Black Grass Bugs
Black grass bugs are an occasional pest of grasslands in South Dakota. They are native to the Great Plains and typically occur in low numbers. However, black grass bug populations can build over time, especially in areas where wheatgrasses are dominant. Large populations of black grass bugs can cause considerable damage (up to 90% forage reduction) to range and pastures. Although we have not received any reports yet this year, it is important to be aware of black grass bugs and to monitor their populations each spring.
Identification & Behavior
There are numerous species of black grass bug in North America. The two most common species are Labops hesperius and Irbisia brachycera. Both species grow to about ¼ inch long, but can easily be distinguished based on their coloration. Irbisia brachycera has a uniformly black body (Figure 1A), and L. hesperius has a black body with tan margins along the outside of the wings (Figure 1B). Note that the wings of the adult black grass bugs are not fully functional, which limits their ability to disperse.
Black grass bugs first appear in early spring (April through May). Eggs that have overwintered in the grass stems will hatch out as soon as the grass starts growing. The immatures, or nymphs, begin feeding and develop over the next 4 to 5 weeks. Adults live for another 4 weeks as they mate and lay eggs that stay dormant until the following spring. There is only one generation of black grass bugs per year.
Injury from the black grass bug appears as white spots on the leaves (Figure 2). This type of injury is called stippling, which reduces forage quality and can deter livestock grazing. Areas with severe infestations may appear 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. The damage caused by black grass bugs is magnified during drought years.
Currently, there are no established thresholds for managing black grass bugs. However, early identification is recommended because these insects are only active in the spring. Check grasslands for patches of discoloration, and inspect plants for stippling damage and the presence of black grass bugs. A sweep net may be helpful for catching the bugs, as they tend to drop off the plant 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. In case of severe infestations, a well-timed insecticide application in the spring can provide black grass bug management for many years. However, insecticides should only be used for severe infestations as their use can potentially reduce beneficial insects as well.
Reference: Hammon, R. W. and F. B. Peairs. 2013. Black Grass Bugs. Colorado State University Extension.