Movement and Survival of Spodoptera litura (Lepidoptera: Noctuidae) Neonate Larvae on Cotton Expressing Bacillus thuringiensis Proteins

Abstract

ABSTRACT

Survival of Spodoptera litura (Fabricius) (Lepidoptera: Noctuidae) larvae has been reported in transgenic Bollgard 3 (BG3) cotton ( Gossypium hirsutum ) fields in northern Australia, despite high mortality when fed BG3 leaves in laboratory bioassays. Larval movement away from plant material expressing Bacillus thuringiensis (Bt) proteins has been proposed as a mechanism enabling lepidopteran larvae to survive in the field. This study examined the movement of S. litura neonates on BG3 and cotton expressing no Bt proteins in no‐choice and choice leaf disc bioassays, and in whole‐plant bioassays under laboratory conditions. In no‐choice bioassays, larvae exhibited greater movement when exposed to BG3 leaf discs compared with control cotton expressing no Bt proteins. However, the pattern of movement suggested that larvae could not immediately detect and avoid Bt proteins in the leaf material on which they were feeding. In choice bioassays, when larvae were placed equidistant between BG3 and control cotton leaf discs, approximately 62% of larvae were observed on BG3 leaf discs after 1 h. Thus, their movement away from BG3 cotton appears to be a post‐ingestion response. Larval survival was significantly higher in choice bioassays (where larvae placed on BG3 leaf discs could move to control leaf discs) than in no‐choice bioassays (where larvae were constrained to BG3 leaf discs). On whole plants, a higher proportion of 1‐day old neonate larvae remained near the site of the egg mass on control cotton plants than on BG3 cotton plants, but more larvae disappeared on BG3 cotton plants compared with control plants. These findings suggest that the Bt proteins expressed in BG3 cotton alter the movement behaviour of S. litura larvae. In the field, dispersal may increase survival if larvae can move from plant structures expressing lethal levels of Bt proteins to plant structures that express sublethal levels.