Monday, August 08, 2011
Corn Rootworms Developing Bt Resistance In Iowa
“That’s not a real surprise.” “It was only a matter of time.” “You can’t put the genie back in the bottle.” Those undoubtedly are the thoughts of many corn farmers, seed corn companies, corn researchers, and others who are learning about the findings of Iowa State University entomologists that corn rootworms have become resistant to a primary Bt gene in the northeastern part of Iowa. Oooops.
USDA’s June 30 Planted Acreage Report indicated that 16% of corn acres were planted with insect resistant hybrids, 49% of corn acreage was planted with stacked gene varieties, and 88% of corn acres were planted with some biotech corn hybrid. It is not really possible to determine how many acres were planted with the Cry3Bb1 Bt hybrid, but that has become a problem for some farmers in northeastern Iowa where researchers report “that fields identified by farmers as having severe rootworm feeding injury to Bt maize contained populations of western corn rootworm that displayed significantly higher survival on Cry3Bb1 maize in laboratory bioassays than did western corn rootworm from fields not associated with such feeding injury.”
They report that the fields with the major problems had been grown for three consecutive years, and the researchers say, “There was a significant positive correlation between the number of years Cry3Bb1 maize had been grown in a field and the survival of rootworm populations on Cry3Bb1 maize in bioassays. However, there was no significant correlation among populations for survival on (other Bt hybrids), suggesting a lack of cross resistance between these Bt toxins.”
This is the first such report of a failure of Bt genetics to control corn rootworm. But why did it fail? One of their theories is that the Bt toxin comes in a low dose, and may not have delayed the development of resistance sufficiently, as would have been the case if the offspring with the resistance, had come from one adult that survived in a refuge, and another adult that had survived the Bt toxin.
Another theory was the insufficient refuges that were being planted. They report that only 50% of Bt corn being planted complies with refuge requirements, and the lack of refuges that control rootworms with conventional insecticide resulted in the quick development of resistance.
The researchers looked at hybrids that have “pyramid” combinations of Bt toxins, which allow one toxin to control the rootworm, if resistance is developed against the other. They say the rootworms that apparently were surviving one, were not surviving the other, and the pyramiding concept is working. They note that so far there is only a small amount of resistance developed, of which they are aware. However, they add, “These recent cases suggest a need to develop more integrated management solutions for pests targeted by Bt crops. A common pattern observed among problem fields in this study was the consecutive planting of the same type of Bt maize over several seasons. Even with resistance management plans in place, sole reliance on Bt crops for management of agriculture pests will likely hasten the evolution of resistance in some cases, thereby diminishing the benefits that these crops provide.”
At the University of Illinois, entomologist Mike Gray says, “Our current approach is insurance-based, fueled by high commodity prices. The results of the western corn rootworm survey (in Illinois, which found less than 1 adult beetle per stalk) attest to the fact that we are applying enormous selection pressure to this insect species. The pressure comes in multiple forms: increasing use of Bt hybrids, neonicotinoid insecticidal seed treatments, and broadcast treatments to corn and soybean fields of pyrethroid insecticides that are frequently tank-mixed with fungicides.”
At Purdue University, entomologist Christian Krupke says, “Hybrids expressing this toxin include those formerly labeled as Yieldgard RW and VT3 hybrids. This toxin is also one of the proteins found in SmartStax hybrids. The good news is that the study tested the other major toxins deployed in North America against this pest, Cry34/35 (found in Herculex hybrids targeting rootworms and also in SmartStax hybrids), and no enhanced survival was found. Although Cry3bb1 and Cry34/35 toxins are different, they are similar enough that cross-resistance (where surviving exposure to one toxin confers some level of survival to another), was a possibility worth investigating.” And he adds, “It certainly is not good news, but it is not a total disaster either. First off, it demonstrates again what a remarkably adaptable pest the western corn rootworm is. There is no “putting the genie back in the bottle,” and resistance in these areas is a problem that won’t go away. This is an alert to keep our eyes open for similar occurrences elsewhere.”
Krupke’s advice to farmers is rather straightforward, “Planting the recommended refuge certainly helps, and although compliance with the refuge requirements has been falling in recent years, this serves as a stark reminder of how important it is. “
Summary:
Corn rootworms in northeastern Iowa have become resistant to one of the Bt toxins commonly inserted in may corn hybrids. They are being controlled if the hybrid has a second toxin. Most of the resistance has come in fields of continuous corn, and in areas where refuge compliance is at a low level.
Posted by Stu Ellis on 08/08 at 12:00 AM | Permalink
Comments
Posted by: James T. Fisher at August 9, 2011 4:04AM
funny:
St Louis Post-Dispatch
July 23, 2001
‘Independent testing of the product at the University of Illinois, Urbana-Champaign, have produced encouraging results, said Kevin Steffey, an entomologist with the school’s department of crop sciences. Under a 1-to-6 ranking system adopted by farmers to measure corn root damage (with 6 being the worst), more than 95 percent of the Monsanto corn tested rated 3 or below, he said, which “is pretty damn good.”
In order for Monsanto to succeed, the product “has to be at least as effective as an insecticide,” Steffey said.
Steffey thinks YieldGuard Rootworm corn will not only compete with insecticides, but one day replace them altogether. But like all pests, he said, the rootworm beetle could also one day develop a resistance to this corn.’
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Posted by: s. morris at January 12, 2012 12:12PM
The European Union and the United States have strong disagreements over the EU’s regulation of genetically modified food. The US claims these regulations violate free trade agreements, the EU counter-position is that free trade is not truly free without informed consent.