Resistance, cross-resistance and stability of resistance to Bacillus thuringiensis kurstaki in Earias vittella (Fabricius) (Lepidoptera: Noctuidae)

Transgenic cotton expressing (Cry) endotoxins from the bacterium Bacillus thuringiensis (Bt) is cultivated in various regions of the world to manage spotted bollworm, Earias vittella, a serious pest of cotton and okra crops. However, the development of resistance to Bt toxins is a major challenge to the management and the current laboratory study was conducted to investigate the development of resistance in E. vittella against Bt toxins, the stability of Bt resistance and the patterns of cross-resistance found to other insecticides. A field-collected strain of E. vittella that showed moderate level of resistance to a commercial spray formulation of Bacillus thuringiensis sub species kurstaki (Btk) (Dipel (R)) was further exposed to Dipel continuously for eight generations (G1-G8). At G1, the resistance ratios (RR) for Dipel, an organophosphate triazophos, the pyrethroids bifenthrin, lambda (lambda)-cyha-lothrin and deltamethrin, the pyrole chlorfenapyr and the avermectin emamectin benzoate were 31, 21, 113, 157, 25, 19 and 14 mu g a.i. /mL, respectively compared with a laboratory susceptible (Lab-PK). At G9, after eight generations of selection with Dipel, the LC50 value for Dipel was significantly greater (P < 0.01) compared with G1, with a corresponding 128-fold increase in the resistance ratios (RR) for Dipel-SEL compared with Lab-PK. In contrast, the LC50 values for triazophos, bifenthrin. lambda-cyhalothrin, deltamethrin, chlorfenapyr and ema-mectin benzoate all decreased significantly (P < 0.01) for Dipel-SEL at G9, indicating a lack of cross-resistance to Dipel. When Dipel-SEL was left unselected for five generations (G9-G13), bioassays at G14 showed a significant (P < 0.01) decrease in the LC50 value for Dipel with a corresponding 3-fold decline in the RR; suggesting that resistance to Dipel in Dipel-SEL was unstable. The apparent instability of Dipel resistance in the Dipel-SEL E. vittella strain and the apparent absence of cross-resistance to various other classes of insecticides provides the opportunity for resistance management program involving the rotational use of such compounds with Dipel.