Facing Bt toxins growing up: Developmental changes of susceptibility to Bt corn hybrids in fall armyworm populations and the implications for resistance management

Developmental changes in target-pest susceptibility to Bacillus thuringiensis (Bt) toxins matter to resistance management in Bt crops, especially in certain refuge modalities. We determined the cumulative, lethal and sublethal effects of Bt corn hybrids during the larval stages of Bt-resistant populations of fall armyworm, Spodoptera frugiperda (Lepidoptera: Noctuidae). The susceptibility to Bt corn traits depended on the fall armyworm population and the developmental stage of the larvae. On Cry1A.105 + Cry2Ab corn, neonate-to-adult survival rates of Cry1F-resistant larvae was 4% when the exposure started in the neonate and 87% when in late instars. Cry1F-resistant larvae feeding on Cry1A.105 + Cry2Ab corn throughout larval development produced viable adults that successfully reproduced without any noticeable disadvantage. In contrast, Bt susceptible larvae had high mortality on Cry1A.105 + Cry2Ab corn, and no adults emerged from the larvae exposed to the toxins, independently of the larval stage. Cry1Ab + Vip3Aa corn killed all larvae in any instar, regardless of the Bt-toxin resistance of the fall armyworm population. These results consistently indicate that Cry1F resistance reduces the susceptibility to Cry1A.105 + Cry2Ab toxins and that Vip3Aa has a high potential to control field populations of Cry-toxin-resistant fall armyworm, even if there was plant-to-plant larval movement to Vip Bt plants in late instars in a scenario seed-mixture refuge. We discuss the implications of our findings for sustainable resistance management.