As summer progresses, the number of adult grasshoppers observed in South Dakota rangeland typically increases (Figure 1). Based on current reports, it would seem that 2016 is following this trend. If grasshopper populations reach high enough densities they can be very destructive to rangeland. For this reason, it is important to monitor grasshopper populations so management actions can be taken before economic damage occurs.
Figure 1. Differential grasshopper adult. Photo by Adam Varenhorst.
Estimating grasshopper density
A simple procedure for monitoring grasshopper densities is the square-foot method used by the United States Department of Agriculture (USDA). This method involves counting grasshoppers in 18 different one square foot areas in the target location. In a pasture, randomly select a point that is several feet from the fenceline. Visualize a one square foot area around the point, and count the number of grasshoppers jumping out of the area while slowly walking towards it. When finished counting, select a new sampling point that is about 50 feet away from the previous one. Repeat this procedure 18 times in a curved pattern that goes away from the fenceline and then circles back again (Figure 2). Add up the total number of grasshoppers observed across all 18 samples and divide this number by 2. The resulting value estimates the number of grasshoppers per square yard.
Figure 2. An example of a sampling pattern for grasshopper scouting using the square-foot method. The dashed line represents the path to be traveled, while the squares represent areas that will be scouted for grasshoppers.
Diagram by Patrick Wagner.
Management thresholds for grasshoppers on rangeland
The threshold for grasshopper densities varies depending on climate conditions, the cost of insecticide treatment, and the value of the area being affected. In general, the base economic threshold is approximately 20 nymphs per square yard, or approximately 10 adults per square yard.
Once thresholds are reached, management action should be taken to avoid extensive damage to the infested pastures. There are several insecticides listed for grasshopper control, including carbaryl, malathion, and diflubenzuron. Diflubenzuron is a growth regulator that is only effective on second to third instar grasshopper nymphs and does not control adults. If pastures are already severely impacted by grasshoppers, exclude livestock for at least one year to allow the vegetation to rest and recover.