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Corn+Soybean Digest

Scientist Says Aphid Invaders Pose Specter Of 'Clone Warfare'

The Chinese soybean aphid is no bigger than 1/16-in. long fully grown, but what it lacks in size it more than makes up for in numbers, warned a speaker at the University of Missouri (MU) Crop Management Conference in Columbia.

"This thing is built as a reproductive machine in that it reproduces asexually," says Ken Ostlie, University of Minnesota entomologist. "They're all basically females, and the mother essentially clones herself. What we face is the challenge of clone warfare."

Minnesota has the worst problems with the soybean aphid, a native Asian pest that appears on soybean leaves by the thousands, seemingly almost overnight, Ostlie says. The current economic threshold is set at 250 aphids per plant. "They can go from 250 to more than 800 in less than a week."

That state's soybean farmers have seen not only significant yield losses, he says. "We've seen changes in soybean quality as well in the form of less oil and smaller beans."

The pest has been moving steadily south, said Tom Clark, MU entomologist. "We first detected it here in 2000, in northeast Missouri," he says. "Last year, we saw a pretty wide expansion. It's probably safe to assume that it's now everywhere in the state where soybeans are grown."

High-pressure weather systems bring northeasterly winds that carry millions of the tiny pests, Clark said. "A lot of them drop out over parts of Missouri."

The soybean aphid "doesn't take long to colonize an entire state," Ostlie says. "These things move pretty quickly." One reason it has thrived in Minnesota is because "the cooler it is, the longer they survive," he says. "Anything 95 degrees Fahrenheit or above is basically a lethal temperature. In Missouri, you're going to have a better chance of having those high temperatures that will hold them back."

Unfortunately, the soybean aphid could be highly adaptable. Its range in Asia stretches from southern Siberia to the tropical climes of Indonesia, Ostlie says. "You'd expect over time to see an adaptation taking place."

Clark agrees. "We have high summer temperatures, and that slows down their reproductive cycle. But who's to say there couldn't be a mutation, so that the species would do better in higher average temperatures?"

Research in Minnesota and Missouri has shown seed treatments and repeated insecticide spraying afford some degree of control. But seed treatments are effective only in the early part of the season, Ostlie says, and soybean aphids "can sense when the residual effect of the pesticides wear off and the plant is safe for re-colonization."

Pesticides also endanger other beneficial insect species that feed on the soybean aphid, Clark says. Many of those insects - damsel bugs, minute pirate bugs, lacewings and parasitic wasps, to name a few - "are already in place in Missouri." The soybean aphid is also known to overwinter here, he adds.

The pest often overwinters on buckthorn, says Wayne Bailey, MU extension state entomologist. "We have at least seven species in the state, and we know that three species serve as overwintering hosts of the soybean aphid."

Despite the influx of soybean aphids into Missouri last year, Clark says, very few fields experienced the economic threshold of 250 bugs per plant, at least partly because of the fierce summer heat and the pest's natural predators.

Nevertheless, Ostlie says, "we just don't know what we're going to see in the long run. There are so many factors. "This is one insect I respect."

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