It's been some time since we have experienced heavy infestations of grasshoppers in Illinois. However, the numbers of grasshopper nymphs being found around the edges of crop fields suggest that we need to pay attention to these avid herbivores this year. Kevin Black, with Growmark, reported that the numbers of grasshopper nymphs are high throughout southern Illinois. Mark Hoard, Extension IPM educator in Mt. Vernon, has received sporadic reports of large numbers of grasshoppers. During a field trip to Cass County on June 21, Kevin Steffey, Extension entomologist at the University of Illinois, observed a significant number of grasshopper nymphs along the road next to a cornfield. There was no sign of grasshoppers having fed on the corn yet, but the numbers of grasshoppers were impressive.|
Grasshopper nymphs on a culvert near a field margin.
If the weather continues to be hot and dry, we can expect grasshoppers to become problematic in some areas. As the weedy hosts on which grasshopper nymphs are feeding dry up, the grasshoppers will begin moving into field edges and feeding on corn or soybean plants. If hot, dry weather stresses the crops, feeding by grasshoppers will exacerbate the stress. Also, hot, dry weather is not conducive to the development and spread of a fungal organism that often suppresses grasshopper populations.
Following is information about grasshoppers and their management in Illinois. For more information (and photos) regarding grasshoppers, go to our IPM Web site (http://www.ipm.uiuc.edu), click on "Publications," click on "Books & Reports," and then click on "Corn Insect Pests." Scroll down to grasshoppers under "Knee-high to tasseling corn (V8 to VT)."
Description. The two most common grasshoppers in Illinois cornfields are the differential (Melanoplus different-ialis) 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.
Redlegged grasshopper adult.
Life history. 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.
Injury. Grasshoppers defoliate both corn and soybeans by tearing off and consuming plant tissue with their mandibles, giving the plant 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.
Old photographs showing extreme defoliation to corn caused by grasshoppers.
Scouting and management guidelines. Because grasshopper nymphs feed and develop first in noncrop areas, watch noncrop areas near field margins during June and July. 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 within 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 represents 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.
We'll provide suggested insecticides for control of grasshoppers in alfalfa, corn, and soybeans in next week's issue of the Bulletin.--Kevin Steffey and Mark Hoard