More Diverse Tactics Needed to Prevent Rootworm Resistance

LAWRENCE, Kan. (DTN) -- Rootworms susceptible to Bt-corn hybrids are a "non-renewable resource" and growers must make changes to ensure they aren't lost forever, entomologists warned in a rootworm webinar hosted by the North Central Integrated Pest Management Program (IPM) on Feb. 20.

"Susceptibility of a pest to a management tactic provides a means to control that pest population, but in the process of managing the pest, that pest susceptibility is used up -- it's expended as that pest develops resistance," Iowa State entomologist Aaron Gassmann explained to an online audience of more than 250 farmers, seed dealers, agriculture input suppliers and fellow scientists.

Gassmann stressed a growing need for farmers to "layer tactics" in their rootworm management system instead of relying solely on Bt-traited corn hybrids, particularly single-trait hybrids. Beyond the standard advice to rotate crops and use hybrids with stacked Bt-traits, Gassmann recommended farmers rotate Bt traits more frequently and consider adding non-Bt corn hybrids in conjunction with a soil insecticide to rotations.

So far, the western corn rootworm has developed resistance to Monsanto's Cry3Bb1 and Syngenta's mCry3A. Both Bt traits are sold individually or as trait stacks by a number of seed corn companies.

Gassmann's lab first documented western corn rootworm resistance in Iowa fields in 2009. Resistance to the Agrisure mCry3A trait was officially confirmed by Gassmann's team in 2013.

Gassmann said cross-resistance has also been found, so if a rootworm is resistant to one trait, the chances of it being resistant to the other trait are higher. Corn hybrids with stacked Bt traits are safer bets for slowing resistance, but no one product is entirely bulletproof, Gassmann stressed.

"One thing to keep in mind with all of the pyramids that are out there, they all contain either Cry3Bb1 or mCry3A, so there are going to be areas where one of the two toxins in any pyramid is compromised," he said. "I think that just highlights the need to use these technologies wisely in an integrated fashion and not put too much pressure on the Bt-toxins even if it is a pyramid."

University of Minnesota entomologist Ken Ostlie agreed. "Pyramids are much more effective in terms of beetle control, but that performance may be affected by cross-resistance to other traits that have been used in that field or even in the surrounding area because of migration of beetles into that field," he said.

The entomologists agreed that crop rotation remains the best way to keep resistance at bay. In 2013, some Illinois growers experienced damage in first-year corn following soybeans. However, entomologists generally agree crop rotation still slows the pest, even in those cases.

"At this point, corn-rootworm resistance to traits is still mainly a corn-on-corn issue," Ostlie noted. "We're starting to see some signs of potential issues with rotated corn, but they're very slight at this moment. The preferred option is rotation if you can work it in."

Gassmann showed suggested rotations to slow rootworm resistance that included a two-year rotation between soybeans and non-Bt corn hybrids followed by a non-Bt corn hybrid paired with soil insecticides or a Bt corn hybrid.

One Wisconsin farmer commented that non-Bt corn hybrids are scarce and the sales programs of local seed companies increasingly dictate rootworm strategies. "We've heard that comment before," University of Nebraska entomologist Bob Wright said. "Of course there are multiple seed companies, so I'd shop around."

If farmers insist on maintaining corn-on-corn rotations, there are some steps they can take to slow resistance and manage rootworm populations, Ostlie said. He recommended switching the Bt events every three to four years, even if they still appear to be working, and avoiding seven if they still appear to be working, and avoiding single-trait hybrids. "Recognize that single traits are going to be phased out down the road," Ostlie told growers.

He recommended switching the Bt events every three to four years, even if they still appear to be working, and avoiding single-trait hybrids. "Recognize that single traits are going to be phased out down the road," Ostlie told growers.

The entomologists stressed that corn-on-corn farmers must be especially vigilant and have a detailed knowledge of the pressure levels in their field, trap crops near where beetles could congregate and survive, and the trait history of that field.

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