Enhanced esterase gene expression and activity in a malathion-resistant strain of the tarnished plant bug, Lygus lineolaris.

Extensive use of insecticides on cotton in the mid-South has prompted resistance development in the tarnished plant bug, Lygus lineolaris (Palisot de Beauvois). A field population of tarnished plant bugs in Mississippi with 11-fold higher resistance to malathion was used to examine how gene regulation conferred resistance to this organophosphate insecticide. In laboratory bioassays, synergism by the esterase inhibitors S,S,S,-tributylphosphorotrithioate (DEF) and triphenylphosphate (TPP) effectively abolished resistance and increased malathion toxicity by more than 80%. Esterase activities were compared in vitro between malathion susceptible and resistant (selected) strains. More than 6-, 3- and 10-fold higher activities were obtained with the resistant strain using alpha-naphthyl acetate, beta-naphthyl acetate, and p-nitrophenyl acetate, respectively. Up to 95% and 89% of the esterase activity in the susceptible and resistant strains, respectively, was inhibited by 1 mM DEF. Inhibition of esterase activity up to 75% and 85% in the susceptible and resistant strains, respectively, was obtained with 0.03 mM TPP. Esterase activities in field populations increased by up to 5.4-fold during the fall season. The increase was synchronized with movement of the insect into cotton where exposure to pesticides occurred. Esterase cDNA was cloned and sequenced from both malathion susceptible and resistant strains. The 1818-nucleotide cDNA contained a 1710-bp open reading frame coding a 570 amino acid protein which was similar to many insect esterases conferring organophosphate resistance. No amino acid substitution was observed between susceptible and resistant strains, indicating that esterase gene mutation was not involved in resistance development in the resistant strain in Mississippi. Further examination of esterase gene expression levels using quantitative RT-PCR revealed that the resistant strain had a 5.1-fold higher level of esterase mRNA than the susceptible strain. The results of this study indicated that up-regulation of the esterase gene appeared to be related to the development of resistance in the tarnished plant bug.