What is in this article?:
- Refine your Refuge | It's More Important Than Ever This Year
- IPM approach needed to preserve Bt technology
Minnesota farmer Gene Stengel and his sons Kevin and Rob will plant a 20% insect refuge this spring, as usual. Although “refuge-in-the-bag” blends of Bt and non-Bt seeds are proving popular with farmers – and could predominate in coming years – many growers will still plant structured non-Bt refuges in 2012.
“Following refuge guidelines is important to manage insect resistance,” says Michigan State University Entomologist Chris DiFonzo, “and even more critical now that Bt rootworm control has been compromised” in some Midwest fields.
To manage corn rootworms in your structured refuge, select the best non-Bt hybrid for each field and consider soil insecticide to kill rootworm larvae, says Jeff Coulter, University of Minnesota Extension corn agronomist.
A significant yield hit to the refuge is not inevitable unless larvae numbers are high, Coulter says. That’s because insect traits don’t add yield, but rather protect yield potential. And “unlike Bt corn for corn borer, Bt corn for rootworm has a moderate dose of Bt,” says DiFonzo. “It has never provided excellent control like corn borer corn.”
So when it comes to managing your refuge, Coulter says, “Don’t lower your expectations – manage for yield just like you do for the rest of your corn.”
The Stengels raise seed soybeans and corn near Granite Falls, MN, and operate a large custom-farming and trucking business. They see both northern and western corn rootworms in their fields. The Stengels’ non-Bt corn often yields about the same as their Bt corn, Gene says, producing around 170 bu./acre, their 10-year average. “But it does take more management, and the right genetics,” says their crop consultant, Dorian Gatchell, Minnesota Agricultural Services, Granite Falls.
Glyphosate-resistant weeds complicate refuge management on the Stengels’ farm, Gatchell adds. Most cropland in the region has some frequency of glyphosate-resistant giant ragweed, waterhemp or both. Just to the north of the Stengels in Swift County, glyphosate-resistant waterhemp is suspected on at least 70% of fields.
To preserve glyphosate’s effectiveness for their high-value seed soybean crops, the Stengels use a glufosinate herbicide program on corn. They plant triple-stack hybrids with the Liberty Link trait, and place their refuges in a block configuration so they can scout and manage them more intensively. They use seed treatments and put down a soil insecticide, Lorsban, in a T-band at planting.
Their biggest refuge-management challengeis getting good non-Bt hybrids. “There are very few non-Bt corn hybrids with the Liberty Link trait for our region,” Gatchell says. For 2012, they had only two to pick from.
“A lot of times, I don’t think we’re getting the best genetics or best-yielding choices” in non-Bt seeds, Gene says. “We have to settle for what’s available.” It’s that, he adds, or “being non-compliant.”
Recent surveys estimate that about 75% of farmers comply fully with refuge size and placement requirements, says Mike Gray, a University of Illinois entomologist who informally tracks refuge compliance in Illinois.
“In the last few years, I hear anecdotally that people are planting less refuge than required,” Chris DiFonzo says. “Sometimes this was intentional, but more often than not, the conventional seed supply was short at planting time, or the conventional options were viewed as poor-performing hybrids.”
This year, as in past years, the Stengels’ seed dealer couldn’t supply a conventional Liberty Link hybrid for the insect refuge, so they had to find another supplier. Dealing with several seed companies can make it trickier to get a good match between the refuge and Bt hybrids, which is important for production efficiency, Coulter says.
For example, growers should keep in mind that “relative maturity ratings are somewhat company specific,” he notes. If you plant your refuge in a series of rows throughout the field, your non-Bt and Bt hybrids should be of similar relative maturity to ensure good pollination – ideally within a couple of days. And different hybrids sometimes have different optimum seeding rates, he adds.
One refuge-management mistake growers sometimes make is placing the non-Bt corn in ground with a different cropping history than the Bt corn, says Iowa State University Entomologist Aaron Gassmann. If you plant Bt corn in fields that were corn the previous year, the refuge must be, too. Otherwise, he explains, there won’t be enough susceptible insects to mate with potentially resistant corn rootworms emerging from the Bt acres.
Gassmann recently confirmed that western corn rootworms in some Iowa fields have become immune to the Bt toxin Cry3Bb1, which is found in YGRW, YieldGard Plus, YieldGard VTRW and YieldGard VT Triple. The affected fields were all in continuous-corn production. Unexpectedly severe root pruning of Bt hybrids has also been reported in Illinois, Minnesota, Nebraska and South Dakota, but it’s not known if resistant bugs were the culprit.
“Now that we have the first case of resistance,” Gassmann says, “it highlights the importance of planting the correct refuge and using integrated pest management.”
Looking ahead, Gene Stengel worries about losing one of his most useful weapons against corn rootworms. Son Rob, age 23 and just beginning his farming career, fears that “we could be facing a train wreck” of pesticide-resistant weeds and bugs, from corn rootworms to soybean cyst nematodes. That’s why refuge compliance “matters to us,” Gene says. “It’s a stewardship issue. We’re doing our part to protect and prolong the usefulness of a technology that works well now.”