Efficacy of transgenic Bt cotton for resistance to the Asian corn borer (Lepidoptera: Crambidae)

Transgenic Bt cotton is expected to provide satisfactory control of several lepidopteran species in cotton. The Asian corn borer (ACB), Ostrinia furnacalis (Guenee), is an important component of the lepidopteran pest complex of cotton in China. Monsanto's transgenic Bt cotton NC 33B, expressing Cry1Ac protein, and a Chinese transgenic Bt cotton GK-2, expressing Cry1A protein, were evaluated for resistance to ACB during 2001-2002. Field trials were conducted with an artificial infestation of ACB at different cotton crop stages, which coincided with the generations of natural ACB occurrence. Damage ratings were significantly reduced in transgenic cotton cultivars both NC 33B and GK-2 compared with their parental non-transgenic Bt cotton cultivars DP5415 and Simian 3, and a major conventional cultivar Shiyuan 321, respectively. In addition, percentage of plants stem-bored and mean number of tunnels per plant were significantly higher on GK-2 than on NC 33B in the second generation. Laboratory bioassays were carried out by exposing neonates to plant tissues collected from the field during the season. Tissues assayed included the new leaves, floral buds, match-head squares, and white flowers, which are the tissues initially attacked by the neonates in the field. Low larval survival rates were observed on NC 33B and GK-2, contrasting greatly to the high number of survivors found on equivalent non-Bt cotton tissue isolated throughout the season. However, larval survival was higher on the tissues isolated from late-season Bt cotton plants than on early season. In addition, higher larval survival was observed on GK-2 than NC 33B in assays with the late season tissues. This may be associated with reduced levels of available toxin in plant tissues as they age. Both laboratory and field data indicated that NC 33B and GK-2 were highly resistant to ACB. The high level of efficacy for Bt cotton against ACB offers the potential for season-long control.