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 weedscomplicate 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 estimatethat 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.”
IPM approach needed to preserve Bt technology
By now, you’ve heard that western corn rootworms in a few Iowa cornfields have overcome the Bt toxin Cry3Bb1.
One thing that all the fields had in common: continuous use of corn with the Cry3Bb1 protein “for at least three and as many as six consecutive years,” says Aaron Gassmann, the Iowa State University entomologist who documented the resistance. “This indicates the need for better integrated pest control, and not relying on a single Bt corn year after year.”
To prevent widespread rootworm resistance to Bt, growers must adopt “a long-term integrated management approach that includes multiple tactics,” says Mike Gray, a University of Illinois entomologist who has documented severe root damage and lodging of Bt hybrids in Henry, LaSalle, and Whiteside counties. You should:
•Comply with refuge requirements. “There are a lot of different refuge scenarios for 2012, so it’s very important to be knowledgeable about the hybrids you are planting and their refuge requirements,” Gray says.
•Rotate to soybeans or another non-host crop.Crop rotation is the best way to reduces rootworm density, Gray says.
•Diversify modes of action. Switch to a hybrid containing a different Bt toxin, Gray says. Or choose a “pyramided” hybrid that expresses more than one Bt toxin targeting corn rootworms.
•Apply conventional insecticidesin fields with known high rootworm pressure. Choices include liquid or granular insecticide applied at planting or egg-laying suppression programs.
However, this does not mean that using “a Bt corn rootworm hybrid plus a soil insecticide should be a standard and routine practice in a single growing season,” Gray emphasizes. Nor should growers routinely plant a Bt corn rootworm hybrid in fields that were sprayed for western corn rootworm adults the previous summer, he says.
“We need to avoid throwing everything, including the kitchen sink, at western corn rootworms in a single growing season,” Gray says. Not only is this “insurance” approach expensive, “but ultimately it may select for resistance more rapidly and lead to unwanted environmental consequences.”
•Monitor root damageand beetle numbers. “I encourage growers to walk fields a time or two during the season to look for unexpected damage,” says Michigan State University Entomologist Chris DiFonzo. If you see significant root pruning or a large number of adult beetles in Bt corn, “contact someone as soon as possible, rather than mentioning it after harvest. This will alert companies and universities to potential problems early enough to take samples.”