Differences between two populations of bollworm, Helicoverpa zea (Lepidoptera: Noctuidae), with variable measurements of laboratory susceptibilities to Bt toxins exposed to Non-Bt and Bt cottons in large field cages

Authors: Little, Nathan; Elkins, Blake; Mullen, Regina; Perera, Omaththage; Parys, Katherine; Allen, Clint; Boykin, Deborah

Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/13/2019
Publication Date: 3/13/2019
Citation: Little, N., Elkins, B.H., Mullen, R.M., Perera, O.P., Parys, K.A., Allen, K.C., Boykin, D.L. 2019. Differences between two populations of bollworm, Helicoverpa zea (Lepidoptera: Noctuidae), with variable measurements of laboratory susceptibilities to Bt toxins exposed to Non-Bt and Bt cottons in large field cages. PLoS One. 14(3). https://doi.org/10.1371/journal.pone.0212567.
DOI: https://doi.org/10.1371/journal.pone.0212567

Interpretive Summary: Results from laboratory bioassays of bollworms on Bt toxins are highly variable and their relevance to survival on transgenic crops is largely unknown. However, broad characterizations of bollworm resistance to Bt toxins are often based almost entirely on findings from these studies. We investigated the link between bollworm survival and damage on dual- and multi-gene transgenic cottons and laboratory estimates of susceptibility to Bt toxins using two bollworm colonies with different levels of tolerance in large field cages. Determining the relationship between laboratory bioassays with Bt toxins and bollworm survival in transgenic cottons is vital for accurate characterization of and any subsequent response to the occurrence of an implied bollworm resistance event. Measurements of decreased laboratory susceptibility to Bt toxins in a feral colony of bollworms ultimately resulted in higher insect survival and damage to fruiting structures of dual- and multi-gene transgenic cottons in the field. Data from this study indicates that supplemental control of bollworm in new third generation cottons that express multiple Bt genes may still be necessary in certain high-pressure situations or when insects with reduced susceptibility to one or more Bt toxins are suspected.

Technical Abstract: 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. Although this study was designed to simulate a “high-pressure” insect event, supplemental control of bollworm in Bollgard III cottons may be required when insects with reduced susceptibility to Bt toxins are suspected.