Bean Leaf Beetles
In June, we discussed how we were observing quite a few bean leaf beetles in the Southeast Region of the state. Now, as we enter August, we are again observing an uptick in bean leaf beetle numbers. Why might this be happening? It is because bean leaf beetles go through two generations in South Dakota. The overwintering generation, which was initially observed this spring, is actually made up of adults that survived the winter and feed on soybean plants after they emerge. The bean leaf beetle adults that we are seeing now are actually part of the first and possibly second generation that occur each year. The first-generation adults usually show up in July and August in South Dakota. In the same areas where the first generation adults were seen, a second generation is typically observed from August to the first hard frost or when soybeans senesce. The Northern areas of South Dakota don’t have a second generation of bean leaf beetles.
Some of the individuals that 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 available leaf area can lead to reduced levels of photosynthesis and lower yields.
Identification & 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 that 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 70-100 beetles per 20 sweeps. This is based on bean leaf beetle populations later in the season.
Adult bean leaf beetles can vary in color from brown (Figure 1), yellow (Figure 2), and orange to red (Figure 3). The distinguishing characteristics of bean leaf beetles are the black triangle located behind their thorax (segment behind black head capsule) and the four spots that are present on their hardened forewings (elytra).
Fig. 1. Brown bean leaf beetle adult. Credit: A. Varenhorst
Fig. 2. Yellow bean leaf beetle adult. Credit: A. Varenhorst
Fig. 3. Red bean leaf beetle adult. Credit: A. Varenhorst
An alternative to directly scouting for populations of bean leaf beetles is to look for the amount of defoliation occurring within the field. This method may be more effective because of the potential for multiple species of defoliators being present. To scout for defoliation, examine 10 plants from five locations spread throughout the field. For each of the plants, estimate the percentage of leaf area that is removed from all of 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 and above this level of defoliation a 3-7% yield loss may occur. Figure 4 has a visual guide for defoliation caused by bean leaf beetles and the associated percentages.
Figure 4. Bean leaf beetle defoliation guide. After flowering the threshold is an average of 20% throughout the field. Adapted from original by Robert Koch, University of Minnesota Extension.