Although some parts of Iowa 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.
Pests, like grasshoppers and spider mites, move into crops when surrounding the vegetation is reduced by mowing or drought stress. 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. There are two common grasshoppers in Iowa: the differential grasshopper and the redlegged grasshopper.
Grasshopper nymphs create irregular-shaped holes, leaving a ragged appearance. Older nymphs and adults can eat more plant tissue but often the tougher veins remain. 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 acceptable food sources all summer. They are usually perimeter pests, similar to Japanese beetle, and estimating field-wide populations can be difficult.
The economic thresholds are based on leaf area consumed or percent defoliation. In soybean, a foliar treatment may be justified if defoliation exceeds 40 percent before R1 (full bloom) or 20 percent 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.