Data Generation and Utilization for Evaluating Helicoverpa zea (Lepidoptera: Noctuidae) Resistance Management in Bt Field Corn and Cotton through Computer Modeling

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Title: Data Generation and Utilization for Evaluating Helicoverpa zea (Lepidoptera: Noctuidae) Resistance Management in Bt Field Corn and Cotton through Computer Modeling
Author: Kurtz, Ryan Warner
Advisors: Fred Gould, Committee Member
J. R. Bradley, Jr., Committee Co-Chair
Clyde Sorenson, Committee Member
John W. Van Duyn, Committee Co-Chair
Abstract: To mitigate resistance development in pest insects to transgenic field corn and cotton expressing Bacillus thuringiensis (Bt) proteins, the USDA and EPA have indicated that a high dose/refuge strategy is most effective, but single toxin Bt crops which predominate the transgenic crop acreage provide only a moderately high dose for one target pest in North Carolina, Helicoverpa zea (Boddie), or bollworm. When a source of susceptible insects is present, moderately high insecticide doses are expected to result in resistance development more quickly than high doses. In theory, introducing a pyramided genotype expressing two different Bt proteins in the presence of a source of susceptible insects can more substantially delay resistance development than a single Bt genotype expressing a high or moderately high Bt dose. Herein, we determine the effects of transgenic field corn (2001 & 2002) and cotton (2003 — 2005) genotypes expressing Bt proteins Cry1Ab/Ac & Cry2Ab, both singly and pyramided, on the development and survival of H. zea. Effects were documented by quantifying larval, pupal, and adult populations for each Bt crop genotype and a conventional sister genotype over time in eastern North Carolina. Each year all Bt field corn genotypes significantly reduced larval populations and production of pupae and adults below that of the non-Bt genotype with the pyramided Bt genotype significantly reducing these populations below each single toxin Bt genotype. Additionally, larval growth rate, pupal weight, and pupal length were reduced and adult eclosion date delayed by Bt field corn genotypes. Adverse weather conditions resulted in no useful data collected from cotton in 2003 & 2004, thus all cotton data reported are from two locations in 2005. All three Bt cotton genotypes significantly reduced the percentage of terminal regions and bolls containing live heliothine larvae and feeding damage compared to the non-Bt genotype with no significant differences among Bt genotypes. All three Bt genotypes also significantly lowered bollworm pupal and adult production per hectare compared to the non-Bt genotype; moreover, the Bollgard II and Cry2Ab significantly lowered pupal and adult production compared to the Bollgard genotype. Adults from the non-Bt genotype eclosed significantly earlier than adults from all three Bt genotypes, but there were no statistical differences between any Bt genotypes. We then use the simulation model originally described in Storer et al. 2003 to evaluate the role of pyramided Bt cotton and corn cultivars, EPA mandated non-Bt refuges, and soybean as a host in H. zea resistance development to Bt. With at least one pyramided Bt cultivar, the model predicts that the 20% sprayed non-Bt cotton refuge's contribution to delaying resistance evolution is greatly supplemented by other non-Bt sources of susceptible moths (i.e. soybean and other non-Bt alternate hosts) and has less of an effect on Bt resistance management than the non-Bt corn refuge. A field study was also conducted in eastern North Carolina to determine what effect seed mixtures of Bt and non-Bt cotton containing greater than 90% Bollgard II cotton have on fruit damage by H. zea and cotton yield. A lab study determined the efficacy of Bollgard II bolls against feral H. zea larvae (≥ 3rd instar) that initially fed on non-Bt cotton. Averaged over years, there were no significant yield differences between any seed mixture despite the 8% non-Bt mixture having significantly higher boll damage than all other treatments; however, when analyzed by year, the 100% Bollgard II treatment had significantly higher seed cotton yield than all seed mixture treatments. In our lab study, H. zea larvae (≥ 3rd instar) were able to survive to adulthood on Bollgard II cotton bolls after initially feeding on non-Bt cotton but at significantly lower percentages than larvae that remained on non-Bt cotton.
Date: 2005-12-02
Degree: PhD
Discipline: Entomology
URI: http://www.lib.ncsu.edu/resolver/1840.16/4348


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