Susceptibility of Field Populations of the Cotton Bollworm to Purified Cry1Ac Protein and Cotton Leaf Tissues Containing Cry1Ac Protein

The cotton bollworm, Helicoverpa zea (Boddie), is one of the most important insect pests of cotton, Gossypium hirsutum L. Cry 1Ac protein expressed in transgenic Bt cotton has been widely used for control of cotton bollworms since 1996. However, extensive and constant exposure of cotton bollworms to this protein may result in a shift towards a reduction in susceptibility. To estimate the susceptibility of the field populations of cotton bollworm to this Bt protein, we compared the survivorship of F1 or F2 generations of field-derived cotton bollworms collected from Mid-South region in the U.S. to Cry1Ac protein with that of a susceptible counterpart using diet incorporated and cotton leaf tissue bioassays. In the leaf tissue assays, expressing only Cry1Ac, the 7-day survivorship of all field collected populations (54.7-66.1%) was significantly higher than that of susceptible strain (26.1%). In the diet incorporated assays, the LC₅₀ of the susceptible strain to Cry1Ac protein was 0.265 µg/g with a 95% confidence interval of 0.207 - 0.339 µg/g. However, dose-response bioassays showed all the field collected populations except for one from Louisiana and one from Arkansas showed a significant resistance level (> 25.5-fold) to Cry1Ac protein with the LC₅₀ > 10 µg/g, relative to the susceptible strain. These results suggest that the susceptibility of cotton bollworm to Cry1Ac protein may have declined in the Mid-South region of U.S.