Structure of populations of Lygus hesperus (Hemiptera: Miridae) with multiple resistance mechanisms to trichlorfon

Five populations of Lygus hesperus Knight collected from alfalfa fields in Utah and Idaho were exposed to a range of concentrations of trichlorfon in a contact bioassay using plastic, self-closing bags. Various degrees of resistance were noticed. Esterase activity and acetylcholinesterase (AChE) insensitivity were assayed in 160 individuals from each population. Esterase activity of the most resistant population from Idaho was more than four times that of the susceptible Utah population, and AChE of the most resistant population was only one-third as sensitive to 10(-7) M paraoxon. We detected a linear correlation between esterase activity and AChE insensitivity levels with the logarithm of the LC50s. Population structures were discerned from distribution patterns of esterase activities and AChE insensitivities, as well as from slopes of the responses. The susceptible population was very homogeneous and the resistant populations were more heterogeneous. Interactions between the multiple resistance mechanisms are discussed.