Issue No. 14, Article 8/June 24, 2005
Grasshopper Nymphs Moving into Field Edges
Randy McElroy, Monsanto Company, reported finding grasshopper nymphs moving into field edges in southeastern Illinois. If the hot and dry weather continues to hold, we can expect grasshoppers to become a problem in some areas. Grasshoppers begin to move into field margins of corn and soybeans when weedy hosts begin to dry up. As hot, dry weather stresses crops, feeding by the grasshoppers intensifies the stress. Also hot, dry conditions are not conducive to the development and spread of a fungal organism that often suppresses grasshopper populations.
The two most common grasshoppers in Illinois cornfields are the differential (Melanoplus differentialis) and redlegged (Melanoplus femurrubrum) grasshoppers. The differential grasshopper is a robust, olive-green to brown grasshopper that reaches 1-3/4 inches in length. The underside of the body is yellow, with complete black chevrons (V-shaped "sergeant's stripes") on the femurs of the yellow hind legs. The redlegged grasshopper is red-brown and smaller (less than 1 inch in length) than the differential grasshopper. The hind legs are red with black spines.
Differential grasshopper adult (University of Illinois).
Redlegged grasshopper adult (University of Illinois).
In late summer and fall, females of both species deposit clusters of eggs in undisturbed soil (noncultivated fields, roadsides, waterways, pastures). A female grasshopper can deposit an average of 200 to 300 eggs in her lifetime. Soil particles adhere to the frothy secretion that covers the eggs, forming a soil-covered egg pod. Both differential and redlegged grasshoppers overwinter as eggs in pods in the soil.
Nymphs begin hatching from eggs in May and June in Illinois. First instars are very small (~ 1/8 inch). Both species have five instars before they become adults. With adequate food and warm, dry weather, nymphal development requires 35 to 50 days. Both differential and redlegged grasshoppers complete only one generation per year.
Grasshoppers defoliate both corn and soybeans by tearing off and consuming plant tissue with their mandibles, giving plants a ragged appearance. Defoliation can be extensive if densities of grasshoppers are large. Later in the season, grasshoppers also may feed on corn ears and soybean pods. Because grasshopper nymphs feed and develop first in noncrop areas, watch noncrop areas near field margins during June and July.
Defoliation of corn plant by grasshoppers (University of Illinois).
While nymphs are feeding in noncrop areas, there is a window of opportunity for management. Nymphs are less mobile than adult grasshoppers because they lack functional wings. As vegetation in uncultivated areas bordering crops is mowed or dries out, nymphs begin to move into adjacent rows of corn, soybeans, or other field crops. If populations of nymphs average 15 to 20 per square yard in noncrop areas bordering a crop field, an insecticide labeled for use in these sites (see Table 1) can be considered.
Another management strategy is to wait until grasshopper numbers and levels of injury exceed established thresholds in the crop field. In soybeans, control might be warranted when defoliation reaches 30% before bloom or 20% between bloom and pod fill. In corn, seven or more grasshoppers per square yard represent a potentially damaging population. In alfalfa, 15 to 20 grasshoppers per square yard might cause economic damage. Nymphs often succumb to fungal and bacterial diseases during periods of warm, humid weather. This wait-and-see strategy allows time for diseases to suppress populations before a decision is made to apply an insecticide.--Kelly Cook