Soybean growers across southern Wisconsin are confronting an unexpected and costly surprise at harvest. Some of their beans are wrinkled, moldy and mottled with dark blotches on the typically creamy yellow seeds.

“We”re hearing from growers and crop advisors about poor soybean seed quality this harvest season,” says Craig Grau, a soybean disease expert at the University of Wisconsin-Madison. Where the problem is most serious, grain elevators are rejecting poor quality soybeans.

When something goes wrong with the soybean crop, it affects the pocketbooks of many state farmers. Soybeans are the second leading cash crop in the state. The Wisconsin Department of Agriculture, Trade and Consumer Protection projects that growers will harvest more than 55 million bushels of beans. At $5.10 a bushel, the harvest value of this year”s crop would be $280 million if the beans were of good quality.

UW-Madison agricultural scientists lay 75 percent of the blame for this year”s problem at the six little feet of the bean leaf beetle. This pest has gradually moved form Nebraska to Wisconsin, according to entomologist Bryan Jensen. “It”s now common in the southern two tiers of Wisconsin counties although it was difficult to find any there just three years ago.” he says. The beetle has also been found as far north as the Fond du Lac and Eau Claire areas. Mild winters and snow-covered fields favor the beetles” survival and increase the chances for crop damage, Jensen says.

The bean leaf beetle is especially good at transmitting bean pod mottle virus, which lives and reproduces in soybean, snap bean and their legume relatives. The bean pod mottle virus decreases pod formation; reduces seed size, weight and number; and causes seed to become mottled, according to Grau. It is also associated with green stem syndrome, a delayed maturity of the stems and petioles, which can make harvesting difficult.

During the past two summers, growers have been keeping an eye out for the soybean aphid, another insect pest that”s new to Wisconsin. But they may have overlooked the steady increase in beetle numbers. “There”s been such an emphasis and awareness of the soybean aphid recently that growers haven”t fully recognized how much damage the bean leaf beetle and bean pod mottle virus can do,” Grau says.

Grau thinks that the virus probably has existed in Wisconsin for many years. Iowa researchers have found it in several native plants. However, until the beetle arrived, the virus seldom damaged the soybean crop.

Bean leaf beetles that survive Wisconsin”s winter, perennial non-crop species, infected seed, and infected forage legumes may all be sources of the virus, according to the experts in the College of Agricultural and Life Sciences. Grau says it is not clear how much infected seed might be contributing to the development of bean pod mottle, because transmission rates via seed are low.

When adult bean leaf beetles become active in spring they survive by feeding on wild legumes, alfalfa and clover, until they move to soybean fields as that crop emerges. After the bean leaf beetles feed on plants infected with the virus, they transmit it to the next plant on which they feed.

As its name implies, the beetle initially feeds on soybean leaves and infect plants with the virus. Soybean plants that become infected with the virus early in the season have the highest percentage of mottled seed. In August, the beetles switch their attention to the pods as they begin forming. Even if the virus is absent, feeding on pods opens them up to fungi and conditions that lead to moldy and wrinkled seed.

Grau says that there is strong evidence from several states that soybeans planted early in the season are at greater risk to bean leaf beetle and bean pod mottle virus. He recognizes that growers plant soybeans early to produce higher yields and wants to remind those growers that they should be prepared to deal with this increased risk through variety selection or an application of insecticide to reduce virus transmission early in the season.

UW-Madison agronomist Roger Borges agrees. “There appears to be a relationship between planting date and the severity of bean leaf beetle infestations,” he says. “Where the bean leaf beetle has been a problem, adjusting planting dates may become part of a larger strategy to manage the crop.”

Opinions differ about the effectiveness of insecticides in reducing virus transmission. Jensen urges growers to check with their local extension office or online crop management newsletter for current recommendations.

After a preliminary look at soybean variety trials in southern Wisconsin, Grau is seeing evidence that some varieties show fewer symptoms to bean pod mottle virus than others. Varieties that are fully resistant have not yet been reported, he says. Grau hopes to have the results of these variety trials available to growers and seed companies by late November.

Growers who seek more information about the soybean seed quality issue, the bean leaf beetle, the soybean aphid, the bean pod mottle virus and other insect-transmitted viruses, may want to follow three websites. Both the UW”s Crop Manager site and Soybean Health Bulletins have information and photos. These sites will provide updates as new results become available. Another good source is the North Central Soybean Research Program”s Plant Health Initiative site.

The research on soybean insects and viruses 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, the USDA North Central Integrated Pest Management program and the College of Agricultural and Life Sciences.