KINETICS OF INSENSITIVE ACETYLCHOLINESTERASES IN ORGANOPHOSPHATE-RESISTANT TOBACCO WHITEFLY, BEMISIA-TABACI (GENNADIUS) (HOMOPTERA, ALEYRODIDAE)

Resistance to organophosphorus and carbamate insecticides in Bemisia t abaci is due to insensitivity of the target enzyme acetylcholinesteras e. Different forms of insensitive enzyme were identified in population s from Israel and Pakistan. The degree of insensitivity varied from 11 6-fold (aldicarb sulfoxide) to 2800-fold (monocrotophos) in the Israel i form, and from 5-fold (aldicarb sulfoxide) to >4000-fold (propoxur) in the Pakistani form when compared with the sensitive acetylcholinest erase of the standard susceptible strain. In bioassays, the Israeli an d Pakistani populations showed moderate resistance relative to the hig h levels of insensitivity. Whereas resistance factors for monocrotopho s were 20-fold (Israel) and 40-fold (Pakistan), the insensitivity fact ors were 2800- and 1400-fold, respectively. The kinetics of the intera ctions between the enzymes and substrate, in both the presence and abs ence of insecticides, are described to explain why high degrees of ins ensitivity can yield relatively low resistance factors. The relevance of our findings to the field performance of insecticides is discussed. (C) 1997 Academic Press.