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2001, A Soybean Odyssey: Scientists Seek Answers For New Pests

With fertilizer and energy prices high this year, Wisconsin growers may plant more soybeans and less corn. But state farmers, who planted 1.5 million acres of beans last year, face new pests that could cost them more than $50 per acre.

“The soybean aphid was first detected in the United States last July at a research trial on a farm near Whitewater,” says John Wedberg, a University of Wisconsin-Madison extension entomologist. “By late fall the aphid had been confirmed from nine states, with the biggest problems in Michigan, northern Illinois and Wisconsin. In Wisconsin the aphid was found as far north as Marinette and Chippewa counties.”

The bean leaf beetle also expanded into Wisconsin”s southern tier of counties last year, moving in from Iowa and Illinois. Both insects feed on soybeans and transmit viral diseases. The insect/virus combination reduces yields and quality.

“Together, the soybean aphid and the bean leaf beetle may have a profound effect on how we manage soybeans in Wisconsin,” says Craig Grau, a UW-Madison expert on crop disease. Grau and Wedberg are part of a College of Agricultural and Life Sciences team trying to learn more about the insects, viral diseases and how best to manage them.

No one is sure what to expect in 2001. Last year”s weather may have been unusually favorable for many aphid species, including the soybean aphid. But the experts fear major problems in soybeans. The aphid alone can reduce yields by more than $30 per acre. Throw in the viruses and the leaf beetle, and damages could easily double, according to Grau and Wedberg.

Although last year”s aphid outbreak was unexpected, the researchers collected some useful information at field experiments set up to evaluate virus problems in beans. For several years, soybeans had developed “green stem,” a disease in which stems fail to dry and turn brown by harvest. Grau thought viruses might cause green stem. In 2000 he was conducting experiments to exclude insects that might transmit viruses to soybean plots so he could evaluate the crop for viruses, symptoms, and yield loss. Those studies became extremely important when the soybean aphid made its American debut.

To provide information about soybean pests, the College team will expand its studies in 2001. In addition to Grau and Wedberg, the team includes entomologists Dave Hogg and Tom German, and weed scientist Chris Boerboom. Hogg, for example, is studying how humidity and predators affect aphids. The results will help scientists develop models to predict how weather and biological control affect the pest.

The soybean aphid is the only aphid that lives on soybeans and damages the crop, according to Wedberg. The aphids suck sap from the plants, stressing them and introducing viruses. They transmit two viruses – soybean mosaic virus and alfalfa mosaic virus – that can distort pods, discolor seed and reduce yields. The aphids concentrate on new leaves and branches as plants grow. After plants flower, aphids move down the plants and feed on the undersides of leaves. Heavily infested plants may not show obvious symptoms of stress.

Studies last year found that aphids began leaving soybeans in late September. They then flew to buckthorn plants. These woody shrubs are widespread in Wisconsin. The aphids mated and laid eggs on buckthorn. The team anticipates that soybean aphids will hatch in spring, live on buckthorn for a period and then again colonize soybean fields.

The soybean aphid is native to China, where it also overwinters on buckthorn. Experts suspect that the aphid was brought into the United States on ornamental buckthorn plants imported from Asia.

Heavily infested soybean plants are stunted, have distorted leaves, and may drop their leaves early, Wedberg says. Research in China has shown that aphid-related yield losses can range from 3 percent to as much as 50 percent. Preliminary trials at the College”s Arlington Agricultural Research Station in 2000 indicated that aphids reduced yields by 11 to 13 percent – 6 to 8 bushels per acre – compared with treated control areas.

“It”s a challenge to separate yield loss caused directly by insect feeding and caused indirectly by the viruses they transmit,” Grau acknowledges. The research team is starting greenhouse and field experiments to separate the two problems.

In a study in 2000 using a single soybean variety, Grau found that yields were 10 bushels per acre lower in plots where 90 percent of the foliage showed viral symptoms than in plots where only 18 percent of the foliage showed symptoms. He says some varieties appear to tolerate viruses better than others. Wedberg also found that soybean aphids were more common on some soybean varieties than others.

“Our initial results indicate that soybean viruses and the soybean aphid form a complex that must be managed together,” Grau says. “We”re just starting to learn what to expect from the aphid-virus complex, so we don”t know enough yet to make definitive recommendations. The standard principles for managing viral diseases are to minimize the source of the viral infections, control insects that spread viruses, and plant varieties that resist viruses.”

The seed that farmers plant may be an important source of viruses. But tests on soybean seed showed little viral contamination in 2000, Grau says. Forage legumes may harbor the viruses the aphids transmit. Therefore, planting soybeans near forage legumes may increase the risk of losses to the viruses.

“We are trying to gather information on which varieties are most resistant to these viruses,” Grau says, “but there are not likely to be highly resistant varieties available now.” Growers and scouts can learn more about the viruses and other diseases of the crop at Grau”s plant health web site.

“Managing the insects that transmit the diseases looks like the best option in 2001,” he says. “Plants that become infected with viruses after flowering suffer far less damage than those infected earlier. Insecticides are most helpful if they delay virus transmission until after flowering.”

Sprays can keep the aphid population in check, Wedberg says. Growers should begin monitoring the crop in spring. They may not see aphids on seedlings, he says. Last year the aphid population exploded during the late vegetative and early bloom stages when plants were growing most rapidly.

Wedberg says sprays will have the greatest impact after the aphids develop colonies and before flowering is over. “Insecticides are most effective early in the season while aphids are feeding at the tops of plants, and before the insects move down the plants to feed,” he says.

Growers may use products that are labeled for soybeans and also for aphid control on other crops. Growers and crop scouts can find detailed management recommendations for possible insecticide treatments at the UW-Extension Wisconsin Crop Manager web site.

To minimize damage from aphids and the viruses they transmit, growers should plant early, use narrow row spacing and avoid herbicide injury. The growers who experienced the worst aphid damage last year were those who planted late in the season or planted soybeans after harvesting an early crop, Wedberg says. Soybean varieties differ in their susceptibility to green stem and the disease becomes more severe the later the beans are planted, according to Grau.

Farmers in southern Wisconsin may be confronted with both the aphid and the bean leaf beetle. These growers will have to select a strategy aimed at managing the greater of the two evils, according to the researchers. An Iowa study indicates that delaying planting will help farmers avoid problems with leaf beetles and the pod mottle virus they transmit. However, delayed planting exposes beans to greater risks from the soybean aphid and green stem.

This year the scientists also hope to learn more about the biological agents that kill the soybean aphid. Aphid predators include the Asian lady beetle, which took a tremendous toll on the aphid population last summer, Wedberg says. But he warns that these predators can”t reproduce quickly enough to keep the prolific aphids from damaging the crop.

An as-yet-unknown fungal disease holds greater promise. It spread among the aphids last August and decimated their numbers. The researchers will be studying how biological control can be incorporated into programs to manage the soybean pests while minimizing any adverse impacts of insecticides.

The research is being supported by: state funding to the UW-Madison College of Agricultural and Life Sciences and UW-Extension Cooperative Extension Service; and grants from the Wisconsin Soybean Marketing Board, the North Central Soybean Research Program, USDA North Central Integrated Pest Management program, and the College of Agricultural and Life Sciences.