Editor’s note: The following was written by Erin Hodgson, Iowa State University associate professor, for the Integrated Crop Management blog July 6.
Although some parts of the Midwest have been catching up with moisture deficits, many areas continue to be in severe drought. Drought stress combined with high temperatures is good news for field crop pests like grasshoppers and spider mites.
We’ve been getting reports of high grasshopper numbers around the state this year. If your area is hot and dry, consider scouting fields now and throughout August.
Usually I hear about grasshopper injury later in the summer (and typically restricted to field edges). But this is the second summer in a row people are noticing them inside crop fields in June.
Grasshoppers chew through green soybean pods (which bean leaf beetles will not do) and destroy the seeds within. They can also feed on developing corn ears and destroy kernels.
Reducing broadleaf weeds within and around fields will discourage adults from feeding and mating in that area for subsequent growing seasons. Grasshoppers are highly mobile pests and will follow food sources all summer. They are usually perimeter pests, similar to Japanese beetle.
The economic thresholds are based on leaf area consumed or percent defoliation. In soybean, a foliar treatment may be justified if defoliation exceeds 40% before R1 (full bloom) or 20% after R1. Consider an insecticide application in corn if grasshoppers are clipping silks or ear tips, or are removing foliage above the ear leaf.
In my experience, grasshoppers rarely reach field-wide economic concerns in Iowa. Border treatments are recommended if infestations are restricted to field edges.