Resistance mechanisms and risk assessment regarding indoxacarb in the beet armyworm, Spodoptera exigua

The beet armyworm, Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae), is a worldwide polyphagous pest with a strong ability to develop resistance. After 12 generations of selection by indoxacarb, a resistant strain (RR-indox) was obtained with a 240.04-fold resistance ratio compared with the susceptible strain (WH-SS), which was derived from the Wuhan Institute of Vegetable Science. The LC50 for the susceptible strain was 0.23 mg l-1 and it increased to 55.21 mg l-1 after 12 selected generations. The estimated realized heritability (h2) of the RR-indox strain was 0.25. The number of generations required for a 100-fold increase in LC50 was estimated to be nine under the 90% lethal dose. The inhibitors piperonyl butoxide (PBO), triphenyl phosphate (TPP), and diethylmeleate (DEM) had synergistic ratios of 0.87-, 0.31- and 0.36-fold in RR-indox, and 0.51-, 0.09- and 0.12-fold in WH-SS, respectively. A 3.75-fold increased level of activity of glutathione S-transferase was found in RR-indox compared with WH-SS, whereas a 1.99-fold higher activity level of carboxylesterase was observed. No significant differences in the cuticular penetration rate were found between RR-indox and WH-SS. A DNA fragment of 257 bp length of the IIS6 region of the voltage-gated sodium channel gene was amplified and sequenced from both RR-indox and WH-SS. An amino acid substitution from leucine (CTT) in WH-SS to phenylalanine (TTT) in RR-indox was observed at site 1014. These results show the resistance mechanisms of S. exigua to indoxacarb to be associated with glutathione S-transferase, carboxylesterase and a Leu-1014-Phe mutation.