Soybean aphid may meet match in another pest

When the soybean aphid first appeared in the United States in 2000, Wisconsin farmers saw what it could do. Where infestations were severe, the aphids and the viruses they transmit cut soybean yields by 10 to 15 percent.

A year ago, even university experts didn’t know if the outbreak in 2000 was an anomaly. But last summer the aphid was back, packing just as big a punch.

Now that the aphid is here to stay, growers need information on the pest, which is native to China. A team of researchers from the College of Agricultural and Life Sciences is building a picture of how the aphid operates in Wisconsin. The researchers want to know what other crops the aphid feeds on, when it colonizes fields, and what growers can do to manage aphid/virus damage.

“We’re starting to learn what to expect from the aphid/virus complex and to identify steps growers can take,” says plant pathologist Craig Grau. “We hope to put together a complete integrated pest management package for growers in the next few years.”

In addition to Grau, the research team includes entomologists Dave Hogg, Tom German and John Wedberg, weed scientist Chris Boerboom, agronomist John Gaska and their students.

Entomology graduate assistant Robb Alleman reports that the soybean aphid can feed and reproduce on several kinds of clover in the laboratory and on red clover in the field. It also will feed on snap beans, causing problems for growers as the aphid transmits viruses to snap beans.

In southern Wisconsin, scattered aphids appear in soybean fields about June 1. Researchers say that the aphid population can then spread across an entire field within two weeks. In late July — when their numbers may exceed 5,000 aphids per plant — the population produces winged adults. Grau and his colleague Mary Lee found that those winged aphids are extremely effective in moving viruses from diseased plants to healthy ones.

To manage diseases, growers are usually told to minimize sources of viral infection, to plant resistant varieties and to control insects that spread viruses. But they don’t have all those options. “We don’t have highly resistant soybean varieties available right now,” Grau says. “There is a flicker of good news. We are starting to learn that there are a few varieties that are at least partially resistant to soybean mosaic virus.”

Meanwhile, Hogg and graduate student Bob Ellingson are looking for ways to control the aphids without pesticides. They’re studying the aphid’s major predator, the multi-colored Asian lady beetle. The beetle, also an Asian immigrant, kills and eats tremendous numbers of aphids, according to Hogg.

In a preliminary field study, Ellingson found that the beetles controlled the aphids in screen-enclosed soybean plots when he added one beetle larva for every seven plants. However, adding beetle larvae at that density did not control aphids on plants outside the screen cages.

Insecticide sprays can decimate aphid populations, according to Wedberg and his coworkers. However, aphid populations rebounded within two weeks, often rising to numbers that exceeded pre-spray levels. Wedberg says repeated insecticide applications would be prohibitively expensive for growers and would still not control the spread of viruses.

The researchers say planting soybeans early — before May 10 — played a key role in minimizing aphid-related losses. Soybeans that grew rapidly suffered less damage than those that grew more slowly, according to the team’s findings.