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Areas on the backs of sheep were sprayed with several insecticides and a comparison was made of the degree and length of protection thus provided against oviposition, and in one experiment against strike, by Lucilia cuprina when the sheep were exposed to a dense population of flies in an insectary. Indole plugs were tied into the fleece to provide conditions suitable for oviposition. Under these conditions, DDT gave better protection than other insecticides. In 2 per cent. concentration it gave excellent protection for 6-8 weeks; in 1 per cent. concentration the protective period was shorter, although valuable protection was still given. A noticeable feature of the DDT treatments was their long partial protection after some oviposition was permitted, an effect which was not as marked with other insecticides. Crude BHC preparations gave valuable protection when applied at 0.3 per cent. gamma isomer. Intermittent artificial rain, amounting to 17 inches, did not affect the protection afforded by 1.0 per cent. DDT or 0.5 per cent. BHC over a 64-day period. Chlordane gave some protection, but it was less effective than DDT, and chlorinated camphene did not give useful protection. In vitro tests indicated that none of the insecticides was ovicidal. However, in larvicidal tests BHC was extremely toxic, the vapour killing larvae rapidly even when contact with the solid material was prevented. None of the other insecticides had a fumigating action, although in high concentrations they were often lethal on contact. |
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