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
ManagementARA 2016 Report and Crop Mix for 2017
December 9, 2016
Editors Paul Schrimpf and Eric Sfiligoj talk about the recent Agricultural Retailers Association meeting and the projected acreage mix for Read More
AgriSync
Matt Hopkins17 Agriculture Apps That Will Help You Farm Smarter In …
December 9, 2016
Ag professionals are working smarter, not harder, than ever before. Smart farming technologies have enabled them to reduce costs, maximize Read More
Ryan Bartlett
Crop InputsCompass Minerals Appoints Vice President, Innovation An…
December 8, 2016
Ryan Bartlett, Ph.D., has joined Compass Minerals as vice president, innovation and product development. Dr. Bartlett will be responsible for Read More
HerbicidesHerbicides 2017: New Cropping Systems Set For Debut
December 7, 2016
Herbicide-resistant weed are inching ever-closer to a potentially frightening saturation point here in the U.S. Heading into 2016, USDA planting Read More
CropLife 100Wilbur-Ellis Acquires Michigan Ag Retailer
December 7, 2016
Wilbur-Ellis’ Agribusiness, a recognized leader in precision agriculture technology and the distribution and marketing of plant protection, seed and nutritional Read More
Fertilizer storage The Andersons
CropLife 1002016 Fertilizer Report: Another Really Rough Year For A…
December 7, 2016
In many ways, the fertilizer category cannot seem to catch a break. During the early part of the 2010s decade, Read More
R4023 Sprayer, John Deere
CropLife 100Ag Retail Equipment Report: The Green Party Continues
December 7, 2016
In the annual race for sales in the ag retail equipment marketplace, the color schemes for participants are a little Read More
Winter wheat
HerbicidesTalinor Herbicide Approved For Wheat And Barley
December 7, 2016
Talinor herbicide from Syngenta has received federal registration from the U.S. EPA, giving wheat and barley growers a new option Read More
Forage Sorghum
UncategorizedMilestone Achievement Continues For Dow AgroSciences, A…
December 7, 2016
New and innovative forage products are on the horizon driven by continued collaboration between Dow AgroSciences, a wholly owned subsidiary Read More
Palmer pigweed seedhead in cotton
HerbicidesNew WSSA Factsheet Explores Weed Seeds And Their Longev…
December 7, 2016
Did you know some weed seeds can lie dormant in the soil for more than a century and then sprout Read More
Crop InputsARA 2016: 5 Developments Worth Monitoring Into 2017
December 6, 2016
You can pretty much set your watch to it (do people even still wear watches? I know I do…but I’m Read More
Tim McCardle, ARA Chairman
CropLife 100BRANDT COO Named ARA Chairman
December 5, 2016
BRANDT EVP and Chief Operating Officer Tim McArdle has been named chairman of the Agricultural Retailers Association (ARA) during a Read More
Crop InputsWilbur-Ellis Receives ResponsibleAg Certification At Mo…
December 5, 2016
Wilbur-Ellis Co., a recognized leader in precision agriculture technology and the distribution of crop protection products, announces the ResponsibleAg certification Read More
Young Corn Closeup
Eric SfiligojThe Read On 2017 For Agricultural Fortunes Is Anybody’s…
December 5, 2016
As I (and others) have written in recent months, the nation has just experienced one of the most offbeat election Read More
Acceleron B-300 seed coating
Crop InputsThe BioAg Alliance Launches New Yield-Boosting Microbia…
December 5, 2016
As part of their commitment to develop and commercialize innovative microbial solutions for farmers through The BioAg Alliance, Monsanto Co. Read More
ManagementCorn and soy planting update; Takeaways from Climate Co…
December 2, 2016
AgriBusiness Global Editor Dave Frabotta Joins Paul Schrimpf for a discussion of global corn and soybean planting trends, and a Read More
Radish cover crop taproot
Crop InputsSoil Health Institute, Datu Research Receive Grant To E…
December 1, 2016
The Soil Health Institute (SHI) and Datu Research have announced a $626,000 grant from the Walton Family Foundation to quantify the Read More
Mike Stern
Precision AgClimate Corp. CEO Talks Retailer Support For Digital Ag
December 1, 2016
CropLife Magazine’s sister publication, AgriBusiness Global, recently sat down with Mike Stern, CEO of The Climate Corp., following the Monsanto subsidiary’s Read More