University of Illinois Extension entomologist Mike Gray says the potential for Bt resistance, high commodity prices and the difficulty in scouting for this insect is driving producers toward an increased use of soil insecticides.
University of Illinois Extension entomologist Mike Gray says the potential for Bt resistance, high commodity prices and the difficulty in scouting for this insect is driving producers toward an increased use of soil insecticides.

PEORIA, Ill. — History shows that western corn rootworm is resilient and going back to a reliance on soil insecticides is not the end-all solution amid confirmation of Bt hybrid resistance.

Western corn rootworm resistance to Bt hybrids expressing the Cry3Bb1 protein was confirmed in Iowa cornfields in 2011 and in northwestern Illinois last year.

The potential for Bt resistance, high commodity prices and the difficulty in scouting for this insect is driving producers toward an increased use of soil insecticides.

“In looking at this renewed interest in soil insecticide, I went back over 20 years ago to see some of the results from our on-farm rootworm research trials,” said Mike Gray, University of Illinois Extension entomologist.

“There was a lot of interest over 20 years ago of how producers can reduce their input costs and can if they use a three-quarter rate or maybe a half rate of soil insecticide.”

He said each year was a hit and miss, “regarding whether or not you’re going an economic infestation out there in a given cornfield.”

“Going back to the late 1980s and early 1990s, roughly 90 percent of all those continuous cornfields received a soil insecticide application every year, despite the fact that prices were not nearly as favorable back then as they are now, but because this is a very difficult insect to scout for and make accurate predictions,” he said.

“Twenty-plus years ago, those soil insecticides represented the state of the art when it comes to rootworm management. We had more products out there.”

Today, there are three proteins in the marketplace to protect corn from rootworm damage — the modified Cry3A, the Cry3Bb1 and the binary protein of Cry34/35Ab1.

“Concern is being generated over the Cry3Bb1 protein that is basically in the YieldGard lineup of products,” Gray said at the recent Corn and Soybean Classic.

This is the first protein that hit the marketplace back in 2003, and the other proteins followed.

“The historic refuge structure has been a 20-percent refuge, but things are shifting very significantly in that area, as well,” the entomologist said.

“In fact, we know that if we pyramid to combine these proteins, we can reduce the refuge from that 20-percent structure to a 5-percent seed blend refuge, as we see with SmartStax refuge-in-the-bag type products.”

Gray said a new product pending U.S. Department of Agriculture approval is the Agrisure Duracade trait that expresses the eCry3.1Ab protein for corn rootworm. It has received U.S. Environmental Protection Agency and U.S. Food and Drug Administration approval.

“If USDA offers full approval, this protein will be pyramided with the Agrisure RW (mCry3A protein) trait and sold with a seed blend refuge,” Gray said. “I would anticipate this protein will join those three probably in 2014. That protein looks very good and offers really nice root protection.

“I’m really excited about that one joining these other products, particularly because of the concern over resistance to Cry3Bb1 in the Corn Belt.”

There is some concern by some scientists that there may be some cross-resistance between the proteins.

“So in areas where resistance has been confirmed already, that does raise a few red flags by some in academics and some in the private sector, as well,” Gray said.

In a letter sent last year to the EPA, 22 entomologists expressed their concern over the development of resistance to the Cry3Bb1 protein, the threatened long-term durability of the Cry34/35Ab1 binary protein, the increasing use of soil insecticides used in conjunction with Bt hybrids and the overall lack of traditional integrated pest management methods in commercial corn production and over-reliance of an insurance-based approach to corn rootworm management.

The concern is the refuge was reduced from 20 percent down to 5 percent, putting increased pressure on the Cry34/ 35Ab1, “and it’s not because we have a lot of these Cry proteins and our disposal — we don’t,” Gray said.

“When you take insecticides and overlay them on top of this transgenic technology, the economic and environmental advantages begin to quickly disappear,” he said.

“When Bt hybrids for rootworm entered the marketplace, the No. 1 selling feature that was touted was that we can eliminate the use of insecticides. That was what many of us thought where we were headed. But I think we tend to forget or ignore some of the lessons from the past with regard to this particular insect.”

He noted that western corn rootworm has adapted over time to several classes of insecticides, crop rotation and new transgenic technology.

“This is a very versatile and highly adaptive fellow,” he said.

The damage caused by corn rootworm can be devastating, as indicated in research from 2005 through 2011. Data was collected via more than 7,000 observations.

U of I research determined that for very one node of root destroyed by western corn rootworm, there is a 15-percent yield loss.

Overall, among the four locations across Illinois used for the research, it was determined that each node of roots pruned resulted in a 28-bushel-per-acre yield loss.

If corn rootworm injury averaged one node of roots pruned across a field and if the market price was $7.50 per bushel for corn, that would result in a loss of $210 per acre due to corn rootworm feeding.

Consequently, as long as producers are concerned about the loss of efficacy by some Bt hybrids, commodity prices remain high and access to elite high-yielding germplasm is perceived by producers to occur most commonly in Bt hybrids, even those with resistance challenges, soil insecticide use in conjunction with Bt hybrids will increase in many areas of the Corn Belt, according to Gray.

“I know we’re all working under the assumption that if I had a soil insecticide, that is an approved insect resistance management strategy — I’m adding yet another mode of action,” he said. “I just want us to think carefully about making some mistakes with those kinds of assumptions.”

He referred to some research published in 1992 that focused on corn rootworm populations from 1983 through 1992, “showing that in some years we actually had more adults emerging from fields where soil insecticides had been used versus no soil insecticides.”

“These are not population management tools,” he said. “So don’t automatically assume that if you use soil insecticide that that’s a good IRM strategy and you’re going to get fewer adults emerging. We need more work in that area and how they’re being perceived as a resistance management tool.”

Producers who have experienced less-than-satisfactory performance with a corn rootworm Bt hybrid should consider several recommendations.

In addition, growers who want to avoid a future problem with a Bt hybrid and, more importantly, prolong the usefulness of this technology also should think through these following recommendations:

* Consider rotation to soybeans or another non-host crop;

* Consider the use of a corn rootworm soil insecticide at planting, along with a hybrid that does not contain a rootworm Bt protein;

* Consider the use of a Bt hybrid that expresses a different corn rootworm Cry protein than one which may have performed poorly in your fields in 2012 or has been in use for several consecutive years;

* Consider the use of a pyramided Bt hybrid that expresses multiple proteins targeted against corn rootworms; and

* Most importantly, consider a long-term integrated approach to corn rootworm management that includes multiple tactics.