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1. Dimethoate was generally more toxic when ingested than when applied topically to similar sized larvae of Mamestra brassicae (L) (4th-instar), Spodoptera littoralis (Boisd) (4th-instar), Pieris brassicae (L) (3rd-instar) and Bombyx mori (L) (2nd-instar). 2. In vivo oxidation of ingested 14C-dimethoate to omethoate decreased in the order of S. littoralis > P. brassicae > B. mori and followed the order of decreasing susceptibility. 3. 14C-dimethoate was regurgitated and excreted and was related to feeding rates of the species. 4. The implications of the above data in relation to causes of variation in interspecies susceptibility with particular reference to concepts of resistance as influenced by polyphagy versus monophagy are discussed.

1. Lindane was generally more toxic when ingested than when applied topically to similar sized larvae of Mamestra brassicae (L) (4th-instar), Spodoptera littoralis (Boisd) (4th-instar), Pieris brassicae (L) (3rd-instar), and Bombyx mori (L) (2nd-instar). 2. In vivo metabolism of ingested 14C-lindane decreased in order of S. littoralis > P. brassicae > B. mori (based on the recovery of metabolised 14C-lindane form the larva and frass) and followed the order of increasing susceptibility. 3. 14C-lindane was regurgitated after ingestion; its effects on insecticidal action are discussed. 4. The causes of variation in interspecies susceptibility with particular reference to concepts of resistance as influenced by polyphagy versus monophagy are discussed.