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Corn+Soybean Digest

New Aphid Attack

Tiny pest from China sucks sap, carries viruses The soybean aphid, a native of China, somehow found its way to U.S. soybean fields. It won't likely dent this year's expected record national yield, but it has scientists more than a little concerned.

Discovered in southern Wisconsin in mid-July, it also had been identified by entomologists in Illinois, Michigan and Minnesota by mid-August.

It poses a double-barreled threat to soybean yield in infested fields. Swarms of the sap-sucking aphids can damage plants, plus they can spread yield-damaging viruses.

The new pest has never before been seen in the U.S., and scientists aren't sure how it entered the country. But they suspect that the most likely vehicle was an ornamental woody plant imported from Asia.

The alarm bells started ringing when Nancy Kurtzweil, a staffer on plant pathologist Craig Grau's team at the University of Wisconsin, brought in an aphid-infested plant from their soybean virus study plots at Whitewater. Grau called on entomologist John Wedberg to identify the bugs. They looked much like cotton/melon aphids, a common species that feeds on several crops but doesn't focus on soybeans.

So Wedberg, in turn, sent them to David Voegtlin, an aphid specialist at the Illinois Natural History Survey, who identified them as soybean aphids. That was soon confirmed by Manya Stoetzel, an aphid expert at USDA in Beltsville, MD.

There are several aphid species in the U.S., but none that are a threat to soybeans. The aphids brought to Wedberg were a total surprise because, he says, there had never been any documentation of aphids colonizing (invading, feeding and reproducing) in a soybean crop.

"This has the potential to be a big new soybean pest," says Wedberg. "Aphids are a really difficult pest to determine pest management and treatment decisions for because of their explosive nature.

"We found hundreds per leaf and thousands per plant in some fields," the scientist adds. "We're convinced they had to be here for awhile before they were discovered because of the numbers we've got right now."

The soybean aphid is smaller than a pea aphid - even smaller than a pinhead, explains Wedberg, but it can be seen with the unaided eye. He recommends that farmers buy good-quality magnifying glasses for crop scouting next year.

"Until now, we haven't had to pay a lot of attention to insects most years in soybeans in Wisconsin and the Midwest," says Wedberg. "But this is different. This is a new year-in, year-out thing that we are going to have to watch for and manage."

Wedberg, Grau and scientists in many other Midwestern states were scrambling in late summer to get a better fix on the problem and to develop management strategies for 2001. Here's what they've learned so far.

In China, these aphids produce 18 overlapping generations per year, Wedberg says. About three generations are spent on buckthorn species, which grow around field edges, especially wooded field edges. Winged aphids then fly to soybean fields to establish colonies.

"Buckthorn is the only known overwintering host, and we have four or five species of buckthorn in Wisconsin," Wedberg notes.

Once colonized in a field, both winged and wingless aphid females produce living young during summer.

Then, in late summer, the winged females fly to buckthorn and produce males. Mating occurs and the females lay eggs, which are hardy enough to overwinter and produce new aphids in spring.

In an old Chinese research study, the aphids reduced yields by 27% in untreated fields. Their optimum temperature range in the growing season is 71-78ø, with humidity of less than 78%. The aphids do best from the early trifoliate stage through flowering, and it appears the population explodes during flowering.

They're found on upper leaves until late bloom stage, then move to the plant's midsection. Populations start to wane during pod stage; then, helped by fungus diseases and parasites, they start to crash.

Late-season plant symptoms typically include distorted leaves, later turning almost black, with white flecks on them. No insecticides for soybeans are labeled for aphids, but several labeled for aphids on other crops are effective, Wedberg says. There'll likely be a scramble by companies to get labels for aphids on soybeans, he adds.

What about the secondary threat from viruses? Grau says flatly: "I don't know at this point. I don't think it's going to cause much damage on a statewide basis, but individual fields will be hurt. In any case, the problem certainly has us concerned."

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