Seed Today - Q1 2008 - (Page 14)

Arizona researchers report limited cotton bollworm Bt resistance Cotton bollworm (Helicoverpa zea) USDA Photo by Scott Bauer. In spite of detailed insect resistance management (IRM) practices, it appears there is evidence of cotton bollworm (Helicoverpa zea) resistance to toxins produced by bacterium Bacillus thuringiensis (Bt). Bt-resistant populations of bollworm were found in more than a dozen crop fields in Mississippi and Arkansas between 2003 and 2006, according to a study released by researchers at the University of Arizona’s BIO5 Institute. Evolution in Action "What we're seeing is evolution in action," says lead researcher Dr. Bruce Tabashnik. "This is the first documented case of field-evolved resistance to a Bt crop.” The bollworm resistance to Bt cotton was discovered when University of Arizona entomologists analyzed published data from monitoring studies of six major caterpillar pests of Bt crops in Australia, China, Spain and the United States. The data documenting bollworm resistance was first collected seven years after Bt cotton was introduced in 1996. "Resistance is a decrease in pest susceptibility that can be measured over human experience," says Tabashnik, head of University of Arizona's entomology department and an expert in insect resistance to insecticides. "When you use an insecticide to control a pest, some populations eventually evolve resistance." Even so, the researchers found that most caterpillar pests of cotton and corn remained susceptible to Bt crops. The major caterpillar pest of cotton in Arizona is a different species known as pink bollworm (Pectinophora gossypiella) which has remained susceptible to the Bt toxin in biotech cotton. "The resistance occurred in one particular pest in one part of the United States.," Tabashnik says. "The other major pests attacking Bt crops have not evolved resistance. And even most bollworm populations have not evolved resistance." The field outcomes refute some experts' worst-case scenarios that predicted pests would become resistant to Bt crops in as few as three years. “The only other case of field-evolved resistance to Bt toxins involves resistance to Bt sprays, Tabashnik says. To delay resistance, non-Bt crops are planted near Bt crops to provide "refuges" for susceptible pests. Because resistant insects are rare, the only mates they are likely to encounter would be susceptible insects from the refuges. generally would be susceptible to the toxin. In most pests, offspring are resistant to Bt toxins only if both parents are resistant. In bollworm, however, hybrid offspring produced by matings between susceptible and resistant moths are resistant. Such a dominant inheritance of resistance was predicted to make resistance evolve faster. The Arizona researchers found that bollworm resistance evolved fastest in the states with the lowest abundance of refuges. The field outcomes documented by the global monitoring data fit the predictions of the theory underlying the refuge strategy, Tabashnik says. Although first-generation biotech cotton contained only one Bt toxin called Cry1Ac, a new variety contains both Cry1Ac and Cry2Ab. The combination overcomes pests that are resistant to just one toxin. The next steps, Tabashnik says, include conducting research to understand inheritance of resistance to Cry2Ab and developing designer toxins to kill pests resistant to Cry1Ac. For more information: Dominate Resistance The hybrid offspring of such a mating Bruce Tabashnik, Ph.D. Tucson, AZ (520-621-1141/brucet@ag.arizona.edu) 14 First Quarter 2008

Table of Contents Feed for the Digital Edition of Seed Today - Q1 2008

Seed Today - Q1 2008 Contents Editor’s Notes Cover Feature: Agrecol® Corporation Reed Barker, Ph.D. Cal/West Seeds Inc. Annual GM Use Report Bt Resistance Reported Camelina Tony Harper Gayle Jacklin According to ASTA Krauter Structures AOSCA AOSA/SCST GM and Greenhouse Gas Seed Associations Ad Index

Seed Today - Q1 2008

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