Kinetics of insensitive acetylcholinesterases in organophosphate-resistant tobacco whitefly, Bemisia tabaci (Gennadius) (Homoptera: Aleyrodidae)

Resistance to organophosphorus and carbamate insecticides in Bemisia tabaci is due to insensitivity of the target enzyme acetylcholinesterase. Different forms of insensitive enzyme were identified in populations from Israel and Pakistan. The degree of insensitivity varied from 116-fold (aldicarb sulfoxide) to 2800-fold (monocrotophos) in the Israeli form, and from 5-fold (aldicarb sulfoxide) to >4000-fold (propoxur) in the Pakistani form when compared with the sensitive acetylcholinesterase of the standard susceptible strain. In bioassays, the Israeli and Pakistani populations showed moderate resistance relative to the high levels of insensitivity. Whereas resistance factors for monocrotophos were 20-fold (Israel) and 40-fold (Pakistan), the insensitivity factors were 2800- and 1400-fold, respectively. The kinetics of the interactions between the enzymes and substrate, in both the presence and absence of insecticides, are described to explain why high degrees of insensitivity can yield relatively low resistance factors. The relevance of our findings to the field performance of insecticides is discussed. (C) 1997 Academic Press.