Comparison of in vitro and in planta toxicity of Vip3A for lepidopteran herbivores

Agricultural pest infestation is as old as domestication of food crops and contributes a major share to the cost of crop production. Transgenic production of Vip3A, an insecticidal protein from Bacillus thuringiensis, effectively controls lepidopteran pests. A synthetic vip3A gene was evaluated its efficacy against Spodoptera litura (cotton leafworm), Spodoptera exigua (beet armyworm), Spodoptera frugiperda (fall armyworm), Helicoverpa armigera (cotton bollworm), Helicoverpa zea (corn earworm), Heliothis virescens (tobacco budworm), and Manduca sexta (tobacco hornworm). In artificial diet assays, the Vip3A concentration causing 50% mortality was H. zea > H. virescens > S. exigua > H. armigera > M. sexta > S. frugiperda > S. litura. In contrast, on vip3A transgenic tobacco the order of resistance (time until 50% lethality) was M. sexta > H. virescens > S. litura > H. zea > H. armigera > S. exigua > S. frugiperda. There was no significant correlation between the artificial diet and transgenic tobacco effects. Notably, the two insect species that are best-adapted for growth on tobacco, M. sexta and H. virescens, showed the greatest tolerance of vip3A-transgenic tobacco. This may indicate synergistic effects of Vip3A and endogenous plant defense mechanisms, e.g. nicotine, to which M. sexta and H. virescens would have greater resistance. Together, our results show that artificial diet assays are a poor predictor of Vip3A efficacy in transgenic plants, lepidopteran species vary in their sensitivity to Vip3A in diet-dependent manner, and that host plant adaptation of the targeted herbivores should be considered when designing transgenic plants for pest control.