CABERY, Ill. — Since the initial findings of rootworm resistance to Bt corn, crop rotation is among the first recommendations, but recent findings have changed that thinking.

Significant western corn rootworm larval injury in first-year cornfields has been confirmed in Livingston and Kankakee counties in Illinois, according to University of Illinois Department of Crop Sciences professor of entomology Michael Gray.

Gray and Joe Spencer, Illinois Natural History Survey entomologist, recently confirmed injury to rotated cornfields that had been planted to Bt rootworm hybrids, VT Triple PRO RIB expressing the Cry3Bb1 protein.

The fields in question were brought to their attention by Bryan Johnston of Cabery Fertilizer in Cabery.

“Bryan indicated that many first-year Bt cornfields in the area had severe root pruning and lodging,” Gray said.

“The fact that rotated corn is now showing susceptibility to rootworm damage, even when planted to certain Bt hybrids, is evidence that crop rotation in central and east-central Illinois does not adequately confer a consistent level of root protection.”

Spencer collected numerous adult western corn rootworms in the damaged corn and adjacent soybean fields.

“The number of beetles in the soybean fields was reminiscent of densities in the late 1990s and early 2000s — very impressive,” Gray said. “The density of western corn rootworm adults in both crops, along with the severe pruning and lodging, was additional evidence that the Bt hybrids had failed to offer the necessary root protection.”

Bioassays on the offspring from the adults collected by Spencer will be required to determine if these rotation-resistant western corn rootworms also are resistant to the Cry3Bb1 protein.

If the bioassays confirm resistance to the Cry3Bb1 protein, Gray said producers across a wide swath of Illinois will have a “formidable insect foe,” capable of overcoming both crop rotation and at least one Bt protein.

Spencer said a couple of growers during Agronomy Day mentioned to him that they had some poor Bt performance in rotated corn.

“So this was really interesting, in part, because the expectations based on published reports from Iowa State University and some populations that Mike was already working on suggested that resistant rootworms were produced as a consequence of continuous corn and using the same trait year after year,” he said.

“So rotated corn seemed like an environment where we probably weren’t going to see Bt resistance, and certainly it wouldn’t be the first place you would find it within a county. That’s what really surprised us.”

Spencer said the Cry3Bb1 protein has been in the market the longest and the one for which resistance has been observed in Iowa and a variety of locations throughout the Corn Belt.

Gray is encouraging producers across east-central and central Illinois to look for corn rootworm injury in their first-year cornfields.

“From the roadside, it is very easy to overlook areas of fields that may be lodged,” he said.

“We were visiting locations in the field that Bryan brought us to and see the injury. I didn’t personally survey the whole field, but one of the things we learned in speaking with at least one of the growers was he was made aware of this problem because one of his neighbors was up using his ultra light airplane and began to see all this downed corn,” Spencer added.

“I think that’s been one of the ways that people have really come to appreciate how much of the problem there is. People who have been up in planes have indicated to me that there’s a lot of downed corn.”

“I suspect that during harvest many producers may be surprised to see pockets within fields that have been severely damaged by corn rootworms,” Gray said. “Looking ahead, now is the time to begin thinking about how to best protect against corn rootworm damage in 2014.”

Producers in the most severely affected areas of central and east-central Illinois should consider the use of pyramided Bt hybrids — hybrids expressing more than one rootworm Cry protein — in 2014, the professor recommended.

“The use of a planting-time soil insecticide should not be needed with a pyramided Bt rootworm hybrid,” he said.

“Producers who elect to plant Bt hybrids expressing only a single Cry rootworm protein may choose to use a planting-time soil insecticide as an added precaution, particularly in areas where single-traited Bt hybrids have failed this season.”

Over history, insects have shown an ability to develop resistance.

“It’s one of these things that it seems as though we’re always surprised when some type of resistance happens. But if we go back and look at history, really the history of insect management is really a history of resistance,” Spencer said.

“Ever annoying, the insects are going to haunt us forever. Just when we think we have something that they can’t beat, they teach us another lesson. I think that may be one of the lessons that we need to learn. If we fail to learn that lesson, the insects will reeducate us every decade or so or at least every generation.

“The insects have an amazing capacity to adapt, especially when you put pressure on them that results in their death — that’s a very significant selection pressure and any individual that processes some trait that gives it a slight advantage is going to over time become more abundant and that’s what has happened.”