Western corn rootworms (CRM) resistant to Bt appear to be gaining ground. Initially, resistance concerns were limited to Cry3Bb1; however Aaron Gassman, Extension entomologist, Iowa State University, has since found resistance to mCry3A and reports problems with Cry34/35Ab1. Early reports out of central and east central Illinois suggest the problem may have taken a turn for the worse in that state, while pressure was down in Minnesota hotspots.

We haven’t confirmed resistance to Cry3Bb1 yet in 2013, but with resistance previously confirmed in northwestern Illinois, it looks suspicious,” said Mike Gray, Extension entomologist, University of Illinois. “There we were looking at long-term continuous corn using the same trait without rotation. In Kankakee and Livingston counties, problems with root feeding and lodging were found in first-year corn with Bt traits.”

Doug Flageole and his son David raise corn and soybeans in a 75:25 ratio near the epicenter of the first-year damage. He said traits continue to work; however, some need the aid of a soil-applied insecticide.

“I can drive down the road and see my neighbor’s corn flat on the ground,” he related. “If they didn’t have the right ‘event’ going on, their corn fields are a disaster. I’ve been scouting my fields. I’ve seen no root feeding, and my corn is standing well.”

Flageole credited planting the bulk of his corn acres to Pioneer hybrids with the Herculex Cry34/35Ab1 trait. Acres planted to DeKalb hybrids with the Cry3Bb1 CRW trait, as well as the 20 percent refuge acres in fields planted to Herculex trait hybrids, received a soil application of Force this year. It is a practice Flageole described as “preparation for resistance,” something he will extend to Herculex acres if necessary.

“I keep a close watch on the resistance test trials Pioneer is running,” said Flageole. “I’m concerned problems could develop with Herculex as well, but so far it is showing no resistance, so I have no plans to change.”


Loss of crop rotation as a viable option is a concern. However, the news that Western CRW resistant to Bt may have developed in another area comes as no surprise to Bruce Tabashnik, population geneticist and head, Department of Entomology, University of Arizona. He advised that the dose rates of Bt traits toxic to corn rootworm make resistance inevitable.

“The immediate problem is Cry3Bb1, but the other toxins have a similar profile in terms of response of the Western corn rootworm,” said Tabashnik. “Unlike the Bt trait for European corn borer, they don’t provide a high dose that kills more than 99.99 percent of the target pest. They kill about 94 percent to 98 percent. Survivors are more likely to be resistant and to mate with other resistant survivors. You’re eliminating nearly all of the susceptibles from the population, but probably not the hybrid off spring (heterozygotes) produced by matings between resistant and susceptible beetles.”

In areas where resistance to both crop rotation and Cry3Bb1 is suspected, Gray advises using pyramid hybrids with their multiple traits or rotating traits. If using a suspect trait, in particular Cry3Bb1, he suggests Flageole’s practice of using an overlay of a soil insecticide for increased protection against lodging.

Bryan Johnston, Cabery Fertilizer, Cabery, Ill., was the one who asked Gray and Joe Spencer, an entomologist with the Illinois Natural History Survey, to look at problem fields.

Cabery has been running trials with granular insecticide on top of traited seed in anticipation of resistance. When rain and winds hit part of Cabery’s trade territory, lodging was severe.

Expanded scouting revealed extensive root feeding and lodging and also demonstrated the benefit of CRW insecticides.

“Fields can look normal when you drive by, but seen from the air or scouting into the fi eld reveals problems,” said Johnston. “It is important to dig the roots, as we had some lodging from hybrid failure due to poor root systems. We saw a major difference in lodging and feeding where a granular insecticide like Force was applied versus no granular. Others in the area reported the same.”


Although Johnston acknowledged the reluctance many growers have to reverting to CRW insecticides, he expects to be recommending a soil-applied insecticide next year.

“Growers have to realize the sure-deal of 100 percent protection by traits is not there anymore,” he said. “Traits can still bring value, but we definitely have to utilize multiple modes of action.”

Gray agreed that multiple modes of action are a good response. However, he advised being selective. “Our land grant community of entomologists is still pretty consistent on the question of insecticide and pyramid Bt hybrids,” said Gray. “It may be a negative when it comes to resistance management.

Where insecticides should be considered is where a trait has failed or been inconsistent and the grower still wants to plant a hybrid with that trait.”

Johnston recommended an insecticide with a pyramid hybrid may be necessary if resistance is suspected with one of the traits. Gray agreed that if a trait in a pyramid hybrid has been compromised, you are back to a single mode of action, little different from simply rotating to the other trait. He warned that even new traits, while welcomed, may not be the answer.

“I anticipate high levels of protection from the new Duracade hybrids that contain ECry3.1AB, a modified form of Cry3Bb1, being introduced by Syngenta in 2014,” said Gray.

“However, there is some evidence building that there may be some cross resistance between it and the Cry3Bb1.”

Dirk Benson, technology manager, Syngenta, cautioned against jumping to conclusions of resistance when it comes to “unexpected damage.” Although the industry and the EPA agreed to the validity of a number of protocols for evaluating resistance, none when used alone are definitive in determining resistance. Benson argued those conditions are radically different from field situations and can depend on the quality of the insect sample taken. He further pointed out the multitude of conditions that can affect Bt efficacy, including synchrony of planting date, larval emergence, root radical development and population pressure. All can affect whether the CRW feed on smaller early root mass or later larger root mass that is better able to support the plant. With all the variables growers face, he challenged them to practice integrated pest management, of which insect resistance management is a component.

“Rotating traits is one thing, but growers need to consider rotating crops as well,” he said. “We need to break the biological cycle of the insect, or we are just running an experiment around resistance development. We all know what the answer will be. Just look at multiple applications of glyphosate without breaking the cycle. Growers need to look long term.”

Rootworms Resistant to Bt Gain Ground


Benson noted that Syngenta appreciates the need for multiple modes of action and new answers to the CRW problem. “We have an active discovery engine in both chemical and seed traits,” he said. “Soil-applied insecticides certainly have their place. Seeds, chemistry and traits all have a role.”

Rich Porter, Amvac, noted that continuous corn growers in many areas are experiencing CRW pressure not seen in years.

“In many situations the pressure has become overwhelming,” he said. “Soil insecticides have challenges, and now areas are seeing traits with quite a few challenges. We are emphasizing to retailers that CRW management will take more diligence and more scouting. They can no longer assume a single technology will meet grower expectations for corn rootworm management, but need to consider multiple strategies to control this difficult pest.”

Having purchased or licensed the bulk of the granular soil insecticides on the market during the rise of CRW Bt traitbased control, including Aztec, Counter and SmartChoice, the company has been preparing for the current situation.

Porter said Amvac has been looking at newer technologies and working with university researchers since 2007, combining granular soil insecticides with traits.

“Where there has been significant rootworm damage, if you have an overlay or second mode of action, there is a significant reduction in feeding damage a high percentage of the time,” explained Porter. “CRW is a very humbling pest. There is no single bullet, no single technology. The trait technology that came on 10 years ago has not been the end solution in continuous corn or, it appears, even in first-year corn.”