Watch for Grasshopper Population Outbreaks
By : Adam J. Varenhorst, Assistant Professor & SDSU Extension Field Crop Entomologist, SDSU Agronomy, Horticulture and Plant Science Department, Courtesy of iGrow.org. Written collaboratively by Adam Varenhorst, Amanda Bachmann, Philip Rozeboom, Patrick Wagner, and Erica Anderson.
Our early prediction of grasshopper populations was that there would be areas where outbreaks would be possible during 2018. We are just starting to receive reports of grasshopper populations that are exceeding recommended thresholds throughout central South Dakota as well as areas in northeast South Dakota that haven’t received much rain this year. So far, these populations are reported as being in grass borders as well as pastures. However, as the summer progresses it is possible that these populations may move into adjacent crops.
Although there are over 70 species of grasshoppers present in South Dakota, not all of them reach pest statuses. The grasshoppers that tend to be the most devastating to row crops in South Dakota include the redlegged grasshopper, migratory grasshopper, differential grasshopper, and the twostriped grasshopper. During a 2017 survey, the most abundant grasshopper specie in South Dakota was the redlegged grasshopper, which accounted for 52% of the grasshoppers that were collected in eastern South Dakota and 26% of the grasshoppers that were collected in western South Dakota.
Redlegged grasshoppers are small to medium sized with a body that ranges in length from 0.6-1.1 inches. As their name suggests, redlegged grasshoppers have a red hind tibia. However, the redlegged grasshopper and migratory grasshopper are very similar in appearance. The distinguishing characteristic is a complete black stripe present on the hind femur of the redlegged grasshopper (Figure 1).
Figure 1. Redlegged grasshopper adult. Courtesy: Adam Varenhorst.
Migratory grasshoppers are 0.8-1.1 inches in length. They are very similar in appearance to redlegged grasshoppers. One of the key differences is that they have an incomplete black stripe on their hind femur. Migratory grasshoppers have a hind tibia that range in color from red to light blue (Figure 2).
Differential grasshopper adults are 1.1-1.6 inches in length. They are one of the larger grasshopper species that are observed in South Dakota row crops. These grasshoppers are typically a light green or yellow color and have black chevron markings on their hind femurs (Figure 3).
Figure 3. Differential grasshopper adult. Courtesy: Adam Varenhorst.
The twostriped grasshopper is also a large grasshopper species that is approximately 1-1.6 inches in length. This grasshopper has a dark green or tan body with two distinct white stripes that originate at their head. The lines run the length of the body but converge into one line on the wings (Figure 4).
Figure 4. Twostriped grasshopper adult. Courtesy: Adam Varenhorst.
The easiest way to scout for grasshopper populations is to use a sweep net. Three sweeps with a 15-inch diameter sweep net is approximately one square yard. The threshold for grasshoppers is 8 adults per square yard.