ARTHROPOD MANAGEMENT Field Performance of Transgenic Cottons Expressing One or Two Bacillus thuringiensis Endotoxins Against Bollworm, Helicoverpa zea (Boddie)

Bollworm, Helicoverpa zea (Boddie), survival on Bollgard cottons is an economic problem and also a concern for dealing with the development of bollworm resistance. Bollgard II cottons that produce two Bt proteins have been developed to provide increased bollworm control. Bollgard and Bollgard II cottons were evaluated along with their conventional sister line for susceptibility against bollworm in North Carolina field studies from 1999-2002. The impact of supplemental pyrethroid oversprays on bollworm control and yields was also evaluated. Comparisons of untreated genotypes averaged across four years demonstrated that both Bt genotypes reduced infestation rates by larvae and damage to squares and bolls compared with the conventional cultivar. Bollgard II had fewer squares and bolls infested with larvae and less square and boll damage compared with Bollgard. Larval infestations and damage levels were also lower in pyrethroidtreated conventional cotton, and both pyrethroidtreated and untreated Bt genotypes, compared with the untreated conventional cultivar. A reduction in larval numbers and square and boll damage was achieved with pyrethroid oversprays in both conventional and Bollgard cultivars, but not in Bollgard II. Except for square damage, larval numbers and damage were comparable among pyrethroid-treated conventional and untreated Bollgard cottons. Pyrethroid-treated Bollgard contained numbers of larvae and damage comparable to that of untreated Bollgard II. Pyrethroid-treated and untreated Bollgard and Bollgard II cottons on average produced similar yields. Only pyrethroid-treated and untreated Bollgard II cottons produced significantly higher yields compared with the pyrethroid-treated conventional cultivar. T he availability of transgenic Bollgard cottons (Monsanto Co., St. Louis, MO), which contain a gene from the soil bacterium Bacillus thuringiensis var. kurstaki encoding for the Cry1Ac δ-endotoxin, has provided a novel alternative for management of certain lepidopteran pests. Two major lepidopteran pests of cotton in North Carolina that are affected by this technology are the bollworm, Helicoverpa zea (Boddie), and the tobacco budworm, Heliothis virescens (Fab.). Although control of tobacco budworm with transgenic cottons has been absolute, bollworm susceptibility to the Cry1Ac protein is lower and more variable. For example, purified Cry1Ac endotoxin and spore/crystal combination LC 50 values for bollworm populations were 4 to 60 times higher than those of tobacco budworm populations (Stone and Sims, 1993). Field trials conducted in North Carolina confirmed that supplemental insecticide oversprays were frequently required to achieve satisfactory bollworm control and avoid yield reductions in Bollgard cottons (Lambert et al., 1996, 1997; Mahaffey et al., 1994, 1995). Survival of a portion of the bollworm population on Bollgard cottons may also be partially explained by the significant drop in the average levels of Cry1Ac protein in cotton fruit at approximately 80 days after planting (Greenplate, 1999; Greenplate et al., 2001), which is coincident with the major bollworm flight into North Carolina cotton. Bollworm survival on Bollgard cottons is not only an economic problem, but also causes concern for the development of resistance. Thus, further advances in Bt cotton technology to decrease bollworm survival are needed to address these problems. Bollgard II cottons (Monsanto Co., St. Louis, MO) produce two Bt endotoxins, Cry1Ac and Cry2Ab, whereas commercially available Bollgard cultivars produce only the Cry1Ac endotoxin. The