Cross-resistance pattern and basis of resistance in a thiamethoxam-resistant strain of Aphis gossypii Glover.

A thiamethoxam-resistant strain of cotton aphid (ThR) displayed a 13.79-fold greater resistance to thiamethoxam than a susceptible cotton aphid (SS) strain. Piperonyl butoxide (PBO) and triphenyl phosphate (TPP) synergistically increased the toxicity of thiamethoxam in the resistant strain, whereas diethyl maleate (DEM) did not exhibit significant synergistic effects. Bioassay results indicated that the ThR strain developed increased levels of cross-resistance to bifenthrin (11.71 fold), cyfluthrin (17.90 fold), esfenvalerate (6.85 fold), clothianidin (6.56 fold), methidathion (5.34 fold) and alpha-cypermethrin (4.53 fold) but did not show cross-resistance to malathion, omethoate, acephate, chlorpyrifos, methomyl, sulfoxaflor or imidacloprid. PBO and TPP increased bifenthrin toxicity in the resistant strain by 2.38 and 4.55 fold, respectively. Quantitative real-time PCR results indicated that the mRNA expression levels of the α1, α4-1, α4-2, α5 and α7 subunits decreased significantly by 3.32, 1.60, 2.05, 5.41 and 1.48 fold, respectively, in the resistant strain compared with those in the susceptible strain. However, significant differences were not observed in the expression of the α2, α3 and β1 subunits. No target-site mutations within the α1, α2 and β1 subunits of nicotinic acetylcholine receptors (nAChRs) were detectable in the ThR strain. In conclusion, the levels of thiamethoxam resistance and cross-resistance to other insecticides observed in the ThR strain are likely regulated by two mechanisms, which include the overexpression of detoxification-related P450s and esterase. These results should be useful for the understanding thiamethoxam resistance mechanism and the management of insecticide-resistant cotton aphids in China.