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Entomologist urges farmers to monitor corn rootworms, soybean aphids

By Jean Caspers-Simmet
simmet@agrinews.com

Date Modified: 07/15/2013 9:57 AM

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KANAWHA, Iowa — The "big dogs" in the insect world this summer are corn rootworms and aphids, said Iowa State University Extension entomologist Erin Hodgson.

Corn rootworm egg hatch is happening now, Hodgson said at last week's field day at ISU's Northern Research Farm at Kanawha.

Due to cool wet weather, egg hatch is about two weeks behind average but a month behind 2012. Delayed corn planting means smaller root systems for larvae to feed.

"Start assessing root injury in mid-July to early August," Hodgson said. "You should be looking in every field, every year."

Hodgson said that some continuous corn fields have larvae that can survive at high numbers and cause severe root injury to Bt corn. Farmers need to use multiple strategies to protect crops.

Aaron Gassmann, ISU entomologist, has found resistance to corn rootworm Bt traits Cry3Bb1 and mCry3A and has also seen problems with Cry34/35Ab1, Hodgson said. Resistance to Cry3Bb1 has been found in Illinois and is suspected in Minnesota and Nebraska.

Western corn rootworm is the predominant species, and most of the problem fields are north of Interstate 80.

"The only time we see high populations is in continuous corn production where producers have used the same trait for three to four years," Hodgson said.

Farmers need to use multiple tactics to prolong the effectiveness of Bt traits, she said.

"The single most effective tool is to rotate corn to soybeans or another crop," Hodgson said. "Breaking the cycle of continuous corn, even once every four to six years, will dramatically improve corn rootworm management."

Corn rootworm cannot survive on non-corn crops so crop rotation will break the pest's life cycle. A farmer should expect nearly zero root injury after rotating back to corn.

If crop rotation isn't an option, Hodgson said farmers should use pyramided corn rootworm transgenic lines that include several Bt traits. Pyramided traits should be rotated every two to three years.

"If rootworm populations have developed resistance to one trait, the second trait is likely controlling most of the larvae," Hodgson said. "Using pyramided traits continuously will increase the chances of developing resistance to both traits which is something we are trying to avoid at all costs."

Farmers could also use a soil insecticide on non-rootworm Bt corn.

"Remember that soil insecticides only protect a small area of roots," Hodgson said. "Soil insecticides will not necessarily eliminate resistance because individual larvae will be able to survive outside the insecticide-treated root zone."

Hodgson recommends rotating single Bt traits annually or at least every two years.

"You should monitor every field, every year for larval corn rootworm injury, regardless of which management strategy you use," Hodgson said.

Farmers also need to watch for soybean aphids, Hodgson said. The pest has been detected in soybeans in Iowa as well as Minnesota, Wisconsin and Michigan in numbers not seen since 2007.

Insecticidal seed treatments should minimize colonization for up to 45 days after planting, and host plant resistance will also discourage exponential growth, Hodgson said. Rag1 resistant seed is commercially available for Roundup Ready soybeans. A pyramid of Rag1 and Rag2 resistance is available in organic seed for Iowa and southern Minnesota producers.

Tank-mixing herbicides and insecticides isn't recommended for soybean aphid control, Hodgson said.

A new soybean aphid field guide has just been published by the North Central Soybean Research Program and these are available from ISU Extension.

Grain aphids have been found in corn and small grains this year, Hodgson said.

Hodgson urged farmers to send her photos and messages describing insect problems they face this summer. Contact her at (515) 294-2847, ewh@iastate.edu or on Twitter at https://twitter.com/erinwhodgson.