Todd Vagts, Iowa State University Extension field specialist-crops, reports that young grasshoppers are causing significant defoliation to the edges of a few soybean fields in Crawford County in western Iowa. He notes that the grasshoppers are defoliating the soybean right down to the midrib.

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Grasshopper in jury to corn during 2001 in Crawford County, Iowa.

There are no good economic thresholds for grasshoppers in either soybean or corn. Old data recommended a threshold of 15-20 grasshoppers per square yard, but I have always found that counting grasshoppers was an exercise in futility because they hide in the foliage or they hop out of the area where you are counting. I suggest that you focus on the degree or intensity of leaf defoliation, combined with a nominal threshold of grasshopper numbers (one based on experience), and combine this information with a little common sense in managing grasshoppers. It is usually not too difficult to determine whether grasshoppers are abundant, so forget trying to count the number per square yard.

In soybean, determine the exact location of grasshoppers in the field and spray only those areas. Grasshoppers are often concentrated along field edges or waterways, but they sometimes occur in large areas in the center of the field, especially if weeds were present last year. Also, soybean fields that are sprayed with herbicides can make a grasshopper situation worse because the insects move from the dead weeds to the soybean plants, so these areas should be closely monitored. Consider treatment if grasshoppers are present and defoliation reaches 40 percent in the preblooming stages or 20 percent in the pod-forming and pod-filling stages. Reductions in yield can occur during any crop stage and pod-forming and pod-filling stages are at greater risk than other plant stages. A 40 percent leaf loss during any vegetative stage will result in only a 3-7 percent yield reduction. Defoliation of 20 percent during the pod-forming and pod-filling stages will result in similar yield reductions.

In corn, grasshoppers usually are more of a late-summer pest. Injury in corn is more likely to occur beginning in late July. Consider treatment if grasshoppers are present and they are clipping silks, ear tips, or removing large amounts of foliage above the ear leaf. Grasshopper problems in corn usually begin on border rows and then move deeper into the field. Determine how many rows are infested and spray only those rows. Control of grasshoppers in mid-to-late summer may require the services of an aerial applicator because of the crop height.

In all crops, remember that grasshopper nymphs will eventually become adults and cause more leaf loss during late July, August, and September, but they should not be sprayed (Table 1) until the injury approaches a level that could cause economic yield loss. This level may not occur until the nymphs become adults. Fortunately, some insecticides provide excellent control of adult grasshoppers.

Insecticides and product rate per acre for grasshopper control in field corn and soybean. Read and follow all label directions.

*Restricted-use insecticide.