Drought + Resistance = surge in rootworm numbers and damage

Written by Jonathan Eisenthal

Alarming signs that the corn rootworm is developing resistance to major GMO corn traits, along with the impact of increasing drought stress and booming rootworm populations, adds up to potential trouble for Minnesota corn producers, according to Prof. Ken Ostlie, a University of Minnesota Extension entomologist, based at the St. Paul Campus of U of M.

Now is a critical time, according to Ostlie, for getting a handle on Bt performance in your fields, and assessing corn rootworm populations. 

“Corn rootworms have been emerging over the last ten days to two weeks, with root injury nearing completion” Ostlie said. “The drought may be aggravating this situation in a couple of ways. Corn rootworm survival is better under drought conditions. Insect feeding on the roots will increase moisture stress and its yield impacts. Also, without thunderstorms, the typical lodging that would be a telltale sign of corn rootworm problems may not be happening, so producers need to get out into the field, dig roots and scout for beetles.

Besides looking for beetles on plants, especially on the ear silks, dig up plants, wash the roots and carefully examine for signs that the worms fed there. “Growers should be concerned when root injury is occurring to a node or more of the roots,” said Ostlie. “In these drought conditions, even loss of half a node may lead to yield loss. Time scale wise, the next week will be crucial to assess what is happening.”

Another concern is that high corn rootworm beetle numbers can trim corn silks so severely that pollination fails. Spraying for corn root worm beetles to prevent ‘silk clipping’ is warranted only on the leading edge of pollination when scouting uncovers the presence of eight-to-ten or more beetles per ear that are keeping silks chewed to within ½” of the ear tip. Minnesota Corn Research & Promotion Council sponsored a radio interview with Ostlie that aired recently on the Linder Farm Network to alert growers to this threat.

For the past six years, with critical funding from the Minnesota Corn Research & Promotion Council, Ostlie has investigated the potential for corn rootworm to become resistant to the main genetic traits used to combat it– genes taken from the bacterium Bacillus thuringiensis and spliced directly into the genetic make-up of “Bt” corn varieties. These Bt genes produce a protein in the plant which is toxic to the rootworm alone. In order to assure that the protein remained effective, producers have planted refuges of conventional corn where the insects could feed. Difficulties in maintaining refuges may have led, in part, to the current resistance, Ostlie said.

“It’s important for farmers to know how the traits are working in their fields,” Ostlie noted. “Scouting how these traits are working in their fields right now will be critical to making good decisions for next year. Producers are beginning to plan what additional root protection measures to take, which seed varieties and pest management resources to purchase for the 2013 growing season.”

Since 2009, Ostlie has tracked a growing resistance problem in Yieldgard VT Triple and Triple Pro, but it may be only a matter of time before resistance impacts other Bt varieties like Herculex Xtra, AgriSure 3000GT or even SmartStax.

“We suggest the fields most at risk are going to be corn planted after corn with a multiple-year history of the same trait,” Ostlie advised. “Those especially would be the ones growers should check–number one looking for signs of excessive moisture stress, number two looking to see if corn rootworm beetles are especially prevalent in a field, and third, digging up plants and washing roots off to see if they had been attacked by the worms.”

Ostlie is working with pest management scientist Bruce Potter, at the Southwest Research and Outreach Center in Lamberton, to develop maps and other information to help growers assess potential rootworm activity in their regions.  We’re particularly interested in reports of performance problems with Bt rootworm traits, Potter says.

Go to this web page to get additional information, or to report Bt hybrid performance problems:  http://www.extension.umn.edu/cornrootworm/

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