Volunteer corn threatens efficiency of modified varieties against pests

WEST LAFAYETTE, Ind. – Bt could stand for “big trouble” in the years ahead if farmers aren’t careful in their use of biotech corn, said a Purdue University entomologist.
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, said Christian Krupke.
“We need to stay a step ahead of rootworm resistance development,” Krupke said. “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.”
Threat. Rootworms are a major threat to corn crops, costing farmers about $1 billion a year in yield losses and control expenses.
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 said.
“Most of that volunteer corn showed up as being Roundup Ready and as having the Bt gene for rootworm.
Different levels. “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.
Options. Farmers have several herbicide options for controlling volunteer corn, said 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 said.
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.
“A refuge is anything that is not Bt corn,” Krupke said. “So when you plant Bt corn for rootworm or corn borer, for every 80 acres you plant of the Bt you have to plant 20 acres of the refuge.
“The thought behind the refuge is that you have some insects in that refuge that are never exposed in their lifetime to Bt. They never have an opportunity to develop resistance to it. The only way insects develop resistance is by exposure.
Resistant. “The more you expose them, the greater the pressure is for them to be resistant. So you want to generate some insects that are never exposed to Bt so that they will mate with the ones that are exposed to Bt to dilute the chances of those offspring being resistant.”
Killing all rootworms by planting 100 percent of acres in Bt corn is neither the objective, nor is it possible, Krupke said.
“If you expose the entire rootworm population at the same time to Bt, the insects will either have to become resistant or go extinct,” he said.
“We have made zero species of insects extinct, so you can figure out which way it is going to go.”
Even in cornfields where refuge acres were planted, Krupke and fellow Purdue entomologists have found troubling signs.
Size matters. “We’ve looked at the relative sizes of rootworm beetles coming out of the transgenic and refuge corn and found some large females coming out of the transgenic blocks,” Krupke said.
“That is important because large females tend to lay more eggs and are preferred by the male beetles because they lay more eggs.”
Farmers need to remain vigilant when they plant Bt corn to ensure the technology is around for a long time, Krupke said.
“If we don’t do the things that we need to do, then we’re eventually going to have products that are not effective against rootworm,” he said.