ELEVATED ESTERASE-ACTIVITY AND ACETYLCHOLINESTERASE INSENSITIVITY IN CITRUS THRIPS (THYSANOPTERA, THRIPIDAE) POPULATIONS FROM THE SAN-JOAQUIN-VALLEY OF CALIFORNIA

Esterase activity levels and the degree of acetylcholinesterase (AChE) insensitivity to organophosphate (OP) and carbamate insecticides were determined for a susceptible strain of citrus thrips, Scirtothrips ci tri (Moulton), from Mexico (ME87) and for several populations of this species from the San joaquin Valley of California, where resistance to dimethoate and formetanate has been documented. Populations differed significantly in their mean esterase activity levels. They exhibited 1 2-45 times the esterase activity of the ME87 strain. Residual AChE act ivity in the presence of insecticide, expressed as the proportion of c ontrol AChE activity, was low for the ME87 strain, both in the presenc e of the carbamate propoxur (0.026) and the OP paraoxon (<0.004). In c ontrast, residual AChE activity among the field populations ranged fro m 0.151 to 0.302 in the presence of propoxur and from 0.188 to 0.598 i n the presence of paraoxon.