By DOUG SCHMITZ

Iowa Correspondent

AMES, Iowa — While bean leaf beetle (BLB) counts are predicted to be low this spring from overwintering, soybean growers are being advised to routinely scout for their presence.

That’s according to Erin Hodgson, Iowa State University associate professor of entomology and Adam Sisson, extension specialist with the ISU Integrated Pest Management (IPM) program and a certified crop adviser, in a May 10 report.

“While BLB adults are susceptible to cold weather and most die when air temperatures fall below 14 degrees Fahrenheit, they have adapted to winter by protecting themselves under plant debris and loose soil,” they said. “Each spring, adult beetles emerge from overwintering habitat and migrate to available hosts, such as alfalfa, tick trefoil, and various clovers.

“As the season progresses, bean leaf beetles move to preferred hosts, like soybeans. While initial adult activity can begin before soybean emergence, peak abundance often coincides with early-vegetative soybeans.”

For example, mortality rates in Iowa are predicted to be variable for the 2018-19 winter, ranging from 73-99 percent. ISU has been tracking BLB mortality predictions since 1989. This winter, northern Iowa experienced colder temperatures, and Hodgson and Sisson expect the 99 percent mortality rate.

“Last winter, the predicted mortality of bean leaf beetle in central Iowa was 87 percent, which is about 10 percent higher than the 30-year average of 71 percent,” they wrote. “It is important to remember insulating snow cover and crop residue can help protect bean leaf beetle from harsh air temperatures. Fluctuating temperatures can negatively influence spring populations.”

The BLB is common throughout the north-central United States and has been recorded from Illinois, Indiana, Iowa, Kansas, Kentucky, Michigan, Minnesota, Missouri, Nebraska, Ohio, South Dakota, Wisconsin, and the Canadian provinces of Manitoba and Ontario.

Doug Johnson, University of Kentucky extension entomologist, said the BLB adult is a 1/8- to 1/4-inch-long beetle, with the body slightly convex and longer than wide.

“BLB color varies from light brown to dark red,” he said. “Black spots and/or stripes may be present on the wing covers. All bean leaf beetles will have a backwards-pointing black triangle behind the head.”

With the return of warm spring weather, he said BLB begin to feed and lay eggs. “The orange eggs are laid in small groups on the soil surface on or near the stems of soybean plants. Eggs hatch in about 11 days and the thin white larvae live in the soil for 35 to 55 days, passing through three growth stages.

“They then pupate and come out of the soil as adults. There are probably two and perhaps a partial third generation each year in Kentucky.”

On average, BLB have the physiological capacity for flying short distances (less than 167 feet) and populations at the field scale are highly aggregated. In fact, the spatial relationship between the first and second generations are highly correlated to each other, with the second generation showing the greatest aggregation pattern in soybean fields.

“The overwintering adults probably didn’t do well this winter, and for the most part, they typically don’t cause a lot of problems,” explained Clarke McGrath, on-farm research and extension coordinator for ISU’s Iowa Soybean Research Center.

He said fields with the following factors may need more attention when scouting:

•Soybeans planted near alfalfa fields or fields with the first-emerging soybean in the area

•Overwintering adults are strongly attracted to soybeans and will move into fields with emerging plants

•Fields with a history of bean pod mottle virus

•Food-grade or seed fields where reductions in seed quality from bean pod mottle virus can be significant

“There are three groups of BLB that we typically run into: the overwintering adults that we see early in the season; then, first generation, which we generally find in July; and second-generation into August and early September,” said McGrath, who's also a certified crop advisor.

“While the overwintering and first-generation populations do not typically cause economic defoliation, they can give us an idea if the second generation is going to be a problem. These second-generation BLBs can be a real pain, as they feed on pods, which can cost both yield and seed quality.”

McGrath advised farmers to use the following methods in scouting for BLB:

•“The drop cloth:” Place a 3-foot-wide strip of cloth on the ground between the rows; bend the plants on one row over the cloth, and shake them vigorously. Count the number of beetles that drop on the cloth, then repeat the procedure four times for every 20 acres of the field. Estimate the average number of beetles per 3 feet of row.

•”The sweep net:” Take 20 sweeps while moving forward (a sweep is defined as a 180-degree pass across two soybean rows or along 3 linear feet within a row). Repeat the procedure four times for every 20 acres of the field; estimate the average number of beetles per 20 sweeps.

“Plug these numbers into the spreadsheet, along with insecticide and application costs, and your expected market value, and it will give you a treatment threshold,” McGrath said.

“While they suffer massive winter mortality some years, (BLBs) don’t go away and seem to bounce back and cause problems once in a while. So the best thing we can do is keep scouting.”