Drop Those Volunteers

The term "Bt" could stand for "big trouble" in the years ahead if growers aren’t careful in their use of biotech corn, says 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, says Christian Krupke.

"We need to stay a step ahead of rootworm resistance development," he 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 growers about $1 billion a year in yield losses and control expenses. For example, about 30 percent 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 growers rotate their cornfields to soybeans the following year, Krupke says. 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," he continues. "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 percent 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.

Growers 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.

"A refuge is anything that is not Bt corn," Krupke says. "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," says Krupke. "They never have an opportunity to develop resistance to it. The only way insects develop resistance is by exposure. 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 says. "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 explains. "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.

"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 says. "That is important because large females tend to lay more eggs and are preferred by the male beetles because they lay more eggs."

Growers need to remain vigilant when they plant Bt corn to ensure the technology is around for a long time, Krupke says. "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 says. "The two primary things would be to continue planting the refuge and, in areas where you are rotating corn with soybeans, clean up any volunteer corn that you have in the field. You need to do the latter because volunteer corn is a host, and that’s where rootworms can develop. There will be a lot of eggs in those first-year soybean fields that were in corn the year before."

(Source: Purdue University)
 

Leave a Reply

Latest News
Kochia
Crop InputsNufarm Launches New Herbicide for Resistant Kochia
July 25, 2016
Nufarm introduces Scorch herbicide for U.S. farmers and ranchers combating a broad range of troublesome broadleaf weeds. A unique premix Read More
ManagementNew Rabobank Report Calls for Farmer ROI Focus
July 25, 2016
As U.S. row crop farmers brace themselves for a third year of negative margins, Rabobank believes farmers must lower the Read More
Syngenta Sign
Crop InputsBloomberg: Syngenta-Chem China Deal on Track for Regula…
July 25, 2016
Syngenta AG, which has agreed to be taken over by China National Chemical Corp. for $43 billion, said talks with Read More
ManagementGROWMARK Meeting Visit, Company Takeover Updates, and R…
July 22, 2016
Editors Paul Schrimpf and Eric Sfiligoj discuss their recent speaking engagement at GROWMARK’s eastern event, crop protection company merger rumors, Read More
Crop InputsMonsanto: EU Approves Roundup Ready 2 Xtend Imports
July 22, 2016
Monsanto Co. announced today that the European Commission has granted import approval for Roundup Ready 2 Xtend soybeans. This milestone Read More
Syngenta headquarters in Basel, Switzerland
Crop InputsSyngenta Announces Alfalfa Seed Split Off
July 22, 2016
On September 1, 2016, Syngenta will transfer sales and distribution of alfalfa seed to the NEXGROW branded business that is Read More
Dow DuPont
Crop InputsDuPont, Dow Shareholders Approve Merger
July 21, 2016
DuPont and The Dow Chemical Company announced that, at their respective special meetings of stockholders held today, stockholders of both Read More
ManagementLand O’Lakes Announces SUSTAIN Business Unit, Oth…
July 20, 2016
Land O’Lakes, Inc. today announced the formal organization of a new business unit, SUSTAIN, and its leadership. SUSTAIN will focus Read More
Monsanto Luling Plant
Crop InputsReport: Bayer Shareholder Proposes Monsanto ‘Host…
July 20, 2016
One of Bayer AG’s top 20 shareholders said Wednesday it may make sense for the life-science company to launch an Read More
Young Corn Closeup
Industry NewsNew Leaf Symbiotics Snags Former Monsanto Microbials Le…
July 20, 2016
NewLeaf Symbiotics, announced today the appointment of Dr. Janne Kerovuo as Vice President for Research and Discovery.  Dr. Kerovuo comes Read More
Corn and ditch
ManagementStudy: Saturated Buffers Reduce Nitrates From Subsurfac…
July 19, 2016
The Agricultural Drainage Management Coalition (ADMC), Agricultural Drainage Management Systems Task Force and Dr. Dan Jaynes with the National Laboratory Read More
Case sprayer nozzle closeup
EquipmentAgronomists, CCAs, Custom Applicators Invited To FSR Ag…
July 19, 2016
Agronomists, Certified Crop Advisers (CCA) and custom applicators can stay current on agronomy issues on the grounds of the Farm Read More
Soybean Field
LegislationOhio Soybean Association Names Brian Hill Legislator Of…
July 19, 2016
The Ohio Soybean Association (OSA) has announced that State Representative Brian Hill (R-Zanesville) has been named one of two Legislators of Read More
Eric SfiligojThe Latest Monsanto Rumors: A Game Of Tones
July 18, 2016
When companies are engaged in the back-and-forth of the acquisition game, the tone of their various announcements is always important. Read More
Nozzle spray close-up
AdjuvantsWest Central Distribution Introduces New Elite Family O…
July 14, 2016
West Central Distribution recently released a new line of Elite adjuvants to work alongside the new herbicide technologies that combat Read More
Bayer sign
Crop InputsBayer Makes New Offer For Monsanto
July 14, 2016
Bayer AG boosted its takeover offer for Monsanto Co. to about $65 billion in a bid to overcome the U.S. Read More
ManagementAn Update on Water
July 14, 2016
CropLife Editors Eric Sfiligoj and Matthew Grassi discuss agricultural water issues in conjunction with a recent field trip in Ohio. Read More
Co-Alliance employee
EmployeesNew Purdue Program To Focus On Talent Management
July 14, 2016
Purdue University’s Center for Food and Agricultural Business is starting a new professional development program to help food and agribusiness Read More