Rooting Out Rootworm Resistance
Entomologists Aaron Gassmann (left), of Iowa State University, and Tom Sappington, with ARS, observe magnified western corn rootworm larvae to measure the size of their head capsules. Larvae exposed to Bt corn roots may grow more slowly than unexposed larvae, and head capsule width is an accurate indicator of larval growth stage.
The western corn rootworm (WCR) is a highly destructive pest that has required more acres of conventional insecticide use per year than any other pest since it invaded the Midwest in the 1960s. Today, WCR continues to be the most important pest of corn production across the Midwest, in part because it has developed resistance to conventional crop rotation, which was once a highly effective management tactic.
Certain types of Bt corn, which have been genetically engineered to produce a particular insect-specific toxin protein derived from the bacterium Bacillus thuringiensis, are effective in the control of WCR. These products have partially replaced the use of conventional insecticides for WCR control. But laboratory tests strongly suggest that if Bt technology is not managed effectively in the field, WCR will acquire resistance to Bt over time.
Entomologists Bruce Hibbard, in the Plant Genetics Research Unit (PGRU) in Columbia, Missouri, and Tom Sappington, in the Corn Insects and Crop Genetics Research Unit (CICGRU) in Ames, Iowa, together with Blair Siegfried, at the University of Nebraska-Lincoln, and Aaron Gassmann, at Iowa State University, are analyzing the evolution of Bt resistance in WCR to help safeguard this technology for the future.
Some Bt corn lines targeting WCR produce a protein toxin known as “Cry3Bb1.” Hibbard and collaborators recently documented that WCR exposed to these corn lines, at low to moderate doses under greenhouse conditions, developed Bt resistance after just three generations. This result was later validated in a controlled field test.
Several colonies of WCR that are resistant to Cry3Bb1 have now been established and are being used by Sappington’s group to identify genetic markers associated with the resistance. Similar studies are being conducted with Bt-corn lines that produce mCry3A and other Cry proteins.
“Development of colonies resistant to distinct Bt products will allow us to evaluate the potential for cross-resistance in WCR,” says Hibbard.
Adult female western corn rootworm, Diabrotica virgifera virgifera, on a corn leaf.
As a strategy to control the evolution of Bt resistance in WCR populations, Hibbard and collaborators are currently testing the concept of using non-Bt plants as insect “refuges.” This method promotes the mating of Bt-resistant insects with Bt-susceptible insects that emerge from refuge plants, which do not produce Bt. In this way, Bt susceptibility is maintained in the WCR population.
In 2006, Hibbard, along with CICGRU and University of Missouri collaborators, documented that without a refuge, WCR populations quickly develop Bt resistance under laboratory conditions. At the university’s Bradford Research and Extension Center, the team evaluated Bt-resistant and Bt-susceptible (control) WCR colonies. Bt corn and non-Bt corn plants were infested with 500 viable rootworm eggs from the resistant or control colonies. The number of Bt-resistant insects recovered from Bt corn was 10 times greater from eggs taken from the Bt-resistant colony than from eggs taken from the control colony. Each colony did equally well on the non-Bt corn.
“This emphasizes the importance of farmers planting refuges, because resistance developed in the absence of a refuge,” says Hibbard. Current work specifically tests the effectiveness of different refuge types.
Gathering Genetic Intelligence To Combat Western Corn Rootworm
At the Ames lab, DNA marker technology is being used to glean valuable genetic intelligence on the rootworms. DNA marker sequences are short stretches of DNA along the pest’s genome that uniquely identify a region of that chromosome. A DNA marker that is located near a gene that confers a trait of interest can often be detected more easily than the gene itself.
Sappington’s team is also using DNA markers to genetically locate the gene or genes responsible for “rotation resistance,” an egg-laying behavior that enables some rootworm populations to survive rotations of corn with soybean, a nonhost crop on which the pest’s caterpillars cannot feed.
Farmers in the Midwest have long used corn-soybean rotations, which allows them to forgo using soil insecticides on their corn. But starting in 1987, reports emerged from eastern Illinois that female rootworms were flying out of corn fields to deposit their eggs in nearby soybean crops—in effect, synchronizing their larval offsprings’ emergence to the next season’s rotational corn crop.
The trait has since spread to parts of Indiana, Ohio, Michigan, Missouri, Iowa, and Wisconsin and to Ontario, Canada. A DNA marker associated with rotation resistance would help alert farmers to when the trait has, or has not, spread to their region.
DNA markers are also being developed for examining genetic variation and gene flow within and between WCR populations.
“Though difficult to achieve, information on the population ecology and genetics of WCR are crucial for designing effective resistance-management strategies to maintain the viability of current control technologies, including Bt corn,” says Craig Abel, CICGRU research leader. “Molecular markers are important tools for obtaining this information.”
Sappington, David Grant, Nicholas Miller, and others at Ames are collaborating with university colleagues and the Institut National de Recherche Agronomique in France to conduct more sensitive analyses of genetic variation in populations. The scientists are taking specimens from WCR’s native range in the Great Plains and trying to get better population-structure data than that obtained previously from the Corn Belt, where the pest appeared more than 40 years ago—not long when it comes to random genetic change in huge populations, says Sappington.
“By analyzing these long-established populations, we hope to estimate how far rootworm adults move per generation,” he says. “Such estimates are important for accurately predicting the development and spread of Bt-resistance and rotation-resistance traits over long distances—almost impossible to do using conventional methods such as mark-release-recapture experiments.”—By Alfredo Flores, formerly with ARS, and Jan Suszkiw, Agricultural Research Service Information Staff.
This research is part of Integrated Agricultural Systems (#207) and Crop Protection and Quarantine (#304), two ARS national programs described at www.nps.ars.usda.gov.
To reach scientists mentioned in this story, contact Jan Suszkiw, USDA-ARS Information Staff, 5601 Sunnyside Ave., Beltsville, MD 20705-5129; (301) 504-1630.
"Rooting Out Rootworm Resistance" was published in the September 2010 issue of Agricultural Research magazine.