A year ago even the experts at the University of Wisconsin-Madison 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.”
Members of the team presented some of their early findings at this year”s Wisconsin Fertilizer, Aglime and Pest Management Conference in Madison on Jan. 17. In addition to Grau, the 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 reported 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, which is causing problems for growers because the aphid transmits viruses to snap beans. Alleman found that the aphid fed very little on alfalfa and not at all on peas.
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 viral diseases, growers are usually told to minimize sources of viral infection, plant resistant varieties and control insects that spread viruses. But they don”t have all those options.
“It”s difficult for growers to minimize sources of the problem viruses because aphids can acquire at least two of them from red clover and other forage legumes,” Grau says. In addition, soybean seed may carry several of the viruses aphids transmit. Planting virus-free seed would be another way to manage viruses. But producing virus-free seed will require a coordinated effort within the seed industry.
“We don”t have highly resistant soybean varieties available right now,” he 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.”
That leaves managing the insects that transmit the diseases as the best option for growers.
Hogg and his 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. Even though aphid populations increased after spraying, there was a yield gain of 8.2 bushels/acre. Wedberg believes that repeated insecticide applications would be prohibitively expensive for growers and would still not control the spread of viruses.
Wedberg is more encouraged by preliminary results with a systemic insecticide applied to soybean seed before planting. Plants grown from treated seed appeared to be healthier and had fewer aphids than plants grown from untreated seed. The systemic insecticide also didn”t harm the predators that eat aphids. While the trial is encouraging, Wedberg cautions that the researchers haven”t yet analyzed the yield results or determined if the systemic insecticide reduces viral problems.
The CALS researchers found that 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.
Practices that result in a rapidly forming soybean canopy also appear beneficial, according to Grau. The researchers found that planting at high densities and avoiding herbicide injury – which may thin the canopy – appeared to reduce the damage caused by aphids and viruses.
Growers looking for the latest news on soybean pests can visit Grau”s web site, and the Wisconsin Crop Manager web site.