AbstractUsing techniques for treating the insects unanaesthetised, knockdown of aduitMusca domesticaby pyrethrin I applied topically to or injected into the thorax was assessed by measuring the ED50s on 13 occasions 1 min‐48 h after treatment. From 1 min to approximately 1 h after treatment, ED50s for topical application decreased quickly with increasing time. During this “knockdown phase” ED50values were probably determined mainly by rate of penetration of the insecticide into the insect, but the rate of elimination of the insecticide probably determined the increasing ED50values during the “recovery phase” which followed. ED50s for injected pyrethrin I were initially much smaller than after topical application but increased continuously until 24 h after treatment. From 100 min onwards ED50s by topical application and injection were similar. These results indicate (1) that the site affected during knockdown is within the insect rather than superficial, and (2) that detoxication and excretion processes determine the ultimate toxicity of the insecticide apparently independently of the method of treatment. Additional tests with a strain of flies having the penetration delaying factor and with the cockroachPeriplaneta americanaL. supported these conclusions.Speed of knockdown after topical treatment was also affected by the site of application and the solvent in which pyrethrin I was applied.Bioresmethrin was about as effective as pyrethrin I in knocking down flies when applied topically or when injected, but because the ED50increased very little during the recovery phase it was ultimately much m
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