The influence of environmental factors on susceptibility of B.t. cottons to bollworm, Helicoverpa zea, and factors affecting resistance to B.t. toxins for bollworm

Field and lab studies were designed to examine the relationship of bollworm, Helicoverpa zea, to B.t. cotton. Bollworm larvae were collected from B.t. sweet corn and selected for resistance to CryIA(c) toxin. After 6 and 10 generations of selection, the colony had developed 50-fold and 100-fold resistance, respectively. LC50 values for control strain, selected strain, and reciprocal crosses suggested that resistance to CryIA(c) was inherited as a dominant or incompletely dominant trait.;Adult female bollworm moths were collected from light-traps. Twenty-four larvae from each female line were screened on normal diet and on diets containing either CryIA(c) or CryIIAa toxin. Growth rate bioassays were performed to isolate nonrecessive B.t. resistance genes and estimate gene frequencies. The gene frequency for CryIA(c) and CryIIAa resistance was estimated at 0.00043 and 0.00039, respectively. These allele frequency estimates along with partial dominant inheritance are important for determining refuge sizes needed to delay evolution of resistance to B.t. cotton.;Field trials were conducted to examine effects of planting date on bollworm population development and on yield in Bollgard cotton. Early, mid and late-planted plots were evaluated to determine fruit damage, larval infestations and yield. Early and mid-planted untreated subplots suffered greater fruit damage on the first scoring date. Mid-planted untreated subplots sustained more damage for the second and third sampling dates. Yields were greater for pyrethroid-treated subplots compared to respective untreated subplots. Early and mid-planted subplots yielded more seed cotton than lateplanted subplots regardless of insecticide treatment. Even though early and mid-planted untreated subplots suffered more bollworm damage, yields were still significantly lower for late-planted subplots. Late plantings of Bollgard cotton in North Carolina do not have sufficient growing season to achieve maximum yield potential.;The performance of various B.t. cotton varieties was evaluated against bollworm in NC. Plant mapping data confirmed that ST4740BG was a significantly later maturing variety compared to the others. B.t. cottons not treated for bollworm sustained considerable damage with yield losses ranging from 6.6 to 31.7% compared to pyrethroid treated B.t. cottons. ST4740BG untreated subplots suffered significantly more damage and greater yield loss than all other untreated varieties.