A team of University of Arizona researchers claims that Bt-resistant populations of cotton bollworm, Helicoverpa zea, have evolved in more than a dozen crop fields in Mississippi and Arkansas.
“What we're seeing is evolution in action,” said lead researcher Bruce Tabashnik. “This is the first documented case of field-evolved resistance to a Bt crop.”
However, University of Arkansas entomologist Randy Luttrell, whose data were used in the study, disagreed with some of Tabashnik's conclusions. “One of the key words is field resistance. If I'm talking to an Arkansas or Mississippi cotton farmer, field resistance means they believe they have a problem in the field that's worse than they had before. That's where I don't necessarily agree (with Tabashnik's conclusions). That doesn't mean I'm ruling out the possibility of it.”
According to a University of Arizona news release, bollworm resistance to Bt cotton was discovered when a team of 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 were 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,” said Tabashnik, professor and head of UA'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.”
Luttrell agrees with Tabashnik on this point. In 1992, he collected a few hundred bollworms from Texas, North Carolina and the Hill region of Mississippi and selected for a colony with LC50s (the dose required to kill 50 percent of the test population) 100-fold those of a susceptible unselected laboratory colony. At the time, Luttrell was a researcher at Mississippi State University working under a research contract with Monsanto.
Ten years later, Luttrell, at the University of Arkansas, looked a little harder at resistance. “There were people talking about bollworms surviving on Bt cotton particularly at the end of the year. We picked up those insects, and indeed they had elevated LC50s just like the colony previously selected at Mississippi State.”
But there was something very unique about these less susceptible insects — their populations weren't stable. “We can't hold them in the laboratory. They have a fitness cost, a few generations and they're gone. I believe when the insect has that resistant gene or genes, it gives up something else. We also have lots of insects floating around the system that are diluting the resistant trait out.
Luttrell stressed that resistance can come from any heritable trait — even a behavioral change. “It's been shown that mosquitoes can be selected to avoid surfaces treated with DDT, apparently a genetically controlled trait.”
For bollworms, “There's the idea that insects have been selected to lay eggs on bloom tags at lower locations in the plant canopy, these eggs hatch, and the larva avoids toxic contact by feeding on non-expressing plant tissue. It is certainly true that some plant tissues have lower toxin expression, but I can't say the literature concludes that we have selected out populations that lay eggs further down in the canopy.”
Luttrell stressed that the scientific and agricultural communities “need to remain vigilant, and realize that resistance can happen. I have seen no evidence that anything in 2007 is different than it was in 2002. Dr. Tabashnik could be very insightful, but I don't believe the current data support a conclusion of field-evolved resistance.”
The University of Arizona researchers write in their report that Bt cotton and Bt corn have been grown on more than 400 million acres worldwide since 1996, “generating one of the largest selections for insect resistance ever known.”
University of Arizona researchers concluded that most caterpillar pests of cotton and corn remained susceptible to Bt crops. “The resistance occurred in one particular pest in one part of the United States,” Tabashnik said. “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, he said.
“The only other case of field-evolved resistance to Bt toxins involves resistance to Bt sprays,” Tabashnik said. He added that such sprays have been used for decades, but now represent a small proportion of the Bt used against crop pests.”
An article on the study was published in the February issue of Nature Biotechnology. His co-authors are Aaron J. Gassmann, a former UA postdoctoral fellow now an assistant professor at Iowa State University; David W. Crowder, a UA doctoral student; and Yves Carrière, a UA professor of entomology. Tabashnik and Carrière are members of UA's BIO5 Institute.
The U.S. Department of Agriculture funded Tabashnik's research.