Grasshoppers jump on soybean

An abundance of grasshoppers is being reported by extension field specialists in crops throughout most of the state except northeast Iowa. Grasshopper nymphs range from newly-hatched up to 3/8-inch long. All reports so far originate in soybean fields.

There are no good economic thresholds for grasshoppers in either soybean or corn. Old thresholds recommended that you count the number of grasshoppers per square yard, but I dislike this method. Counting grasshoppers in either corn or soybean is almost impossible because they are either partially hidden in the foliage or they constantly hop out of the area you're trying to count. I suggest that you focus on the amount of leaf defoliation, and use a nominal threshold (based on experience) and common sense about managing grasshoppers. It usually is not too difficult to determine if grasshoppers are abundant, so forget trying to count the number per square yard.

Grasshopper nymphs are common in soybean fields.
Grasshopper nymphs defoliating soybean leaves.

In soybean, consider treatment if grasshoppers are present and defoliation reaches 40 percent in the vegetative stages or 20 percent in the pod-forming and pod-filling stages. Determine the exact location of grasshoppers in the field and spray only those areas. Grasshoppers often are 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. Reductions in yield can occur during any crop stage, although 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 3­7 percent yield reduction. Defoliation of 20 percent during the pod-forming and pod-filling stages will result in similar yield reductions.

Grasshoppers on pretassel-stage corn.

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 are clipping silks or ear tips, or are removing large amounts of foliage above the ear leaf. Grasshopper problems I've seen in corn usually begin on border rows, then move deeper into the field. Determine how many rows are infested and spray only those rows.

In all crops, remember that grasshopper nymphs eventually will become adults and cause more leaf loss during late July, August, and September but they should not be sprayed until injury approaches a level that could cause economic yield loss. This may not occur until 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 hopper populations and economic damage may never occur in the field.

Recommended insecticides and product per acre for grasshopper control in corn and soybean.

Product Rate per acre
Asana XL 0.66EC* 5.8-9.6 ounces
Cygon 400 1 pint
Furadan 4F* 0.25-0.5 pint
Lorsban 4E 0.5-1 pint
Penncap-M* 2-3 pints
Sevin XLR Plus 2-3 pints
Warrior 1EC* 2.56-3.84 ounces (corn)
3.20-3.84 ounces (soybean)

*Restricted-use insecticide

This article originally appeared on pages 127-128 of the IC-478(16) -- July 7, 1997 issue.

Updated 07/06/1997 - 1:00pm