Reduced sensitivity of cholinesterase as a factor of resistance in leptophos selected strain in the Egyptian cotton leafworm.

Larvae of a leptophos resistant strain of the Egyptian cotton leafworm Spodoptera littoralis (Boisd) showed varying levels of cross resistance to several organophosphorus insecticides and to the carbamate matacil. Enzymatic determinations revealed that levels of cholinesterase activity were almost identical in larvae of susceptible and resistant strains. Resistant larvae, which remained susceptible to the carbamate methomyl possessed cholinesterase of nearly normal sensitivity to this compound. On the other hand, there was a considerable degree of insensitivity of cholinesterase to inhibition by leptophos-oxon, methyl paraoxon, cyolane, cytrolane, monocrotophos, and matacil in larvae of the resistant strain. It was concluded that reduced sensitivity of cholinesterase enzyme could be considered as an important factor which complements other variables contributing to the mechanism of resistance in larvae of the Egyptian cotton leafworm.