AbstractComparisons were made between the amounts of abamectin and total radioactivity recovered fromSpodoptera littoralisandHeliothis armigeralarvae after topical application of [3H]abamectin. Penetration (as shown by wash‐off experiments) did not differ significantly between theSpodopterainstars. Significantly more abamectin was recovered from ventral nerve cord samples of larvae showing symptoms of poisoning than from larvae not showing these symptoms. Fifth‐instarS. littoralislarvae had a significantly lower proportion of radioactivity as abamectin in the ventral nerve cord than in sixth‐instarS. littoralisor fifth‐ and sixth‐instarH. armigera. The proportion of radioactivity present as abamectin (but not total radioactivity) was significantly increased when the fifth‐instarS. littoralislarvae were pre‐treated with piperonyl butoxide (PB) suggesting that the relative insensitivity of fifth‐instarS. littoralislarvae to abamectin is due at least in part to greater metabolism, particularly by microsomal oxidases. Fat body samples consistently had a greater proportion of radioactivity as abamectin than the nerve cord and the former may act as storage site
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