Bean Leaf Beetle Activity Noted in 2016

August 4, 2016
ICM News

In April 2016, an ICM News article showed a prediction for higher survivorship of overwintering bean leaf beetles in Iowa. Not surprisingly, I have been finding more bean leaf beetles in my research plots and hearing about adults in commercial soybean this summer. Most people have reported minor defoliation from overwintering and first generation adults, but some scouts are wondering about the potential for second generation injury. The second generation adults should be emerging in a few weeks (Fig. 1). Scouting for bean leaf beetle and other soybean defoliators should continue through seed set.



Figure 1. An example of bean leaf beetle population dynamics in Iowa. The second generation can be an economically important defoliator in soybean.

Sampling


Bean leaf beetles are in the leaf beetle family Chrysomelidae. The adult is about 1/5 inches long and dark in color. The forewings have four rectangular spots with a dark triangle behind the “neck” (Fig. 2); the forewings can range in color from yellow to tan to red.



Figure 2. Bean leaf beetle adults have variable forewing color, but typically have four rectangular spots. Photo by Lynette Schimming.

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


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.

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

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


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.


Management


The overwintering and first generation populations do not typically cause economic defoliation, but can be a useful predictor of the second generation. Bean leaf beetles feeding on soybean pods can lead to significant reductions in seed quality and yield throughout Iowa (Fig. 3). It is important to recognize bean leaf beetle injury. Managing bean leaf beetles in soybean during the pod set and pod fill 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.



Figure 3. Second generation bean leaf beetles can feed on soybean pods.
Photo by Mark Licht, Iowa State University Extension and Outreach.

To help make treatment decisions easier for first and second generation bean leaf beetles, a dynamic Excel spreadsheet has been created. 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.


To make these calculations easier and always up-to-date with the changing market, follow this link to a downloadable spreadsheet. By saving the spreadsheet to your personal computer or tablet, it can be used repeatedly as market values fluctuate. There are a few examples to demonstrate the tools.

Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Integrated Crop Management News, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on August 4, 2016. The information contained within may not be the most current and accurate depending on when it is accessed.

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Erin Hodgson Associate Professor

Dr. Erin Hodgson started working in the Department of Entomology at Iowa State University in 2009. She is an associate professor with extension and research responsibilities in corn and soybeans. She has a general background in integrated pest management (IPM) for field crops. Dr. Hodgson's curre...