Black Grass Bugs May Cause Issues if Drought Persists

 BY Patrick Wagner

In western South Dakota, black grass bugs are a common spring pest of forage grasses. This native insect first appears in late-April to early-May, when grasses begin to emerge from dormancy. Black grass bugs feed on fresh green growth, which can result in stunted plants and decreased forage quality. With adequate moisture, most grasses can tolerate this feeding damage. However, drought stress reduces the plants’ ability to recover and can make the impact from black grass bugs more severe. Monitoring and potential management of this pest is of increased importance since South Dakota currently ranges from moderate to extreme drought conditions (Figure 1).

Figure 1. U.S. Drought Monitor for South Dakota. Released Thursday, April 28, 2022. Courtesy: U.S. Drought Monitor

Profile

Black grass bugs are small bugs that are either uniformly black or black with tan margins along their sides (Figure 2). They are approximately ¼ of an inch long and have large eyes that protrude from the sides of their head. Black grass bugs only have one generation per year. They overwinter as eggs and hatch out as soon as grass begins to green up in the spring. Upon hatching, nymphs feed on tender new grass and mature over the next 4 to 5 weeks. As adults, they live for several more weeks to mate and lay eggs for the following year.

Feeding damage appears as light-colored spots on the leaves, called stippling (Figure 3). Black grass bugs primarily feed on grasses but can eat broadleaf plants as well. They prefer wheatgrasses, including crested and intermediate. Road ditches or pastures composed of wheatgrass monocultures are most susceptible to infestation. Severe infestations may lead to black grass bugs migrating into nearby wheat fields, although injury is often limited to the field edges.

Scouting and Management

There are no established thresholds for black grass bug management. However, early detection is recommended, as large populations can quickly cause widespread damage. Check pastures and wheat fields for areas of discoloration. Closer inspection of the plants can reveal characteristic stippling damage and black grass bug nymphs or adults. A sweep net is 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. For pasture and range situations, 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. In areas that have been heavily impacted by black grass bugs during the previous year, applying an insecticide, such as Malathion, during spring emergence can be very effective. It is not necessary to manage black grass bugs every year, as it takes several years for their populations to build up.