ARTHROPOD MANAGEMENT Efficacy of Seed Mixes of Transgenic Bt and Nontransgenic Cotton Against Bollworm, Helicoverpa zea Boddie

Field trials conducted in northeastern North Carolina in 1994 and 1995 examined the effects of plantings of pure and blended genotypes of Bollgard cotton on (i) the larval population development of caterpillar pests, (ii) fruit damage, and (iii) yield. Treatments in 1994 included (i) 100% Bt seed:0% non-Bt seed; (ii) 0% Bt:100% non-Bt; (iii) 90% Bt:10% non-Bt; (iv) 85% Bt:15% non-Bt; (v) 80% Bt:20% non-Bt; and (vi) 75% Bt:25% non-Bt. The same treatments were tested again in 1995, with the omission of the 90% Bt:10% non-Bt and 80% Bt:20% non-Bt seed blends. Bollworms made up 95 to 98% of larval pest populations in 1994 and 1995. The 0% Bt:100% non-Bt plots had significantly higher larval numbers and damaged fruit and significantly lower yields than all other seed treatments both years. The treatment containing 100% Bt seed had lower percent larval infestation and percent damaged fruit and higher yields than all other seed blends in both years. In general, mean percent larval infestation and mean percent damaged fruit increased in both years as the percentage of Bt seed in the blends decreased. Conversely, seed cotton yields decreased as the percentage of Bt seed in the blends decreased. In these experiments, the treatments incorporating blends of Bt and non-Bt seed sustained too much fruit damage and yield loss for the blended seed concept to be practical. Arevolutionary insect-management technology was provided to the cotton industry in 1996 when Monsanto commercialized Bollgard cotton. Transgenic Bollgard cottons express the deltaendotoxin protein cry1Ac from Bacillus thuringiensis Berliner var. kurstaki (Bt), which is toxic to several of the most important lepidopteran larval pests of cotton. A concern associated with the widespread planting of Bollgard cottons is that lepidopteran pests may develop resistance to the cry1Ac endotoxin as they have to many traditional insecticides (Graves et al., 1991). Although studies with Bollgard demonstrated excellent control of tobacco budworm, Heliothis virescens F., several studies confirmed that bollworm, Helicoverpa zea Boddie, is much less susceptible to the cry1Ac endotoxin than tobacco budworm is (Stone and Sims, 1993; Sims, 1995). Documentation that a proportion of bollworm larvae could survive on Bollgard cotton confirmed that cry1Ac was not expressed at a concentration high enough to provide a “high-dose strategy” approach to resistance management (Hardee et al., 2001). Bollworm survival on Bollgard cotton was shown to be of such magnitude that deployment of resistance-management strategies other than the high-dose refuge strategy was necessary. One approach would be to increase the refuge size and/or refuge efficiency. McGaughey (1990) suggested alternating insecticides, using multiple toxins, providing untreated refuges, selecting appropriate doses, or treating selected plant parts as a tactic for preventing resistance development to Bt toxins. Arpaia and Ricchiuto (1993) examined alternative strategies for potato pest management and suggested using mixtures of plants, including non-Bt types, as a refuge. Gould (1991) also suggested the use of untreated pest habitats offering refuge in or near the crop as a means of delaying resistance when multiple toxins were used. One means of providing such refuges would be to mix seed of Bt cotton with AGI ET AL.: COTTONSEED MIXES AND BT AGAINST BOLLWORM