The relationship between susceptibility of the first laboratory generation of field-collected colonies of bollworm, Helicoverpa zea (Boddie), to the Cry1Ac toxin from Bacillus thuringiensis (Berliner) (Bt) and the crop structure in the surrounding landscape was examined from 2002–2006 in Arkansas. At lesser concentrations of Cry1Ac (10 and 30 μg/ml diet) in diet-incorporated bioassays, colonies of bollworm established from an area with two Bt crops, cotton, Gossypium hirsutum L., and corn, Zea mays L., were less susceptible to Cry1Ac than those from an area with only Bt corn. There were no differences in susceptibilities of colonies established at different months in the areas. On a local scale, there was a positive relationship between susceptibilities of bollworm colonies at greater dosages in bioassays (50 and 100 μg of Cry1Ac/ml of diet) and the percentage of Bt corn acreage within a 0.8-km radius of collection sites. Colonies tended to be less susceptible to Cry1Ac as the amount of Bt cotton increased around collection sites, but these relationships were not significant.
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