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Crop Tech: Rootworms Grow Up Resistant

August 27, 2011
corn rootworm
Western corn rootworm adults may be surviving on Bt corn hybrids that express the Cry3Bb1 toxin.  

Rootworms are rascals. The latest evidence of this comes in the first documented case of resistance by a field population of western corn rootworm in northeastern Iowa.

An Iowa State University team led by Aaron Gassmann responded to producer concerns that Bt hybrids expressing the Cry3Bb1 protein were exhibiting significant larval damage. Gassmann’s report in the PLoS ONE scientific journal reveals how he collected western corn rootworm adults from those fields and found that the progeny from those parents able to survive on Bt corn hybrids expressing the Cry3Bb1 toxin at levels higher than western corn rootworm from fields not exhibiting significant larval damage.

Hybrids with the Bt Cry3Bb1 toxin include Monsanto Company’s VT3 technology and SmartStax hybrids marketed by Dow AgroSciences and Monsanto. The good news is the progeny from the corn rootworm adults remained susceptible to the Cry34/35 Ab1 protein (also found in SmartStax and Herculex hybrids), suggesting there is an absence of cross-resistance.

Todd DeGooyer, Monsanto’s corn traits lead, says the company is working with Gassmann to assess how the lab results impact field scenarios. Farmers had used Bt hybrids expressing the Cry3Bb1 protein for three or more consecutive years in the fields in question.


"While we don’t like to see this kind of situation arise, it is not surprising given the rootworm pressure in that region and the selection pressure given a continuous Bt corn scenario," DeGooyer says.

"We encourage growers to follow an integrated approach to rootworm control that includes crop rotation. Those dedicated to continuous corn should use stacked hybrids containing multiple modes of action to protect against belowground pests, such as Genuity SmartStax," he adds.

Flash in the Field

Here’s a news flash … those lime green or yellow soybean leaves on the top of the crop late in the season weren’t an illusion. "Yellow flash" is a temporary chlorosis of newly emerging soybean leaves that sometimes follows an application of glyphosate to glyphosate-resistant cultivars.

yellow flash
Yellow flash or yellowing of emerging soybean leaves sometimes occurs after herbicide application.

University of Illinois weed scientist Aaron Hager says the flash typically shows up when applications are made late in the season, when the soybean is in the reproductive stage of growth. It’s usually noticed most in areas of sprayer overlap or where growers fire up the boom.

A few days after glyphosate is applied, all leaves on the soybean plant remain green except the newest leaves at the top. These leaves continue to grow, but chlorophyll production is reduced, leaving a yellow color caused by carotenoid pigments becoming visible in the absence of chlorophyll.

Mark Bernards, a Western Illinois University agronomist, says the temporary yellowing usually happens when the plants have undergone rapid growth and environmental conditions are warm.

"The symptoms generally last for about a week to 21 days after [glyphosate] treatment and no decreases in yield have been reported," Bernards says. If it is dry when the yellow leaves appear, those leaves might stay yellow until the crop resumes growth after rain releases the crop from stress.

There are other reasons for yellowing of leaf tissue in soybean plants. Nitrogen deficiency, manganese deficiency, potassium deficiency and soybean cyst nematode have been shown to play a role in turning soybeans yellow, but all are separate issues with yield consequences.

The good news is that yellow flash is largely environmental and, while not advisable to try to induce, is more of a curiosity than a concern.

Infection-Fighting Peptides

Soybean rust has shown an uncanny ability to outsmart genetic defenses. University of Missouri–Columbia researchers have identified peptides that have the potential to protect against the devastating disease.

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FEATURED IN: Farm Journal - September 2011
RELATED TOPICS: Crops, Management, Research, Inputs

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