Written collaboratively by Adam Varenhorst, Patrick Wagner, Philip Rozeboom, Ruth Beck, and Aaron Hargens.
Grasshoppers continue to be an issue in some areas of South Dakota. The hotspots seem to be along the Missouri River, but it is a reminder that everyone should be monitoring their crops for grasshopper feeding. This week, we observed very large grasshopper populations southwest of Pierre in a sunflower research plot. A week ago, the plots were fine, but now they are overrun with grasshoppers and most of the leaves have been removed (Figure 1).
Grasshopper populations are likely increased in these areas due to the abundant 2019 food supply, which allowed grasshopper populations to increase. In addition, many areas of South Dakota had 2019 fall conditions and 2020 spring conditions that were conducive to grasshopper population increases.
The best way to scout for grasshoppers in sunflower is to visually estimate the number of grasshoppers present in one square yard and repeat the process for several areas of the field. Field margins can be scouted using a sweep net. For the field, 30-45 nymphs or 8-14 adults warrant an insecticide application to reduce the populations. If grasshoppers are observed feeding on the sunflower heads (Figure 2), populations are likely above threshold. This type of feeding will reduce sunflower yield. When applying insecticides to reduce adult grasshopper populations we recommend using the highest labeled rate for the selected products. For a list of products available for treating sunflower, review the latest South Dakota Pest Management Guide: Alfalfa and Oilseeds.