Figure 1. Soybean trifoliate after grasshopper feeding. Courtesy: C. Dierks
We have been monitoring grasshopper populations throughout much of Eastern South Dakota. This week, we noticed large populations in the Southeast part of the state, which may be of some concern. Specifically, populations of grasshoppers that exceed the economic threshold were observed near Beresford, SD. While standing in a soybean field, more than 40 grasshoppers could be seen moving within a square yard. There were so many grasshoppers in the area that entire soybean plants were swaying from them jumping from plant to plant.
Although the observed plant defoliation isn’t at or above the recommended 20% threshold yet, it may be only a matter of time. This is due to the dense grasshopper populations as well as the other defoliating insects that are being found in soybean. Some of the individual trifoliates that we observed within the field were at 40-50% defoliation (Figure 1), but the plant average was still 5-10% defoliation. Because the populations exceeded the grasshopper per square yard threshold, our recommendation was to treat the field to prevent additional defoliation from occurring.
The grasshopper population in the field appeared to be made up of about 3 grasshopper species; the differential grasshopper (Figure 4), redlegged grasshopper (Figure 3), and possibly the migratory grasshopper (Figure 2). It is difficult to tell the redlegged and migratory species apart in the field, as they are very similar in size and coloration. Both species tend to be smaller and have yellow-brown bodies with red hind legs. Differential grasshoppers are easily identified due to their large size and green color with dark chevrons on their hind legs.
Fig. 2. Migratory grasshopper adult. Credit: A. Varenhorst
Fig. 3. Redlegged grasshopper adult. Credit: A. Varenhorst
Fig. 4. Differential grasshopper adult. Credit: A. Varenhorst
When evaluating defoliation, remember that the entire plant must be considered and not just a few trifoliates. In addition, if other defoliating insects are present, there is the potential for defoliation to reach and exceed the threshold much faster (Figure 5). For a list of foliar insecticides that are labeled for grasshopper management please refer to the current edition of the South Dakota Pest Management Guide: Soybean.
Figure 5. Soybean defoliation chart.
Developer: M. E. Rice