Impacts of Decreased Measurements of Laboratory Susceptibility to Cry1Ac on Bollworm Survival and Damage to Dual- and Multi-Gene Cottons in Large Field Cages

Interpreting variable laboratory measurements of bollworm susceptibility to Bt toxins has been challenging due to a lack of clear evidence to document declining field control. Research that links laboratory measurements of susceptibility to survival on Bt crops is vital for accurate characterization of and any subsequent response to the occurrence of an implied bollworm resistance event. In this study, bollworm survival and the resultant damage to plant fruiting structures of non-Bt, Bollgard II, and Bollgard III cottons from two insect colonies with differing levels of laboratory susceptibility to Bt toxins were evaluated in large field cages. Laboratory bioassays revealed resistance ratios of 2.04 and 622.14 between the two bollworm colonies for Dipel and Cry1Ac, respectively. Differences between the two bollworm colonies measured via bioassays with Bollgard II and Bollgard III cotton leaf tissue in the laboratory were not discernable. However, there was a trend for numerically lower larval mortality in the feral relative to the laboratory colony in both Bt cottons. Although total bollworm larval numbers in cages infested with the laboratory susceptible colony did not differ between the two Bt cottons, there were fewer larvae per 25 plants in Bollgard III than in Bollgard II cotton in cages containing tolerant insects. Cages infested with tolerant bollworm moths had higher numbers of total larvae than those containing the laboratory susceptible colony in both Bollgard II and Bollgard III cottons. Both Bt cottons received more total damage to plant fruiting structures in cages infested with tolerant insects relative to those containing the laboratory susceptible colony. The damage inflicted to fruiting structures on Bollgard III cotton by a feral bollworm colony with decreased measurements of laboratory susceptibility to Dipel and Cry1Ac indicate that the addition of Vip3A to third generation Bt cottons may not provide sufficient control in certain situations.