This month, we received reports of high numbers of black grass bugs showing up in parts of Southwestern South Dakota. However, the bugs were mostly isolated to grassy areas dominated by crested wheatgrass. This is expected, as black grass bugs typically prefer wheatgrasses over other types of grass. The drought conditions experienced throughout South Dakota in recent years have forced many farmers and ranchers to rest some of their pastures. The reduced grazing and haying activity has allowed black grass bug populations to steadily increase during this time. Large populations of black grass bugs can cause considerable damage (up to 90% forage reduction) to pasture and range, making it important to be aware and monitor their populations each spring.
Black grass bugs (Figure 1) are native to South Dakota and the Great Plains region. They are approximately ¼ inch long with most species being uniformly black in color. Black grass bugs overwinter as eggs and hatch out in April and May. Upon hatching, the nymphs immediately begin feeding on the tender new grass and mature over the next 4 to 5 weeks. Once they become adults, they live for several more weeks to mate and lay eggs. Black grass bugs are only active in the spring, having just one generation per year.
Injury from the black grass bug appears as light-colored spots on the leaves (Figure 2). This type of injury is called stippling, which reduces forage quality and can deter livestock grazing. Areas of grassland with severe infestations may appear yellowish brown or white. Black grass bugs primarily feed on grasses, but can also feed on broadleaf plants as well. They prefer wheatgrasses including crested and intermediate. This makes pastures that are composed of wheatgrass monocultures the most susceptible to infestation. Severe infestations may cause black grass bugs to move into nearby fields of small grains, although injury is usually minimal and limited to the field edges.
Currently, there are no established thresholds for managing black grass bugs. However, early detection is recommended because these insects can quickly cause widespread damage. Check grasslands for areas of discoloration, and inspect plants for stippling damage or the presence of black grass bugs. A sweep net may be helpful when scouting, as the bugs usually drop to the ground when disturbed.
Black grass bugs can be managed effectively through proper grazing management, hay removal, burning, or foliar insecticides. Intensively grazing livestock in the spring can help reduce black grass bug feeding injury and the survival of egg-laying females. Furthermore, intensive grazing in the fall will remove stems containing eggs, which results in lower populations the following year. This type of seasonal management can also be achieved through mowing or burning practices. Applying an insecticide, such as those containing the active ingredient malathion, during spring emergence can also provide effective management. However, insecticides should only be used for severe infestations, as their use can potentially impact beneficial insects as well.
- Blodgett, S. L., A. W. Lenssen, and S. D. Cash. 2006. Black grass bug (Hemiptera: Miridae) damage to intermediate wheatgrass forage quality. Journal of Entomological Science. 41: 92-94.
- Hammon, R. W. and F. B. Peairs. 2013. Black grass bugs. Colorado State University Extension.