Ishaaya, I., Department of Entomology, ARO, The Volcani Center, Bet Dagan 50-250, Israel
Mendelson, Z., Department of Entomology, ARO, The Volcani Center, Bet Dagan 50-250, Israel
Ascher, K.R.S., Department of Toxicology, ARO, The Volcani Center, Bet Dagan 50-250, Israel
Casida, J.E., Pesticide Chemistry and Toxicology Laboratory, Department of Entomological Sciences, University of California, Berkeley, CA 94720, United States

Pyrethroid esterases are a major factor in the tolerance of adults of the whitefly Bemesia tabaci to cypermethrin and related pyrethroids. This conclusion is based on the substrate specificity of these esterases and their inhibition by organophosphorus compounds acting as synergists. Whitefly esterases hydrolyze trans-permethrin faster than its cis-isomer or its α-cyano analogs trans- and cis-cypermethrin and deltamethrin. With trans-permethrin as the substrate, these pyrethroid esterases are sensitive to in vitro inhibition by monocrotophos and methamidophos (the active metabolite of acephate) with 50% inhibition at 9 × 10-7 and 10-5 M, respectively. The potency of cypermethrin under glasshouse conditions is synergized 5- to 50-fold by monocrotophos, acephate or methidathion and it is also greatly increased by profenofos, with synergist:pyrethroid ratios ranging from 1:8 to 8:1. Cypermethrin toxicity under field conditions in cotton is strongly synergized and the effective period for whitefly control is prolonged by adding an equal weight of monocrotophos or acephate or 8 parts of methidation to 1 part pyrethroid. Mouse liver pyrethroid esterases hydrolyzing cis-cypermethrin are inhibited by low intraperitoneal doses of profenofos and acephate but not monocrotophos and methidathion. The high magnitude of cypermethrin synergism in whiteflies is not repeated in mice, perhaps due to differences in the toxicological importance and inhibitor specificities of esterases involved in detoxification. © 1987.