Leaf Tissue Assay for Lepidopteran Pests of Bt Cotton

Abstract

Laboratory measurements of susceptibility to Bt toxins can be a poor indicator of the ability of an insect to survive on transgenic crops. We investigated the potential of using leaf tissue for evaluating heliothine susceptibilities to two dualgene Bt cottons, Gossypium hirsutum L. A preliminary study with different cotton leaf disk combinations was performed to determine the best procedure for using cotton tissue to assay bollworm, Helicoverpa zea Boddie, and tobacco budworm, Heliothis virescens F. Neonate larvae from a laboratory colony were assayed simultaneously on leaf disks of non-Bt and two dual-gene transgenic cottons in addition to meridic diet overlaid with discriminating doses of a commerciallyformulated Bt product. The bollworm colony in the laboratory was more tolerant than tobacco budworm to the commercially-formulated Bt product. When corrected for the non-Bt check, percentage of bollworm larval mortality was 42.1 and 71.9 on leaf disks of the two dual-gene Bt cottons. No tobacco budworm survived on dualgene Bt cotton leaf disks in the study. Assays using transgenic cotton leaf disks may compliment current meridic diet-based methods by providing a linkage to insect survival on Bt cotton plants.