Bean Leaf Beetles: The pest that keeps on giving

Bean leaf beetles are usually the first insect pest to attack soybeans early and come around later for another bite as well. Bean leaf beetle is a pest that attacks the soybean throughout the growing season. It is important to learn to identify it early, and keep watching for its effects as the crop develops.

As soon as planting winds down and soybeans begin germinating and emerging, soybean pests begin their assault. One of the most visible early pests of soybean is the bean leaf beetle (BLB). The adult BLB is about ¼ inch long and its coloration can be green, tan or red. The distinctive identifying feature of the BLB is a black triangle on the wing covers just behind the thorax of the beetle. Although common, their numbers are usually low enough to not be a major concern.

Bean leaf beetles overwinter as adults in woodlots and fencerows, and are attracted to the earliest emerging soybean fields. With the mild winter we’ve just experienced it’s safe to assume that BLB survival was higher than would be expected with colder, more typical weather conditions. As the first soybeans emerge, BLB adults leave their overwintering sites and move to seedling bean fields. They begin feeding on the emerging cotyledons, unifoliate and first trifoliate leaves of soybeans. If severe, feeding on cotyledons or the growing point can lead to stunting and even death of the seedling. The soybean plant is extremely resilient at this stage, unless the cotyledons or the growing points are destroyed.

A University of Nebraska publication indicates that at current soybean prices and treatment costs, beetle counts of 2 to 4 beetles per plant at VC to V1 development stage may warrant treatment. In recent years, seed treatments containing neonicotinoid insecticides have shown the ability to reduce BLB feeding on seedlings. I have seen dramatic differences in treated vs. non-treated seed in high BLB feeding years. There is concern about the possible effects on pollinators when using these products. As Illinois CCA and fellow Soy Envoy Stephanie Porter said in her most recent blog, “If you are worried about neonicotinoid dust coming off the seed and harming pollinators, use Bayer® Fluency Agent seed flow additive.”

In addition to early foliar feeding, BLB can be a vector for soybean pod mottle virus. This viral disease can affect soybean plants by causing yield losses and reducing seed quality. Dr. Craig R. Grau, retired plant pathologist, UW-Madison, has said the BLB is the “most consistent pathological cause of green stem in soybeans.” Green stem is most often exhibited as plants with mature pods whose stems remain green, slowing harvest.

Mid-season, the first new generation of BLB becomes one of the many defoliators, joining Japanese beetles, green clover worms, cabbage loopers and grasshoppers to feed on soybean leaves. It is difficult to differentiate the feeding by these pests, and I’m sure they’ll be the topic for another Soy Envoy blog this season.

The second generation of adult BLB peaks in August and September. Once the maturing foliage becomes less attractive to beetles, they feed on the green tissue of pods, leaving a thin membrane over the seed. During pod maturation this membrane often cracks, leaving an entry hole for air-borne plant pathogens. These pathogens may cause discolored, moldy, shriveled and/or diseased beans.

Sources: University of Nebraska Cropwatch, Scout Early Emerging Soybeans for Bean Leaf Beetles, April 2012

Purdue University Field Crops IPM Bean Leaf Beetle
Coolbean.info

Photos:

Bean leaf beetle adults vary in color and markings, but can be identified by the distinctive black triangle behind the thorax. Photo from University of Illinois Extension

Bean leaf beetle pod feeding. Photo from Kelly Estes, Illinois State Water Survey

Bean leaf beetle feeding on cotyledons and unifoliates can result in stunted growth or kill soybean seedlings. Photo by Kevin Black.

CCA Kevin Nelson is a 4R NMS at Northern Partners Cooperative. Kevin works with the Agronomy Sales Team and growers in North Central Illinois. He is a 2017 Illinois Soybean Association CCA Soy Envoy.