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Corn+Soybean Digest

Welcome, Western Bean Cutworm

When western bean cutworm (WBC) moths first started showing up in Wisconsin traps in 2004, it was a novelty. The pest had been known to creep into an area, then retreat — for reasons unknown to entomologists.

Regular trapping and additional signs of ear feeding from the pest became more evident during the summers of 2006 and 2007, although insect numbers didn't approach those seen in Iowa. But the pest gained a foothold, and the state soon saw a dramatic increase in WBC infestations.

“This (2008) was the first year that several factors came together,” explains Eileen Cullen, entomologist at the University of Wisconsin. “Pheromone trap numbers were up, we saw more evidence of WBC corn ear feeding and, in general, the larval populations of WBC were higher and more widespread throughout the state.”

The WBC isn't a new pest to North America. In fact, it was first discovered in the desert southwest in the late 1800s, and economic damage had generally been confined to the western and mountain states.

On occasion, Iowa and Minnesota have seen some flare-ups of WBC, but damage was usually light, and the pest usually retreated back to Nebraska.

That changed beginning around 2000. Entomologists soon began seeing the pest move farther east…first Minnesota and Iowa, then Illinois and Missouri, then Indiana, Michigan, Ohio and Wisconsin. It's even been reported in Ontario.

THE WBC LARVAE damage corn ears, feeding on the silks, corn pollen and the corn ear. The pest shows a preference for sandy soils, although that may not always be the case.

So why is WBC on the march east? No one knows for sure, although research at Iowa State University and South Dakota State University points to the possibility that the exceptional control of European corn borer — and to a lesser extent the suppression of the corn earworm — from various transgenic corn hybrids may have given the western bean cutworm the opening it needed.

“We have seen an increase in the WBC, so as entomologists we first asked ourselves what has changed dramatically in the Corn Belt to influence this shift,” says Marlin Rice, Extension entomologist at Iowa State University. “And we think that possibly the biggest factor was the increase in Bt corn, first planted in 1996. As the acreage planted to Bt corn increased over the years, it may have influenced competing pest species.”

Nature abhors a vacuum. Control one insect, and another will soon move in to take its place. Severely impact the European corn borer population over a wide area, and a void has been opened up…enough for another insect to move in.

During Rice's studies he found out something else: The corn earworm, which is suppressed by Bt products on the market, may not be a major economic pest during most years, but it's the King Kong when it comes to insect pests. Put a corn earworm in the same arena with a western bean cutworm, and the corn earworm will kill the western bean cutworm. It's an aggressive insect,so Rice believes that if the corn earworm populations are suppressed, the WBC gets another chance to thrive in the cornfield ecosystem.

“We've seen this activity in the lab,” Rice says. “We are working to develop trials in the field to confirm our theories. And while this may not be the single reason why we're seeing more western bean cutworms, we do think this is part of the equation.”

Take one pest out, another one will move in. “Insects are highly adaptable, and our agricultural practices can influence their populations,” Cullen says.

One example of an insect's adaptability is a change in the behavior or life cycle of certain variants of the corn rootworm. Cultural control practices — crop rotation — have resulted in a behavioral change in some populations of the WBC to where today there are rotation-resistant strains that lay eggs in soybean fields, and a life cycle change in extended diapause populations of the northern corn rootworm that overwinter in soybean fields.

NEW INSECTS GAINING a foothold are nothing new. Yet with today's powerful transgenic insect-control products, it's a race to stay one step ahead of the next pest, and find multiple modes of action for insect control that keep today's technology viable.

That's one reason for refuge acres: Give some insects a chance to survive and reduce the possibility of the pest developing resistance. It's also why we're seeing newer transgenic pest-control products on the market.

SmartStax will combine the Bt events from YieldGard (Monsanto) and Herculex (Dow AgroSciences) for broader insect control from?multiple modes of action for above- and below-ground insect control. In addition, the recently announced Agrisure Viptera (Syngenta) will be combined with the company's other Bt products in new corn hybrids. Pending final regulatory approvals, producers should see corn hybrids with these traits in 2010.

“INSECTS ARE PRETTY adaptable,” says Ed King, global technology transfer leader with Dow AgroSciences. “That's why different modes of action are important, and why as companies we can't become complacent with the technologies now on the market. New modes of action will further reduce the chances of resistance and expand the number of insects controlled.”

And companies continue to research products that can provide the broadest spectrum of control. “There is a very active research effort in the area of insect control to broaden the mode of action and control more insect pests,” says Wayne Fithian, business lead for Syngenta Seeds. In fact, Syngenta is on target to release its second-generation corn rootworm event in 2012.

It's a race of sorts to stay one step ahead of corn pests. It's also critically important that companies ensure the valuable Bt traits currently on the market continue to work. There's a lot of money that's been invested to bring these products to market, and companies want them to have a long, long shelf life.

It's a high-tech game of cat and mouse, figuring which will be the next pest, then developing a control before that pest can reach an economic threshold. Michael Catangui, entomologist at South Dakota State University, was on the heels of the WBC more than five years ago. “That's when we first started to notice the numbers and the range increasing,” Catangui says. “It was our first indication that there was definitely a shift in the species composition of pests in corn.”

Today, his research focuses on what pest could cause a major impact five to 10 years down the road. “Current Bt products are very powerful, but when you get rid of one pest, there are other secondary pests that suddenly find a whole field to themselves,” he says. “We continue to look at what's occurring in the spectrum of pests in the field, if there are any shifts occurring, what that could potentially mean to producers and what producers need to do to prepare.”

Cullen adds, “The take-home message to producers is to continue to scout their fields. Current traits offer crop-protection advantages, but it's important to know that there still can be pests outside of the current controls that may pose a problem.”

PRODUCERS NEED TO understand what pests are in their fields, and the efficacy of traited corn hybrids they are currently planting. “Field scouting and integrated pest management should not be overlooked when planting a Bt corn hybrid. Each trait on the market has a different set of insects that it controls or suppresses, so knowing what insect pests are out there on your farm and crop damage potential based on economic thresholds will help growers decide if a Bt trait is suited to their farm's pest management plan, and if so how to make the best selection”

One thing is for sure. The WBC is one pest that's established a foothold. “The western bean cutworm has expanded its historical geographic range,” Rice says. “Now it is here to stay.”

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