Earlier this year, we observed a large emergence of overwintering bean leaf beetles throughout much of the state. We are again observing quite a few bean leaf beetles beginning to emerge. The first population that emerges in the spring is referred to as the overwintering generation and is made up of adults that survive the winter and feed on soybean plants after they emerge when the temperatures warm up and soybean are present.
The bean leaf beetle adults that we are observing now are the result of those overwintering beetles mating and laying eggs. Thus, the beetles we are seeing are called the first generation. The first-generation adults are typically observed in July and August in South Dakota. A second generation of adults may be observed during the summer from August to the first hard frost or when soybeans senesce. The Northern areas of South Dakota don’t generally have enough degree days to 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 the later fall. But 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 reduced yields.
Identification and Scouting
Of the different insects that you may find in soybean, bean leaf beetles are some of the shyest. This can make scouting and identifying bean leaf beetles difficult in soybean that have canopied.
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 include the black triangle located behind their thorax (segment behind black head capsule) and also the four spots that are present on their hardened forewings (elytra).
The best method for scouting for bean leaf beetles is to use a sweep net and collect 20 pendulum swings from four locations within the field.
The thresholds based on bean leaf beetle populations later in the season is 70-100 beetles per 20 sweeps.
An alternative to scouting populations of bean leaf beetles is to look for the amount of defoliation that is occurring within the field. Due to the potential for multiple species of defoliators that may be present this may be more effective.
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 soybean in South Dakota are pretty variable in growth stage due to this spring, the threshold defoliation before flowering is 30% and the threshold after flowering 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.