Knockdown by pyrethroids: Its role in the intoxication process

Using techniques for treating the insects unanaesthetised, knockdown of aduit Musca domestica by 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” ED50 values 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 ED50 values 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 cockroach Periplaneta americana L. 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 ED50 increased very little during the recovery phase it was ultimately much more toxic.