Figure 1. Bean leaf beetle defoliation. Courtesy: Adam Varenhorst.
With soybean starting to emerge throughout South Dakota, one of the early season pests to scout for is the overwintering population of bean leaf beetles. We have started getting reports of bean leaf beetles causing defoliation near Mitchell. Although a previous article estimated that bean leaf beetle survival in South Dakota was likely close to zero due to cold air temperatures, there is always the possibility that suitable cover prevented the high levels of mortality.
The question we are often asked is “What is the threshold for early season bean leaf feeding?” For bean leaf beetles, we recommend the defoliation threshold of 20% prior to flowering. However, this can be tricky to estimate when soybean are just starting to emerge. An alternative method is to count the number of adult bean leaf beetles per plant.
To scout for early season bean leaf beetles, examine 20 plants in five different areas of the field. If you are using the defoliation threshold, look for holes in the leaves as well as feeding on the stems or cotyledons (Figure 1). Management is recommended when an average of 20% of the leaf tissue is removed from the scouted plants (Figure 2).
Figure 2. Bean leaf beetle defoliation chart. Courtesy: Robert Koch, University of Minnesota.
If you are scouting by using visual counts of bean leaf beetle adults, look at 20 plants from five different areas throughout the field. When counting adults, it is important to avoid disturbing the plants as bean leaf beetles will often drop to the ground. This includes casting a shadow on the plants that are being scouted. The adult bean leaf beetles can vary in color from brown, orange, yellow, red and varying shades of those colors. Adults can be identified by the black triangle present behind their thorax (Figure 3). Adults will also have black heads and will generally have four spots on their hardened forewings (elytra). The thresholds for VC to V1 soybean are listed in Table 1.
Figure 3. Bean leaf beetle adult. Courtesy: Adam Varenhorst.
Table 1. Economic threshold for management of bean leaf beetle adults based on adult counts.
|*Adapted from University of Nebraska-Lincoln|