Part 4 of our "Resistance Reality" story, featured in the Farm Journal 2012 Seed Guide. Click here to start reading from the beginning.
Throughout history, Mother Nature has overcome almost all man-made pest control strategies.
"Corn rootworm has shown tremendous ability to adapt, and they’re going to adapt to Bt eventually," says DuPont Pioneer’s Pilcher.
Variants already exist within the rootworm family, of which there are four main species: western corn rootworm, northern corn rootworm, Mexican corn rootworm and southern corn rootworm, also referred to as the spotted cucumber beetle.
A variant of the western corn rootworm species is particularly troubling. Females of this variant have adapted to laying their eggs in soybean fields so their offspring hatch in corn fields the following year. This pest is primarily taking up residence in portions of Illinois and Iowa.
"Rotating to soybeans as a rootworm management tool is practically lost in those areas where this variant is an issue," Araba says.
A variant of the northern corn rootworm species also exists. It has developed an extended diapause, meaning its eggs remain viable two to three years in the soil before hatching. This variant, Pilcher says, is in northwest Iowa, eastern South Dakota and southwest Minnesota.
Rootworm biology and the pests’ ability to withstand the Bt technology complicate control measures, explains University of Illinois’ Gray. Because Bt toxins are low- to moderate-dose technology, roughly one in six of the pests routinely survives the control measure. Furthermore, rootworm larvae don’t usually travel more than a few feet from where they originally hatch. The pests stay congregated in the same field through the larval stage, potentially contributing to the resistance issue.
"It’s a homegrown problem—I’m reaping what I’m sowing when it comes to corn rootworm," explains Patrick Porter, Lubbock-based Texas AgriLife Research and Extension entomologist at Texas A&M University.
The frequency of resistance increasing across the Corn Belt if farmers keep planting the same variety of Bt toxic corn in the same field is "somewhat concerning" for entomologist B. Wade French.
Based in Brookings, S.D., French hasn’t observed widespread resistance in his work for USDA’s Agricultural Research Service. "There probably are small pockets of resistant beetles scattered across the state, which will happen when the same variety is planted annually," he adds.
Texas A&M’s Porter says he is not surprised by the development of field-evolved rootworm resistance.