Scout soybeans for grasshoppers

Jim Jensen, extension field specialist in crops, reports finding grasshopper nymphs in soybean fields in southeastern Iowa. I also have seen them in central Iowa. No economic damage has been reported, but their appearance in soybeans should serve as an early warning to scout soybean fields during the next couple of weeks.

Grasshopper nymphs defoliating soybean leaves.

There are no good economic thresholds for grasshoppers in either soybeans or corn. Old thresholds recommended that you count the number of grasshoppers per square yard, but I have always found this to be an exercise in frustration. Counting grasshoppers in either corn or soybeans is a near impossibility because they hide in the foliage or they hop out of the area you're trying to count. I suggest that you focus on the amount of leaf defoliation, combined with a nominal threshold (one based on experience), and mix this information with a little common sense in managing grasshoppers. It is usually not too difficult to determine if grasshoppers are abundant so forget trying to count the number per square yard.

In soybeans, 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 out 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.

Grasshopper nymphs are common in soybean fields.

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 plant height.

Grasshoppers on pretassel-stage corn.

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 until the injury approaches a level that could cause economic yield loss. This may not occur until the nymphs become adults. Fortunately, some of the insecticides provide excellent control of adult grasshoppers. Another consideration before spraying is that a naturally occurring fungus can reduce grasshopper populations and economic damage may never occur in the field.

This article originally appeared on pages 123-124 of the IC-482(16) -- June 28, 1999 issue.

Updated 06/27/1999 - 1:00pm