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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.”

Out of the blue

After Monsanto launched its YieldGard RW trait in 2003, Dow AgroSciences and DuPont Pioneer launched Herculex RW hybrids containing the Cry34/35Ab1 protein in 2005. Syngenta followed with its Agrisure RW trait containing the mCry3A protein in 2006.

Pyramids featuring two rootworm control modes of action soon surfaced. Dow and Monsanto launched SmartStax hybrids featuring the Cry34/35Ab1 and Cry3Bb1 proteins. Syngenta unveiled an Agrisure 3122 pyramid containing the mCry3A and Cry34/35Ab1 proteins. DuPont Pioneer's Optimum AcreMax XTreme pyramid contains Cry35/34Ab1 and mCry3A proteins.

Hybrids with the rootworm-resistant traits sold faster than beer at a sweltering summer softball game. Corn acres planted to Bt corn rose from 29% in 2003 to 67% in 2012.

Under a traits-control strategy, corn rootworm still nibble at roots but die quickly after ingesting the toxic protein. This plan nixes resistance, as resistant survivors mate with susceptible ones from a 5% to 20% refuge of conventional corn.

All was well until 2009. Goosenecked corn — the calling card of excessive corn rootworm root feeding — started appearing in some Minnesota cornfields planted to hybrids with rootworm-resistant traits.

“It was out of the blue,” says Ostlie. “In 2010 and 2011, we had scattered performance problems in some fields. But in 2012, we saw a dramatic increase in the geographical extent and intensity of the problem.”

These findings echoed confirmation of rootworm resistance to the Cry3Bb1 protein in Iowa in 2011 and Illinois in 2012. These findings, along with subpar performance in other Corn Belt states, occurred mainly in fields protected by the Cry3Bb1 protein. But corn rootworm also damaged Minnesota fields planted to hybrids with the Agrisure and Herculex trait and the SmartStax package.

“Some fields that originally had Cry3Bb1 performance problems now demonstrate performance issues with other traits,” says Ostlie. “If we lose one trait in the field, we may lose more than one trait in terms of effectiveness.”

Don't blame the traits

remember that rootworm-resistant traits worked in most fields in 2012. For example, six years of Dow AgroSciences internal data over 34 sites show its SmartStax product keeps rootworm damage below levels where lodging can occur 99.5% of the time.

“It is the repetitive use of the same trait over and over again that leads to problems,” says Ostlie.

That principle applies to all controls, not just hybrids containing the Cry3Bb1 protein. The growth in continuous corn coincides with continuous use of the same trait.

“Continuous corn is the focal point for these performance problems,” Ostlie adds.

Weather likely aggravated performance problems. “Last year was a mild winter, so there was little winter (rootworm) mortality,” says Luke Samuel, Monsanto corn insect product development manager. A warm and dry spring also ensured high populations.

“There are some fields where pressure is so great that any Bt product out there will see some challenges,” he says.

Subpar performance reports prompted Bakken to apply a three-fourths soil insecticide rate on top of the rootworm-resistant trait in 2012. He believes a pounding rain that occurred after he planted his rootworm-devastated field played a role.

“It rained 3 inches right as I was pulling out of the field, and that was the moisture for the whole growing season,” he recalls. He planted the remaining 160 acres three days later.

“We still had a few issues with the refuge corn, but everything looked considerably normal,” Bakken says. He theorizes the pounding rain likely washed insecticide away. If the rootworm-resistant trait was compromised, the insecticide was his last opportunity for control.

Rotate, rotate, rotate

Refuge compliance is another concern. It's been trending downward since rootworm-resistant corn debuted. Released in 2009, a survey by the Center for Science in the Public Interest showed refuge size and distant requirement compliance for rootworm-resistant corn plunged from 89% and 82%, respectively, in 2006 to 74% and 63% in 2008, respectively.

“Historically, there has been a lack of refuge compliance, but I don't think that is the sole factor behind what we are seeing,” says Ostlie.

One of the first fields with poor trait performance he studied had a split-planter refuge. “This is probably the best refuge configuration we have,” he says. Yet, control was poor.

“The most important factor is continuous use of the same trait,” Ostlie adds. “If you have used the same trait for three to five years, you should seriously monitor that field. If beetle numbers are high, you should be thinking about what you can do to manage that field differently.” 

Channel your inner Gladys Kravitz

Back again

Corn rootworm trait resistance and performance issues are spurring a soil-insecticide renaissance.

“There is inevitable resistance to traits, as Mother Nature finds a way to adapt,” says Joe Short, Midwest marketing manager for AmVac.

