Bean leaf beetle activity bears watching in 2016

Bean leaf beetle activity bears watching in 2016

Some Iowa soybean fields are beginning to show damage from bean leaf beetles this summer

In April 2016, an article in the Iowa State University Integrated Crop Management newsletter showed a prediction for higher survivorship of overwintering bean leaf beetles in Iowa. Not surprisingly, ISU Extension entomologist Erin Hodgson has been finding more bean leaf beetles in her research plots this summer. She’s also hearing about adult bean leaf beetles in farmers’ soybean fields this summer.

BEAN LEAF BEETLE: Mid- and late-season defoliation from first and second generation bean leaf beetle must be assessed to determine whether an insecticide application is necessary. Late in the season you have to observe whether beetles are feeding on the pods.

“Most people have reported minor defoliation of soybean plants from overwintering and first generation bean leaf beetle adults, but some crop scouts are wondering about the potential for second generation injury,” says Hodgson. “The second generation adults should be emerging in a few weeks.” The graph accompanying this article projects the timing of the generations, when they usually show up in Iowa.

“Scouting for bean leaf beetle and other soybean defoliators should continue through seed set on the soybean plants,” says Hodgson.

How to sample soybean fields for bean leaf beetles

Bean leaf beetles are in the leaf beetle family Chrysomelidae, says Hodgson. The adult is about one-fifth of an inch long and dark in color. The forewings have four rectangular spots with a dark triangle behind the “neck”  the forewings can range in color from yellow to tan to red.

Soybean fields with plants in the reproductive stages can be sampled for bean leaf beetle by using either a drop cloth or a sweep net, says Hodgson. Scout each field and each variety separately and walk into the field at least 100 feet before sampling.

Using the drop cloth method

• 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.

• Repeat the procedure four times for every 20 acres of the field.

• Estimate the average number of beetles per 3-foot of row.

Using the sweep net method

• 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.

Managing bean leaf beetles during pod set and pod fill

The overwintering and first generation populations do not typically cause economic defoliation of soybean plants, but can be a useful predictor of the second generation, says Hodgson. Bean leaf beetles feeding on soybean pods can lead to significant reductions in seed quality and yield throughout Iowa (see photo 2).

It is important to recognize bean leaf beetle injury. Managing bean leaf beetles in soybean during the pod set and pod fill stages can be frustrating to growers and crop advisers because adults may be feeding on pods for a couple of weeks before the population reaches the economic threshold to justify treatment with a foliar insecticide.

Spreadsheet can help you make insecticide treatment decision

To help make treatment decisions easier for first and second generation bean leaf beetles, a dynamic Excel spreadsheet has been created by ISU Extension. These calculations use the expected market value (bushels per acre) and cost of control (dollars per acre) to determine the treatment threshold. This calculation assumes bean pod mottle virus is not an issue for seed sale, says Hodgson. To make these calculations easier and always up-to-date with the changing market prices for soybeans, you can follow this link to a downloadable spreadsheet. By saving the spreadsheet to your personal computer or tablet, it can be used repeatedly as soybean market values fluctuate. The spreadsheet also provides a few examples to demonstrate the tools.

TAGS: Soybean
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