The return of the billion-dollar bug
Last July, Mark Moeller drove into Peter Bakken's yard near Beaver Creek, Minnesota, and delivered sobering news.
Moeller, a Centrol crop consultant, told Bakken that western corn rootworm beetles were ravaging one of his pollinating cornfields. “They were chewing up the silks like crazy,” says Bakken. “We figured we had just 20% pollination in that field.”
Prior to 2003, this damage would have upheld the corn rootworm's reputation as the billion-dollar bug. Each year, the pest cost U.S. farmers $1 billion in lost yields and control measures.
That changed, though, in 2003, when corn rootworm traits supplanted former control measures of crop rotation and soil-applied and aerial-applied insecticides.
Before 2012, Bakken derived excellent rootworm protection by planting hybrids containing a Monsanto rootworm-resistant trait containing the Cry3Bb1 protein. This trait package, derived from the soil bacterium Bacillus thuringiensis (Bt), killed corn rootworm and European corn borer.
Last year was a different story, though. Bakken quickly salvaged what was left of the field by aerially applying a foliar insecticide that killed about 85% of the beetles. When they could enter the field again, Bakken and Moeller discovered 80 out of the 240 acres were annihilated. Root digs revealed excessive root damage feeding. However, damage paled compared to the 20% of the field planted as a resistance management refuge. Protected only by the soil insecticide, the mangled refuge corn yielded just 10 bushels per acre. “This brought down the field average to around 80 bushels per acre,” Bakken says.
Even more puzzling were 160 acres of corn that flanked the rootworm-damaged field. “It was some of the better corn we had,” he says. Despite the drought, excellent rootworm protection occurred on these acres, which averaged a 150-bushel-per-acre yield.
“You could pretty much draw a line as to where the damage was and where it wasn't,” says Bakken.
Similar situations surfaced last year in other rootworm trait-protected fields in southwest Minnesota, east-central South Dakota, and northeast Nebraska.
“We got hit pretty hard in some fields, mostly in continuous corn,” Moeller says. “The abundance of western corn rootworm beetles was just amazing. On some ears, there were 20 western corn rootworm beetles.”
Root checks in these fields revealed extensive feeding.
“In some cases, there just weren't any roots,” he says.
Bakken notes that Monsanto has been responsive to the situation, and he, Moeller, and Monsanto officials will closely monitor fields in 2013. Still, he's baffled over this hot spot in the middle of otherwise good-yielding corn.
“In Minnesota, we have had a Lake Wobegon perspective, where all women are strong, all men are good looking, all kids are above average, and Bt rootworm (resistant) corn always works,” says Ken Ostlie, University of Minnesota Extension entomologist.
No more. “We may end up returning to a point where we look at what's happening in a field and make more field-specific decisions,” he says. “I think the ‘easy’ is going out of corn rootworm management.”