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Corn+Soybean Digest

Costly Combo

Beetles and virus deliver double whammy The bean leaf beetle, left, carries the bean pod mottle virus, delivering death and destruction to soybeans.

Recent wimpy Northern winters are translating into giant headaches for some soybean growers. Mild winters help both insects and diseases survive.

The result this year? Plant-damaging bean leaf beetles spreading bean pod mottle virus, ripping yields and seed quality in many fields.

That was especially the case in Iowa, where the problem surfaced seriously last year. This year, in mid-to-late August, aerial applicators worked from dawn to dusk spraying the beetles in heavily impacted areas.

"This is ugly," says Marlin Rice, Iowa State University (ISU) entomologist. He's referring not only to the plant damage from the insects, but also the virus.

"We've really got a compound situation here, with physical plant damage from the insects and plant and seed damage from the virus," says Rice.

Bean leaf beetles are about 31/416" long, with either a red or yellow background and rectangular black spots.

How bad were these bugs in impacted fields?

"I was in a field Sept. 8, where I took 10 sweeps with an insect net and, as best I could count, there were 229 beetles in 10 sweeps," he reports. "That is a phenomenal number - between four and five times the economic threshold. Obviously, that was an extreme case, but I could find the beetles in virtually any field I walked into."

Research has shown that physical damage to soybeans can cause up to a 50% yield loss in extreme cases, Rice points out. But, especially when combined with damage from the bean pod mottle virus, losses of 20% or more wouldn't be surprising, he says.

The other side of the equation - the virus - was equally prevalent and severe in some Iowa fields, notes John Hill, ISU plant pathologist.

Plants may be malformed and have green-to-yellow mottled or "blistered" leaves and produce "scuzzy-looking seed," Hill says.

"We've got a virtual virus epidemic out here," noted Hill in late August. "I think it's going to be bad news when they start running combines through affected fields. We have a major problem on our hands.

"In fact, in 30 years of working virus diseases of field crops, I have never seen anything like it. When the seed comes out of those fields, a lot of it is going to look terrible. The virus can also knock yield 20-30% in beans infected early in the season."

At the mid-September deadline for this issue, it was too early to give an accurate assessment of yield loss due to the beetle and virus problem in Iowa and other north-central states. The beetles and virus are present in several states.

Researchers will be crunching numbers and developing management strategies early this winter, and Soybean Digest will provide those answers in a more detailed article when they're available.

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