Bt corn refuge discussions continue
By Larry Dreiling
Two years ago, some of the top university entomologists in the United States said it was critical that farmers alter their planting decisions to cope with the 2009 discovery of field-evolved corn rootworm resistance to a Bt
transgenic corn hybrid in the continental U.S.
This “group of 22” also told the Environmental Protection Agency that reducing the resistance situation must occur through an integrated approach toward effective long-term corn rootworm management and sustainable use of
Bt hybrid technology that is not overly reliant on any single tactic.
Currently, there are Bt
varieties that use three different genes that act as toxins for corn rootworm. It was in Bt hybrid expressing the Cry3Bb1 gene (developed by Monsanto under the trade named YieldGard RW) that was first shown in 2009 to have greater than expected rootworm damage.
By 2011, the scientists wrote, problem areas had been reported in northwestern and north-central Illinois, northeastern Iowa, southern Minnesota, northeastern Nebraska, and eastern South Dakota. Common features of affected fields in these areas included a history of continuous planting to corn and the use of Cry3Bb1-expressing hybrids for multiple years.
Earlier this year, an article published in the Proceedings of the National Academy of Sciences, reported that during 2011, injury to Bt corn in the field expanded to include mCry3A corn (developed by Syngenta under the Agrisure CB/LL/RW, Agrisure 3000 GT, Agrisure 3111, and Agrisure 3122 Refuge Renew trade names) in addition to Cry3Bb1 corn.
Also, laboratory analysis of western corn rootworm from these fields found resistance to Cry3Bb1 and mCry3A and cross-resistance between these toxins. Resistance to Bt corn has persisted in Iowa, with both the number of Bt fields identified with severe root injury and the ability of western corn rootworm populations to survive on Cry3Bb1 maize increasing between 2009 and 2011.
Bt corn targeting western corn rootworm does not produce a high dose of Bt
toxin, and the magnitude of resistance associated with feeding injury was less than that seen in a high-dose Bt crop.
The new data indicating these first cases of resistance by western corn rootworm highlight the vulnerability of Bt corn to further evolution of resistance from this pest and, more broadly, point to the potential of insects to develop resistance rapidly when Bt crops do not achieve a high dose of Bt toxin.
Moreover, corn engineered to produce multiple Bt toxins—so-called stacked varieties—won’t do much to slow the evolution of rootworm resistance, as was originally hoped.
Pyramiding of two Bt toxins delays resistance because individuals that harbor resistance alleles to one toxin are killed by a second toxin, with greater delays in resistance arising when the frequency of resistance alleles is low and there is an absence of cross-resistance between Bt toxins, the study indicated.
“Unless management practices change, it’s only going to get worse,” said Aaron Gassmann, Ph.D., an Iowa State University assistant professor of entomology and co-author of a March 17 study in the Proceedings of the National Academy of Sciences describing rootworm resistance. “There needs to be a fundamental change in how the technology is used,” Gassmann was quoted in an article on wired.com.
Gassmann was one of the 22 entomologists who signed on to the comment to the EPA in March 2012. Another of the entomologists was Elson Shields, Ph.D., professor of entomology at Cornell University. Shields wrote last year about how the less-than-high-dose of Bt toxin in the plant allowed insects with a low level of resistance to survive the toxin, molt into adult insects, mate and lay eggs.
“The higher the toxin dose, the fewer insects initially survive and generally the longer it takes for the insect to develop resistance. The planting of untreated refuges produce large numbers of unexposed beetles to dilute any genetic resistance, thereby keeping the frequency of the resistance very low,” Shields said.
“The lack of Bt-free refuges allows the Bt-toxin survivors to inter-mate and concentrate the genetic basis for resistance, allowing a larger portion of the population to survive the toxin each year, thus increasing the inter-mating between individuals with a lower level of resistance.”
As a result, Shields said, individual (insects) in each succeeding generation have an increased level of resistance to the toxin and have an increased survival. The cycle continues with each subsequent year.
Approval of these trait pyramids for management of western corn rootworm was accompanied by a reduction in non-Bt refuges to 5 percent. However, the presence of resistance to one toxin in a pyramid diminishes the effectiveness of a pyramid to delay resistance, and, coupled with reduced refuge size, may hasten the evolution of resistance, the new study indicated.
“In light of resistance to Cry3Bb1 and mCry3A, the 5 percent refuge associated with current Bt pyramids targeting western corn rootworm may do little to delay resistance, and larger refuges should be considered as a tactic to delay resistance,” the study said.
Additionally, cultivation of Bt corn should be better integrated with other strategies for management of western corn rootworm, such as crop rotation, which will reduce selection for resistance and may help delay the further evolution of Bt resistance by this pest.
Indeed, all companies with Bt varieties encourage farmers to follow recommended Integrated Pest Management practices, including cultural control tactics, scouting and the appropriate use of pest thresholds and sampling.
For those fields with greater than expected corn rootworm damage, the implementation of very specific best management practices are recommended.
Perhaps the lead BMP all companies—and the independent entomologists—prefer is rotating the field to a non-host crop such as soybeans, which breaks the corn rootworm cycle.
Also recommended is rotating Bt corn hybrids to expose rootworms to different Bt toxins, and using conventional corn hybrids to avoid “prophylactic” planting of Bt
corn in fields where there is no corn rootworm problem.
Perhaps the final sentence of the 2012 statement by the entomologists continues to be true: “Understand that Bt resistance has real economic and environmental costs, not just to themselves, but to the nation, so following the stewardship rules is vital.”
Larry Dreiling can be reached by phone at 785-628-1117 or by email at firstname.lastname@example.org.