The effect of phenological stage and different plant structures of Bt cotton on the development and survival of Spodoptera litura (Lepidoptera: Noctuidae).

Variability in Bacillus thuringiensis (Bt) protein expression, during plant development and between different plant structures, may contribute to the survival of Spodoptera litura (Fabricius) in transgenic Bt crops. This study examined the survival of neonate and second instar S. litura larvae when fed leaf material from field-grown BG3, expressing Cry1Ac, Cry2Ab, and Vip3A, and non-Bt (NBT) cotton plants at different phenological stages (squaring, first flower and/or peak flowering, and cut-out) or different plant structures (leaves, buds, flowers, bolls, and bracts). The effect of alternating diets, where second instar larvae received different combinations of BG3 plant structures and/or NBT leaves was also tested. High mortality levels (>98%) were recorded at day 5 when neonates were fed BG3 plant material from plants of all phenological stages and on all plant structures. Similarly, mortality was high (>99%) when second instar larvae were fed plant material from different BG3 plant structures. However, when larvae were fed an alternating diet of BG3 and NBT leaves from second instar onward, the proportion surviving to adults significantly increased compared with larvae fed exclusively BG3 cotton, irrespective of plant structure combination. These findings indicate that access to a NBT food source, providing intermittent respite from BG3 cotton, can increase larval survival. Further research on NBT host plants, such as weeds or volunteer plants within BG3 fields, is needed to clarify their role in facilitating S. litura survival in BG3 cotton systems.