In this year's hot and dry weather, plants did not tolerate as much feeding during pollination.
By Jean Caspers-Simmet
Date Modified: 09/10/2012 3:09 PM
NASHUA, Iowa — One of the hottest topics this summer, especially in northeast Iowa, is corn rootworms, said Erin Hodgson, Iowa State University Extension entomologist.
"We've had this pest for a very long time, and we've managed it in many different ways with rotation, trait selection, incorporation of Bt, but the last couple of years, especially in northeast Iowa, corn rootworm populations are overcoming certain traits that were once very effective," Hogdson said at last week's field day at ISU's Northern Research Farm at Nashua. "This year with the warm winter and exceptionally warm spring, all insect development was accelerated. We had adults feeding on silks to the point where they were interfering with pollination."
With adult corn rootworms active before silking this year, Hodgson recommended keeping an eye on fields in order to protect yield. Growers were told to consider a foliar insecticide if there were five or more beetles per plant, silks had been clipped to less than one-half inch of the ear tip, and pollination wasn't complete.
Adult corn rootworms often feed on leaves and cause scaring, but this will have little effect on yield, Hodgson said. Adults can cause yield loss if they are present when corn is silking. Adults are attracted to silks and will congregate on plants to feed and mate. Adults that trim back silks to the husk during pollen shed will interfere with pollination and could cause yield loss.
Weather plays a big role in how plants respond to silk feeding, Hodgson said. Under ideal moisture conditions plants could tolerate 15 beetles per plant, but that number is reduced to just five under drought stress.
A grower near Carroll contacted Hodgson because his corn had lodged and thought that the seed for his 20 percent block refuge had been reversed with the seed that had the Bt trait.
"The majority of his corn was down," Hodgson said. "It had small roots and the brace roots were not there. Meanwhile the 20 percent planted to the refuge where he had used an insecticide was still standing, and it had just one node destroyed."
Hodgson did a tissue test in the field to confirm the expression of Bt. The farmer's single trait Bt hybrid had three root nodes destroyed.
"It's a scenario that's becoming too common in Iowa," Hodgson said.
Last year, ISU corn entomologist Aaron Gassmann confirmed Western corn rootworm resistance to the Cry3Bb1 Bt protein, Hodgson said. He visited commercial corn fields with heavy root damage and collected emerging adults for use in a lab bioassay.
"All the fields with severe root damage had been in continuous corn for at least three years with Cry3Bb1 trait," Hodgson said.
Gassmann continues to sample and has collected from problem fields across the northern half of Iowa already in 2012.
"Illinois confirmed resistance to Cry3Bb1, and the same situations are happening Minnesota, Nebraska, South Dakota and even starting to have those conversations in Indiana," Hodgson said. "It's not just Iowa any more. Those isolated fields are becoming more and more common. We are recommending a few integrated pest management alternatives to sustain the effectiveness of these traits."
Hodgson said that rotating to soybeans for just one year will make a big difference.
"If you grow soybeans, the larvae have nothing to feed on," she said.
She advises following refuge requirements and assessing root injury in every corn field every year. Rotate Bt proteins or use a hybrid with pyramided Bt traits. Add soil-applied insecticides to problem fields.