Originally Submitted: July 22, 2022
Despite the prediction of low overwintering survival we have observed bean leaf beetles in soybean this week. In South Dakota, bean leaf beetles go through two generations each year. The overwintering generation, which is initially observed in the spring, is made up of adults that survived the winter and feed on soybean seedling once they emerge. Before soybeans emerge, the overwintering bean leaf beetles feed on alfalfa. The bean leaf beetle adults we are seeing now are part of the first generation that occur each year. The first-generation adults usually show up in July and early August in South Dakota.
In the same areas where the first-generation adults are observed, a second generation can be observed from late August up to the first hard frost or when soybeans senesce. The northern areas of South Dakota do not have a second generation of bean leaf beetles.
Some of the individuals we are currently observing may make up the population of overwintering adults. These individuals will seek out leaf litter and cover in late fall. While these adults are in soybean, they can cause significant amounts of defoliation to the leaves. A reduction in the available leaf area can lead to reduced levels of photosynthesis and result in lower yields.
Bean Leaf Beetles
Pest Identification
Adult bean leaf beetles can vary in color from brown (Figure 1-A), yellow (Figure 1-B) and orange to red (Figure 1-C). Bean leaf beetles have a black triangle located behind their thorax (segment behind black head capsule) and four black spots, which are present on their hardened forewings (elytra).
Scouting
Bean leaf beetles avoid disturbances, which makes them one of the more-difficult insects to scout for in soybeans. Scouting and identifying bean leaf beetles can be especially difficult in soybeans, which have canopied. The best method for scouting is to use a sweep net and collect 20 pendulum swings from four locations within the field. The economic threshold for bean leaf beetles is 70 to 100 beetles per 20 sweeps. This is based on bean leaf beetle populations later in the season.
An alternative to directly scouting for populations of bean leaf beetles is to look and determine the amount of defoliation occurring within the field. During this time of the season, this method is often more effective than individual insect counts, because there can be multiple species of defoliators active in soybean. To scout for defoliation, examine 10 plants from five locations spread throughout the field (50 plants per field). For each of the plants, estimate the percentage of leaf area that is removed from all the leaves (i.e. defoliation). Record this for each of the examined plants and calculate the field average. Since the majority of soybean are past the initial flowering stage, the economic threshold for defoliation is 20%. At or above this level of defoliation a 3 to 7% yield loss may occur. Figure 2 has a visual guide for defoliation caused by bean leaf beetles and the associated percentages.
Pest Management
If defoliation percentage or bean leaf beetle counts are above the threshold, an application of foliar insecticides is warranted to reduce the bean leaf beetle population and other defoliating insect populations. Refer to the most-recent South Dakota Pest Management Guide: Soybean for a list of labeled products.
Bean Pod Mottle Virus
If you are finding bean leaf beetles in the field, you will also need to scout for bean pod mottle virus (BPMV), which is primarily vectored in the United States by the bean leaf beetle. When bean leaf beetles feed on BPMV-infected soybean plants, they also ingest the virus and become a carrier of the virus (viruliferous). The virus can be obtained with a single bite of an infected plant. Virus transmission occurs rapidly with the next feeding. As the beetle moves and feeds throughout the field, it spreads the virus to healthy plants. The beetle eventually loses their infectivity unless they feed and acquire the virus from infected plants again.
Disease Identification
Bean pod mottle virus symptoms are commonly confused with herbicide injury and can resemble symptoms of other viruses. Symptoms associated with BPMV include mild to severe chlorotic mottling or mosaic and rugosity (distortion or wrinkling) on foliage (Figure 3), stunting, and delayed maturity. Symptom severity may lessen during hot weather or with maturity; however, the plant remains infected with the virus.
One effect of delayed maturity is green stem disorder. This is where the stem remains green after the soybean pods have matured. Infection by BPMV decreases pod formation and reduces seed size, weight and number. Seed coat mottling (the discoloration of the seed due to a black or brown pigmentation bleeding from the hilum) is another symptom caused by this virus. Grain with discolored seeds may be docked at the time of sale. BPMV is also associated with increases in seed infection by Phomopsis spp.
Disease Management
Once the plants have become infected there are no in-season BPMV management recommendations. If only a few plants appear symptomatic, they can be removed to slow the spread of the virus, however this will not stop the spread of the virus. It simply reduces the highly symptomatic inoculum for beetles to be acquiring the virus. Therefore, proactive management practices such as planting tolerant varieties, using insecticide seed treatments to manage the overwintering population if in an area where winter bean leaf beetle mortality is low are recommended. In-season insecticide spraying for bean leaf beetles will not control the virus.
