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United States Department of Agriculture

Agricultural Research Service

Research Project: EFFECT OF RESISTANCE ON INSECT PEST MANAGEMENT IN TRANSGENIC COTTON

Location: Southern Insect Management Research Unit

Title: Risk assessment for Helicoverpa zea (Lepidoptera: Noctuidae) resistance on dual-gene versus single-gene corn

Authors
item Edwards, Kristine -
item Caprio, Michael -
item Allen, Clint
item Musser, Fred -

Submitted to: Journal of Economic Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 21, 2012
Publication Date: February 1, 2013
Repository URL: http://handle.nal.usda.gov/10113/58243
Citation: Edwards, K.T., Caprio, M.A., Allen, K.C., Musser, F. 2013. Risk assessment for Helicoverpa zea (Lepidoptera: Noctuidae) resistance on dual-gene versus single-gene corn. Journal of Economic Entomology. 106(1):382-392. DOI:http://dx.doi.org/10.1603/EC12203

Interpretive Summary: The acreage planted to transgenic crops expressing insecticidal toxins from a soil bacterium (Bt) has increased dramatically since their introduction of Bt cotton in 1996. The original Bt crop varieties possessed a single toxin for the control of various caterpillar pests. Along with the introduction of these transgenic crops were EPA regulations which mandated the planting of a refuge of non-Bt plants at a location to provide a source of non-exposed insects and reduce possible selection for resistance to this toxin. This consisted of planting a minimum percentage of the total acreage of particular crop to non-Bt varieties in a location. Since that time, crops expressing two different Bt toxins for caterpillar control have entered the market and replaced the original single-gene varieties. With the introduction of these two-gene crop plants, the EPA has reduced the allocation of the refuge of corn from 50% to 20% in cotton growing areas and eliminated the required cotton refuge for cotton varieties expressing two Bt toxins. A computer model (POPGEN) incorporating the biology of the corn earworm was used to examine the risk of resistance evolution of this insect to Bt crops expressing two Bt genes in lieu of these relaxed refuge requirements. Expert opinions on the mortality of corn earworm to several transgenic corn and cotton varieties were captured and used to assign probabilities to different scenarios in the risk assessment. At least 350 model runs with randomly drawn parameters were completed for each of four risk assessments. In the case of single-gene corn and cotton with no volunteer corn, 22.5% of the simulations resulted in evolution of resistance within 30 years. When volunteer corn, used as a source of C4 moths in late summer, was added to this assessment, risk of resistance evolving within 30 years declined to 13.8%. When two-gene Bt-cotton, planted with a natural refuge and single-gene corn planted with a 50% refuge was simulated, resistance to neither toxin occurred within 30 years, but in 38.5% of the simulations, resistance evolved to the toxin present in single-gene Bt-corn (Cry1A). When both corn and cotton were simulated as two-gene products, cotton with a natural refuge and corn with a 20% refuge, 3% of the simulations evolved resistance to both toxins within 30 years, while 10.4% evolved resistance to the Cry1A toxin.

Technical Abstract: Recent changes in EPA regulations have prompted concern in some experts that transgenic corn expressing two lepidopteran-active genes from the soil bacterium Bacillus thuringiensis (Bt) (dual-gene) may result in more rapid selection for resistance in Helicoverpa zea (Boddie) than corn expressing a single lepidopteran-active Bt toxin (single-gene). The concern is that longevity of Bt toxins could be significantly reduced with the recent adoption of a natural refuge for dual-gene Bt-cotton and the concurrent reduction in the corn refuge from 50% to 20% in dual-gene corn hybrids. A population genetics simulation framework (POPGEN) was applied to the assessment of this risk. POPGEN simulates complex landscapes that vary spatially as well as temporally over the course of a growing season and applies stochastic functions to mortality, fecundity, mating, and dispersal. Expert opinions on the effectiveness of several transgenic corn and cotton varieties were captured and used to assign probabilities to different scenarios in the assessment. For each parameter, a probability distribution based on these expert opinions was specified. A PERT-Beta distribution specifying maximum, minimum, and most likely values was used for the distributions. Numerous model realizations were run, randomly drawing new values for each parameter from the specified probability distributions. At least 350 replicate simulations with randomly drawn parameters were completed for each of four risk assessments. In the case of single-gene corn and cotton with no volunteer corn, 22.5% of the simulations resulted in evolution of resistance within 30 years. When volunteer corn, used as a source of C4 moths in late summer, was added to this assessment, risk of resistance evolving within 30 years declined to 13.8%. When dual-gene Bt-cotton, planted with a natural refuge and single-gene corn planted with a 50% refuge was simulated, resistance to neither toxin occurred within 30 years, but in 38.5% of the simulations, resistance evolved to the toxin present in single-gene Bt-corn (Cry1A). When both corn and cotton were simulated as dual-gene products, cotton with a natural refuge and corn with a 20% refuge, 3% of the simulations evolved resistance to both toxins within 30 years, while 10.4% evolved resistance to the Cry1A toxin.

Last Modified: 12/20/2014
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