.H.C. 33 weeks. In addition to the properties mentioned, however, on which the duration of protection depends such compounds must comply with other demands necessitated by the environmental factors encountered on the bodies of sheep. To start with such protecting agents must not stain or damage the wool fibres and secondly they should be stable in a slightly alkaline medium in order to produce the maximum residual effect in all situations such as even in the soiled wool of the crutch region. From this point of view Parathion and EPN 300 must be excluded for practical purposes as they discolour the treated wool imparting to it a strong yellow-green tint Furthermore, their general use cannot be recommended on account of their toxicity to warm-blooded animals. The only phosphorus insecticide which seems to comply with all the requirements of a protecting agent for sheep against blowfly strike is Diazinon. Its larvicidal action as well as its diffusing power in the fleece are superior to insecticides like Dieldrin and B.H.C. which have proved to be very efficient protecting agents in practice. In the light of the properties studied Diazinon may be expected to produce a longer period of protection than B.H.C., which is not stable in an alkaline medium; and it may be found even to surpass Dieldrin in this field of application. Field experiments to demonstrate the actual period of protection of Diazinon in comparison with Dieldrin and B.H.C. are well under way in some of the worst blowfly areas of the Karoo and will be reported on in due course.