Enzymes mediating resistance to lambda-cyhalothrin in Eriopis connexa. (Coleoptera: Coccinellidae)

Resistance to widely used insecticide, lambda-cyhalothrin, was recently reported in the predatory lady beetle Eriopis connexa (Germar) (Coleoptera: Coccinellidae). However, to understand whether metabolic mechanisms underlie such resistance, synergism bioassays and in vitro studies were carried out by using inhibitors and model substrates for enzymatic assays, respectively. The LD(50)s estimated for susceptible and resistant populations rig of lambda-cyhalothrin/insect, and thus, a 22-fold difference in resistance ratio. Synergism ratios for the susceptible population with piperonyl butoxide (PBO), diethyl maleate (DEM), triphenyl phosphate (TPP), and S,S,S-tributylphosphorotrithioate (DEF) were respectively 33.8-, 0.24-, 0.35-, and 4.25-fold, while for the resistant population, they were 1463.0-, 0.79-, 0.85-, and 282.6-fold, respectively. The synergized resistance ratios were 0.50-, 2.00-, 6.75-, and 8.77-fold with PBO, DEF, DEM, and TPP, respectively, while resistance was virtually suppressed with DEF. The esterase exhibited 4.16-, 4.03-, and 5.38-fold greater activity towards formation of alpha-naphthol, beta-naphthol, and 4-nitrophenol in the resistant population of E. connexa than in the susceptible population. The activity of esterase depended on concentrations of DEF applied, either using alpha-naphthol or beta-naphthol, which completely inhibited the activity at 636 eta M. The PBO inhibited the beta-naphthol formation in approximately 50%, suggesting it as inhibitor of esterases. The activities of glutathione-S-transferase were similar and corresponded to 0.36-0.47 eta mol(-1) min(-1) mu g of protein, for S and R populations, respectively. Similarly, the activities of cytochrome P450-dependent microsomal monooxygenases were 0.04 and 0.05 eta mol(-1) min(-1) mu g of protein. The native gel indicated that the formation of beta-naphthol was completely inhibited by methyl-paraoxon, but only partially inhibited by eserine, TPP, and PBO. Although other studies with DEF and PBO have demonstrated strong inhibition of type B carboxylesterase associated with insecticide resistance, the results reported here do not rule out metabolism by cytochrome P450-dependent microsomal monooxygenases as a factor conferring E. connexa resistance to lambda-cyhalothrin and confirmed that PBO may also act by inhibiting esterases of insects. (C) 2014 Elsevier Inc. All rights reserved.