Variation in plant resistance to cotton bollworm, Helicoverpa zea in selected Bt cotton varieties

Field and lab tests were conducted in 1998-2000 to determine if high nitrogen or high vegetative growth could reduce plant resistance to bollworm in Bt cotton and their recurrent parent, conventional varieties. High nitrogen and vegetative growth were related to higher bollworm damage in the field. In 1998, Deltapine90 and Deltapine90B plots with high nitrogen or vegetation had twice as many bollworm damaged squares as plots with both low nitrogen and vegetation. In 1999, high nitrogen and vegetation plots had four times more squares infested with bollworm as low nitrogen and vegetation plots of NuCotn33B, Deltapine5415, Deltapine90B, Deltapine90 and Paymaster1215. In 2000, Deltapine5415 had approximately twice as many damaged squares in low compared to high nitrogen plots. Deltapine90 and Deltapine90B also had approximately three times more damaged squares in high vegetation compared to low vegetation plots. Egg counts in the field and laboratory bioassays suggest that higher damage in highly vegetative plots was due to higher oviposition rates rather than inherent differences in plant resistance. High vegetation treatments and taller varieties had significantly more bollworm eggs in 1998. The tallest varieties DP90 and DP90B had 2-3 times more eggs than the mid-sized NuCotn33B and Deltapine5415. Only one egg was found in the shortest variety Paymaster 1215. Bioassays of field collected leaves did not indicate any difference in bollworm mortality between high and low vegetation plots, On the other hand, leaves from high nitrogen plots did have lower bollworm mortality in some bioassays. This, and high vegetation associated with high nitrogen, suggests nitrogen can directly and indirectly increase bollworm damage in both Bt and non-Bt cotton.