Black grass bugs reported in Montana

According to a MSU Extension Ag Alert, damaging numbers of black grass bugs were reported in range and pasture locations in Teton and Big Horn Counties. Black grass bugs were also reported in Fallon, Powder River and Prairie Counties. Black grass bugs are plant sap feeders that target many introduced range grasses, including crested wheatgrass, intermediate wheatgrass and orchardgrass.

Black grass bugs are dark insects, often with buff-colored markings on the head and the body margin. They are less than a quarter inch long. Both adults and immatures have wide heads and prominent eyes that are expanded outward from the side of the head.

Black grass bugs feed through straw-like mouthparts on plant sap. Feeding causes dry, whitish stipples and streaks that look like chatter marks on the leaf blades. Heavy feeding reduces yield, plant height, seedhead production and forage protein. Damage first appears early in the growing season, resulting in delayed green-up. Affected acreage appears gray and dried out with stunted plants.

Monocultures of host grasses are more susceptible to damage during black grass bug outbreaks, but CRP mixes are also very vulnerable. Black grass bugs are not usually a problem in cultivated small grains like spring or winter wheat, although they can affect the edges of small grains fields that are planted adjacent to infested pasture or range. However, in Teton County, one grower is currently seeing broad distribution of black grass bugs throughout his winter wheat crop, most likely a result of rotating out of infested CRP late last fall.

Black grass bugs are an early-season problem, with only one generation per year. Eggs start hatching as soon as grasses begin to grow in May, and numbers should decline sharply by the end of June. There are no established economic treatment thresholds for black grass bugs. Summer grazing and late-season burning of dead grasses may suppress egg hatch next spring. Insecticidal control in forage grasses is not always economically feasible, but there are some insecticidal products that are reported to be effective on range forage, including carbaryl, lambda-cyhalothrin and malathion.

For more information on black grass bugs or other plant pests, contact your local MSU Extension Office. If you would like to subscribe to MSU Extension’s Ag Alerts, visit: https://mtagalert.org/.