Written collaboratively by Adam Varenhorst, Philip Rozeboom, Patrick Wagner, and Brad McManus.
Originally Submitted: May 19, 2023
Grasshoppers have the potential to cause defoliation issues throughout much of the growing season in South Dakota. Thanks to recent rains and snowfall during the winter, the drought status in much of South Dakota is less than it was during the spring of 2022 (Figure 1). Although there were areas in South Dakota where grasshopper populations caused issues during 2022, it is possible that grasshoppers won’t be as much of a concern in 2023. However, areas that experienced large grasshopper populations during 2022 may still experience large populations in 2023 that will be capable of causing economic loss in cash crops and reduce forage in range and pasture lands.
In 2022, the U.S. Department of Agriculture Adult Grasshopper Rangeland Survey detected large populations of grasshoppers in northwestern and west central South Dakota (Figure 2).
Although counties in eastern South Dakota are not surveyed by the U.S. Department of Agriculture Animal and Plant Health Inspection Service, large populations were observed in the southeast. With drought conditions still present in this area of the state, it is possible that grasshoppers will again cause defoliation issues.
Grasshopper Identification
In South Dakota, there are three main species of grasshoppers that are being reported in the areas with large grasshopper populations: Two-Striped, Redlegged, and Differential.
Two-Striped
Of the three most-common grasshopper species, the most common is the two-striped grasshoppers (Figure 3). These grasshoppers get their name because of the two light-yellow lines that run on the back of their head to their abdomen, where they converge near the middle of the wings. Two-striped grasshoppers are normally tan to brown in color. They are approximately one inch to one-and-a-half inches long.
Redlegged
Another grasshopper that has been reported is the redlegged grasshopper. These grasshoppers get their name from their red hind leg segment (Figure 4). Their bodies range in color, from green to tan, with black markings, and they are two-thirds of an inch to 1 inch long.
Differential
The other large grasshoppers that are being reported are the differential grasshoppers. These vary in size, from one-and-one-eighth to one-and-a-half inches long. Their bodies are green to yellow in color, and they have black chevron markings on their hind legs (Figure 5).
Scouting and Management Options
The two methods for determining if populations are at or above thresholds are to use visual counts or a sweep net. For visual counts, estimate a square yard in front of you and count the number of grasshoppers jumping out of the area as you slowly walk towards it. For sweep netting, use a 15-inch diameter sweep net and capture grasshoppers in four pendulum swings (approximately one square yard of area) and count them. With either sampling method, we recommend repeating it several times in a pattern, such as the example in Figure 6, and then calculate the average for increased accuracy.
The threshold for grasshopper populations that was established by the U.S. Department of Agriculture for grasslands is 15 to 20 nymphs or 8 to 14 adults per square yard.
For crops, we recommend scouting for nymphs and adults both in field margins and within the fields. For field margins, management should be considered when nymph populations reach 50 to 75 per square yard and adult populations reach 21 to 40 per square yard. In the field, management should be considered when there are 30 to 45 nymphs per square yard or when there are 8 to 14 adults per square yard. If you observe silk feeding, or grasshoppers feeding on ear tips or developing kernels, insecticide management should be considered. For soybeans, if grasshoppers cause 20% defoliation after flowering, or are feeding on developing pods, insecticide management should be considered.
If thresholds are exceeded, an insecticide that is labeled for grasshopper management may be considered. A current list of insecticide sprays for use in soybean, corn and alfalfa can be found in the latest South Dakota Pest Management Guides.
In pastures and range, spreading poison grasshopper baits or baits containing Nosema locusta spores may be used to reduce populations. If treating earlier in the season while grasshoppers are still growing, an insect growth regulator containing diflubenzuron may be applied to inhibit grasshopper development. However, please note that insect growth regulators have no effect on adult grasshoppers and should only be used to manage developing nymphs.
It’s also important to remember that large grasshopper populations can lead to increased populations of blister beetles during the following season. Areas that experienced increased grasshopper pressure in 2022 should be monitored for increased blister beetle activity, especially in alfalfa fields.
