Figure 1. Adult redlegged grasshopper. Note the red hind leg.
Credit: A. Varenhorst
We have recently been observing certain areas of South Dakota that are experiencing high grasshopper population densities. The first area where grasshoppers were observed that exceeded the thresholds of 21-40 grasshoppers per square yard in field margins and 8-14 grasshoppers per square yard within the fields was near Fort Thompson. The second area where thresholds are being reached was near Beresford. Although these populations were not being managed at the time, it is possible that management will be necessary.
Much of South Dakota is experiencing drought. These conditions cause grass in field margins and other areas to become less attractive to grasshoppers, which causes them to search for alternative food sources. In turn, grasshopper populations often show up in the edges of fields where they can cause severe defoliation to crops. Dry conditions also favor grasshoppers because the naturally occurring pathogens that reduce their populations often require warm and humid conditions.
Figure 2. Adult differential grasshopper. Note the black chevron markings on the hind leg. Credit: A. Varenhorst
Scouting & Management
Grasshoppers can be scouted using visual searches for the insects or their defoliation, or sweep nets. Visual searches can prove difficult as grasshoppers move rapidly within crops. The same grasshoppers may be recounted during the search, leading to inaccurate counts. Another method of scouting in field crops is by estimating the percentage of defoliation on leaves. Keep in mind that defoliation thresholds are crop specific. The easiest scouting method is to use a sweep net. For this method, three pendulum sweeps should be conducted in five areas within the field, and the number of grasshoppers present in each set of swings should be determined. For information on grasshopper management in crops, please refer to the current editions of the South Dakota Pest Management Guides for Soybean and Corn.