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Volunteer corn can be a haven for Bt resistance, specialists say

Agriculture.com Staff 01/08/2008 @ 7:28am

Bt could stand for "big trouble" in the years ahead if farmers aren't careful in their use of biotech corn, says a Purdue University entomologist in a university report.

Corn varieties containing Bacillus thuringiensis, or Bt, genes to control corn rootworms and corn borers, and genetically modified to withstand Roundup herbicide, could become more susceptible to rootworms unless growers keep soybean fields free of volunteer corn and continue planting refuge acres, says Christian Krupke.

"We need to stay a step ahead of rootworm resistance development," Krupke says. "If there's one thing we know about insects, it's that they figure out a way to adapt to whatever we throw at them."

Rootworms are a major threat to corn crops, costing farmers about $1 billion a year in yield losses and control expenses. About 30% of Indiana's estimated 6.45 million corn acres were planted to multi-trait biotech varieties this year, including the Bt/Roundup "triple stacks."

While transgenic varieties have helped growers boost corn yields, those varieties could unintentionally produce stronger, tougher-to-control rootworms when farmers rotate their cornfields to soybeans the following year, Krupke said. Rootworms feeding on volunteer corn -- maverick plants that grow from seed produced by the previous year's crop -- are exposed to Bt but at less-than-toxic levels.

"What we found was that in areas where triple stack corn was planted in 2006 and soybeans in 2007, we had a great deal of volunteer corn in some of those fields," Krupke says. "Most of that volunteer corn showed up as being Roundup Ready and as having the Bt gene for rootworm.

"The problem is that the Bt, for whatever reason, isn't expressed at the same level as Bt that you'd get in off-the-shelf corn. So you get a lot of rootworm larvae eating that volunteer corn, and they are able to survive on it. That's a concern because now you're getting insects exposed to sub-lethal doses of Bt that survive to mate and lay eggs and possibly develop stronger offspring. That is exactly what we don't want."

Volunteer corn is considered a weed and is usually controlled with herbicides. Controlling that corn becomes more difficult when it is both resistant to glyphosate, the active ingredient in Roundup, and growing in Roundup Ready soybeans. In recent years, about 90% of Indiana soybean acres have been planted to Roundup Ready varieties.

"Most soybean growers have relied on Roundup as their No. 1 -- and sometimes only -- weed control for a long, long time," Krupke says.

Farmers have several herbicide options for controlling volunteer corn, says Bill Johnson, Purdue Extension weed scientist.

"To control volunteer Roundup Ready corn in soybeans, farmers should use Assure II, Select Max, Fusion or Raptor tank mixed with glyphosate," Johnson says.

Another factor that could hasten rootworm resistance to Bt corn is improper or insufficient planting of refuge corn. Planting refuges alongside Bt corn crops is required by law.

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