Grasshopper populations develop during dry springs following long, warm autumns. Under moderate or high moisture, fungal diseases normally keep grasshopper populations in check. Grasshoppers tend to prefer to lay their eggs in untilled soil, such as roadsides and ditches. Damage, therefore, will likely first occur at the margin of fields. An exception is soybeans planted in last years soybean or alfalfa fields; certain grasshopper species will lay eggs in both cropping systems. Grasshopper nymphs look very much like adults, but lack fully developed wings. Grasshoppers feed on leaves and, as soybeans mature, on developing pods.
Scouting for grasshoppers should start early in the growing season (late April, early May), because early detection is often instrumental in control. Scouting should start at field edges, fence rows, dirt roads, and ditches. Consider field-edge applications unless grasshoppers occur throughout the field. Thresholds can be based on either grasshopper numbers or soybean defoliation. Thresholds based on grasshopper populations can be estimated by scouting the field (see Scouting) and treatment decisions made according to Table 1. Thresholds based on defoliation include treating when defoliation inside the field exceeds 30% prebloom, or 20% blooming-to-pod-fill. (Be aware that certain species of grasshoppers will lay eggs in soybeans and alfalfa.)
Table 4. Summer phenology of 3 major cropland grasshoppers
in Minnesota
May June July August
Grasshopper early/lmid/late early/mid/late early/mid/late early/mid/late
Twostriped (adult) (nymph)
Redlegged (nymph) (adult)
Differential (nymph) (adult)
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