Fitness Studies and Cross Resistance Evaluations of an Eastern North Carolina Cotton Bollworm Strain (Helicoverpa zea) (Boddie) Tolerant to the Bacillus thuringiensis delta endotoxin Cry1Ac

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Title: Fitness Studies and Cross Resistance Evaluations of an Eastern North Carolina Cotton Bollworm Strain (Helicoverpa zea) (Boddie) Tolerant to the Bacillus thuringiensis delta endotoxin Cry1Ac
Author: Marcus, Maria Adalita
Advisors: J. R. Bradley, Jr., Committee Chair
Abstract: A component of insect resistance management is the use of alternative insecticides to delay the onset of resistance to one type or class of toxin. Because cotton bollworm (Helicoverpa zea) larvae have been found to survive on Bollgard plants in the field, concern has been raised over the possible development of resistance to Cry1Ac. A Cry1Ac tolerant bollworm strain (XYZ) was initially collected from Bt cotton plants in eastern NC in summer 2002 and selected against Cry1Ac for 12 generations. Dose-mortality bioassays were conducted to determine response to selection, the highest LC50 recorded at 884.9 μg/ml for generation F12. Cross resistance of this resistant strain was evaluated against Bt endotoxins Cry1Ab, Cry1F, and Cry2Ab. Cross paired matings were made for susceptible (HZ 02) and resistant (XYZ) bollworm strains to obtain F1s for testing against a five fold serial dilution insect diet blend for the Cry1Ac, Cry1Ab, Cry1F, and Cry2Ab toxins. Mortality and weights were assessed after a 10 day incubation period at 27°C and 14:10 L:D photophase. Based on mortality and growth results there was evidence of cross resistance for H. zea to Cry1Ab, negative cross resistance to Cry1F, and no cross resistance to Cry2Ab. Cotton plant tissue and surface treated diet bioassays were performed to determine the extent of cross resistance to the novel insecticidal protein Vip3A, in Cry1Ac tolerant tobacco budworm (Heliothis virescens) strains YHD2, KCBhyb, and CXC, and bollworm strain XYZ. Control H. virescens strain YDK and H.zea strain HZ 02 are susceptible to the delta endotoxin. All insect strains were subjected to cotton plant tissue and surface treated diet assays containing the vegetative insecticidal protein, Vip3A. Purified Vip3A and Cry1Ac proteins were used in surface treated diet assays, the plant tissue assays included three types of insecticidal expression Vip3A, Cry1Ac, or Cry1Ac+Cry2Ab. Control material did not contain any of the insecticidal proteins. Surface treated diet evaluations indicate the budworm resistant strain, YHD2, had lower survival and lower average larval weight on Vip3A than the control strain, YDK. However, the KCBhyb strain had somewhat lower mortality and average higher weight than YDK on the Vip3A. The resistant H. zea strain XYZ, had lower mortality than the control strain, HZ 02, for the surface treated assay. However, HZ 02 had higher average weights than XYZ on Vip3A. Plant tissue bioassays based on mortality, consumption, and weight data showed the Cry1Ac tolerant budworm strains were not significantly different in mortality from the susceptible YDK strain when compared on cotton varieties expressing Vip3A protein. Similar findings for mortality and weight were observed for the H. zea control strain, HZ 02, compared to resistant strain XYZ based on the plant tissue assay. Our preliminary results from both plant and diet bioassays indicate there is no strong cross resistance of the Cry1Ac resistance H. virescens or H. zea strains to Vip3A. Based on the results from this study, there are a number of insecticidal alternatives available to delay evolution of resistance in cotton bollworm and tobacco budworm to Bt cotton. Widespread use of transgenic cotton Bollgard has raised concern for development of resistance in cotton bollworm (Helicoverpa zea). If there were a fitness cost present in individuals carrying the allele for Cry1Ac tolerance, a delay in resistance development could be enhanced. Fitness comparisons between a Bt tolerant (XYZ) and control (HZ 02) bollworm strain were made through exposure to a technical grade pyrethroid, growth on unadulterated insect diet, and growth on secondary plant compound, gossypol. Intergenerational growth responses on an unadulterated diet were measured through larval weight for both H zea strains. Significant differences in weight between the two strains were not found. For the pyrethroid evaluations, third instar larvae of both strains over several generations were treated topically with 1 μl of technical grade cypermethrin and allowed to incubate at 27°C for 72 hours at which time mortality was assessed. Results indicate there were no statistically significant differences between the strains within generation. In the gossypol evaluations, first instar larvae were exposed to a diet incorporated blend of varying concentrations and allowed to incubate at 27°C and 14:10 L:D photoperiod. After 10 days larvae were weighed to assess growth and mortality was recorded. Mortality and growth results suggested no differences between the two strains. Fitness costs for cotton bollworm are not apparent for Cry1Ac resistant individuals. This information may be used in developing strategies for managing resistance to transgenic Bt crops.
Date: 2005-07-26
Degree: MS
Discipline: Entomology
URI: http://www.lib.ncsu.edu/resolver/1840.16/230


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