Survival And Development Of Spodoptera Frugiperda And Chrysodeixis Includens (Lepidoptera: Noctuidae) On Bt Cotton And Implications For Resistance Management Strategies In Brazil

In Brazil, Spodoptera frugiperda (J. E. Smith) and Chrysodeixis includens (Walker) are important cotton pests and target of control of Bollgard II (Cry1Ac/Cry2Ab2) and WideStrike (Cry1Ac/Cry1F) cotton technologies. To subsidize an insect resistance management program, we conducted laboratory studies to evaluate the toxicity of these Bt cotton plants throughout larval development of Spodoptera frugiperda and Chrysodeixis includens. In bioassays with leaf disc, the efficacy of both Bt cotton plants against neonates was >80% for Spodoptera frugiperda and 100% for Chrysodeixis includens. However, Spodoptera frugiperda larvae that survived on Bt cotton had >76% of growth inhibition and stunting. In bioassays with Spodoptera frugiperda and Chrysodeixis includens larvae fed on non-Bt near-isoline during different time period (from 3 to 18 d) and then transferred to Bollgard II or WideStrike leaves showed that larval susceptibility decreased as larval age increased. For Bollgard II cotton, in all Spodoptera frugiperda instars, there were larvae that reached the pupal and adult stages. In contrast, on WideStrike cotton, a few larvae in fifth and sixth instar completed the biological cycle. For Chrysodeixis includens, some larvae in sixth instar originated adults in both Bt cotton plants. In conclusion, Bollgard II and WideStrike cotton technologies showed high efficacy against neonates of Spodoptera frugiperda and Chrysodeixis includens. However, the mortality of these species decreases as larval age increase, allowing insect survival in a possible seed mixture environment and favoring the resistance evolution.