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One Tough Bugger

Corn rootworm knows how to thwart the control measures in its path.
By Gene Johnston

Nature always wins. Want proof? Consider the corn rootworm. “This bug eventually has found a way to resist nearly every control technique we throw at it,” says Bruce Hibbard, a USDA-ARS research entomologist based at the University of Missouri. This pest doesn’t give up easily. The latest weapon, Bt technology, promises to deliver a knockout punch bred inside the plant itself. It’s showing vulnerability, however. Corn in some Corn Belt fields is succumbing to yield-robbing root damage, even when a Bt hybrid is planted. Bt will play a strong role in rootworm control for years to come, says Hibbard. Still, if we want to maximize its long-haul effectiveness, we may need to revert to previous technology in multiple-mode warfare.
Here’s his checklist of rootworm control technologies – some from the past – that can help you reduce the overall levels of infestation. 

1. ROTATE YOUR CROPS  For the first 50-plus years of the last century, this was the only rootworm defense. “It breaks the cycle of the pest,” Hibbard explains. Rootworm pupates into the adult stage late in the growing season. Females then lay eggs that overwinter and hatch again the next spring. Corn-on-corn really plays into their game plan. If no corn roots exist in the field due to planting soybeans or another crop, the larvae starve and the population is greatly reduced.  In many places, rootworm has found a way to get around crop rotation – but not completely. Usually, rotation helps to reduce root feeding. If it is possible to rotate crops, you may have to do that to help maintain high corn yields, Hibbard says.

2. USE GRANULAR SOIL INSECTICIDES  This second-generation control technology started back in the 1940s and 1950s. Initially, farmers broadcast chlorinated insecticides on as a seed treatment. Resistance developed relatively quickly. Organophosphates and carbamates were then developed, but they didn’t offer an easy system to manage. They also were relatively expensive, so farmers banded them in the row furrow with the planter.  This technology and the chemicals are as effective as ever, says Hibbard. Rootworm has shown little or no resistance to the carbamate class of chemicals in 40 years of use. Banding provides a natural refuge where rootworm can survive between rows outside the insecticidal bands.  “The fact that resistance has not developed to this man- agement tactic documents that refuges work for delaying resistance,” he says. “There are places where you can use it to advantage now.”

3. USE DIFFERENT MODES OF ACTION  This tactic is similar to rotating weed chemicals to prevent resistance development. With Bt corn, there are now second- and soon-to-be third-generation Bt modes of action that work in different ways than the original Bt. Thus, rootworm that resists one Bt mode is susceptible to another.  If you are rotating crops and soil insecticides where possible, Hibbard advises also rotating the type of Bt corn that comes in your seed package. The goal is to keep rootworm one step behind you in its fight for survival.

4. STACK THE MODES  If rootworm resists a Bt mode of action, don’t plant seed with solely that Bt mode, Hibbard says. If crop rotation or granular soil insecticides are not possible, plant Bt corn hybrids that stack the modes of action with more than one source. That improves the effectiveness, and fewer rootworms should survive to reproduce.  Hibbard says, some help is on the way for this one in the form of new weapons. Bayer CropScience, Monsanto, and Syngenta are working on new modes of action coming down the pike later this decade (see sidebar below).

THE BOTTOM LINE  There are more management issues to rootworm control than ever. “There are issues with these options in some areas, and you need to manage around them,” Hibbard says. “Our best new techniques don’t always work. And if we use them alone, they won’t work forever. It is going to take great diligence from all of us – growers and scientists. Understand what is happening in your own fields and manage rootworm populations accordingly.”

Future Rootworm-Resistant Products:
Like it or not, resistance is inevitable for a control that is heavily used. That’s why companies continually research new modes of action to replace products showing resistance. Here’s what’s new on the rootworm-resistant trait front:
• Bayer CropScience is developing a new generation of
broad-spectrum insect traits featuring multiple action modes later this decade. Part of this research is a mode of action for a corn rootworm trait that is completely novel compared to current control measures, says Brian Vande Berg, Bayer CropScience senior scientist for corn and soy research.
• Monsanto has a corn rootworm technology in the works for later this decade that will use a Bt protein and an action mode using RNA interference to kill corn rootworm.  “The complementary level of control between the Bt protein and RNA interference technology will bring unprecedented control for corn rootworm,” says Bob Riter, vice president of breeding technology for Monsanto.
• Syngenta is planning a new mode for rootworm resistance on tap for 2014, pending regulatory approval. It’s called Agrisure Duracade. It expresses the eCry3.1Ab protein that has an action mode that binds differently to the insect than other rootworm traits. Syngenta plans to stack it in its AgrisureViptera trait pack- age and refuge stacks in 2014, pending regulatory approval. It will be teamed with Syngenta’s Agrisure RW trait, giving the stack two rootworm modes of action.  “It uses a different technology that is more efficacious, a game changer in the rootworm business,” says Dane Bowers, technical support representative for Syngenta.

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