That's where soil insecticides come in. By supplementing traits, insecticides are another mode of action to control corn rootworm.

In 2011, AmVac officials say tests across several Midwestern universities showed three-fourth rates on top of traits in 80 comparisons had an average 5- to 10-bushel-per-acre yield increase. Overall, a yield advantage occurred 73% of the time in 2011 tests.

“It is intriguing,” says Mike Gray, University of Illinois Extension entomologist. “When Bt products hit the marketplace, a big selling point was that it would reduce or eliminate soil-applied insecticides. This is a sharp reversal of that earlier prediction.

“We often hear that adding a soil insecticide is a good Integrated Resistance Management tactic that adds another mode of action,” adds Gray. “In reality, it's not that simple.”

That's because soil insecticides are designed to protect roots in a 7-inch band. In effect, the untreated area between the bands acts as a refuge. “This promotes rootworm survivorship and explains why corn rootworms did not develop resistance to planting-time soil insecticides,” says Gray.

University research from the early 1990s confirmed that in some years, more western corn rootworm beetles emerged from fields treated with a soil-applied insecticide than untreated fields. Using a soil-applied insecticide on top of traited corn where a resistant population is present could result in prolific beetle emergence and resistance that spreads more rapidly, says Gray.

Soil insecticides are essentially lodging insurance, as opposed to a kill strategy, adds Ken Ostlie, University of Minnesota Extension entomologist. “It is not unusual to get 40% to 60% control with soil insecticides,” he says. “In a sense, they are sort of a bandage, allowing us to limp through the current year's corn crop. They haven't changed the underlying issue, which is resistance. That has been one of the dilemmas of soil-applied insecticides.”

What to do

Here's a look at six ways to forestall corn rootworm trait resistance.

1. Rotate Crops. “The most important strategy — and because of economics, farmers may not like to hear it — is crop rotation,” says Ken Ostlie, University of Minnesota Extension entomologist. “Rotating to a crop like soybeans reduces adult beetle numbers. That's the most important way to reduce the pressure on traits. As the beetle population builds, it may pose issues for the current trait used and also for other traits that could be planted in the future.”

Rotation isn't foolproof. Corn rootworm beetles have managed to survive the year in soybeans and come back the following year to annihilate corn.

Rotating to soybeans also isn't practical for livestock producers who need corn. “We have considered going to more of a 50-50 rotation and have considered selling beans to buy corn. But the economics aren't there to make it work,” says Peter Bakken, a Beaver Creek, Minnesota, farmer. “We can still grow corn cheaper than we can buy it.”

2. Rotate to a different trait. “When you bring in another mode of action, it reduces the chances of resistance,” says Josh St. Peters, marketing manager for DuPont Pioneer.

3. Rotate to a pyramided trait package. Pyramided Bt hybrids express multiple Cry proteins for rootworm control. If one mode of action fails, another control mode is available in a pyramided package. Examples of pyramided products currently on the market include:

  • Dow AgroSciences's Refuge Advanced powered by SmartStax
  • Monsanto's Genuity SmartStax RIB Complete Corn
  • Syngenta's Agrisure 3122 E-Z Refuge trait stack
  • DuPont Pioneer's Optimum AcreMax XTreme

4. Apply a soil insecticide. This, coupled with a pyramided hybrid, is what Mark Moeller recommends to farmer-clients who have had problems in fields with poor corn rootworm trait performance.

“There are some livestock feeders who need the corn,” says Moeller, a Centrol crop consultant based in Luverne, Minnesota. “For them, rotating with soybeans is not an option, so they are pulling out both barrels.”

5. Select tolerant hybrids. Although it's no substitute for tools like traits, hybrid selection can garner farmers some tolerance to rootworm root feeding, says Clint Pilcher, DuPont Pioneer scientific affairs director.

“Some hybrids have rooting systems that are more fibrous in nature,” he says. “They recover better than other hybrids from rootworm root damage.”

A hybrid with a lower lodging score is a good indicator of a stronger, more fibrous rooting system, he adds.

6. Control adults. A foliar insecticide application to control adult beetles during the growing season can halt silk clipping and prevent egg laying. It's a last-ditch effort to salvage a field, however.

“The challenge with adult control is that beetles have a long emergence period, lasting from six to eight weeks,” says Ostlie. “None of these foliar insecticides has a very long life on the foliage. You are lucky to get a week's worth out of it. That means if you knock down enough of the population to have enough impact on egg laying in the following year, you are looking at two to three applications. The cost can ramp up significantly, not to mention the frequency of scouting.”

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