The protection of sheep against blowfly strike. I. An evaluation of certain organic insecticides

1. Higher concentrations of the new organic insecticides are necessary to achieve a complete kill of first instar larvae of Lucilia cuprina Wied., when contact with the insecticides is short than is necessary for mature larvae. The former are able to free themselves rapidly from the toxicant in the nutritional medium owing to their feeding habits. 2. The larvicidal action of eleven synthetic insecticides has been studied and the minimum lethal dose determined on young maggots within a nutritional medium. 3. A new biological assay method for determining the larvicidal value of wool treated with various insecticides has been developed. 4. Insecticides of equal toxicity to first stage blowfly larvae afford different lengths of protection against strike even when their active ingredients are of the same concentration. 5. The importance of the larvicidal properties of insecticides in protecting sheep over long periods is stressed as of very much greater importance than the more transitory repellent properties to adult blowflies possessed by such insecticides. 6. The biological assay method has revealed that a number of insecticides such as gamma BH.C., Dieldrin, Aldrin and to a lesser extent Chlordane and Toxaphene possess properties of diffusion which permit of their penetration from previously treated wool on sheep into the constantly expanding zone of growing wool. Compounds of the D.D.T. group do not diffuse along the wool fibres. 7. The duration of protection of sheep afforded by any insecticide is dependent upon the larvicidal value and diffusion power of the particular insecticide. For this reason gamma B.H.C., Dieldrin and Aldrin have been found to be the compounds most suitable for this particular purpose.