Utah Pests News Summer 2008

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Black Grass Bug Flares in Utah Grasses 

  Black grass bug feeding results in chlorotic blotches along the leaf blade.
This spring, we’ve had several calls about black grass bugs in rangelands.  Although black grass bugs are native to North America, they only started causing significant damage to western rangeland when wheatgrass planting started in the 1930s.  Insecticidal treatments are usually not warranted because damage typically occurs to field margins adjacent to pasture or rangeland.  However, there have been severe populations of black grass bugs in Utah’s Box Elder, Sanpete, and Beaver counties this year.

Several different species of black grass bugs occur in Utah.  The most common is Irbisia pacifica, and the second most common is Labops hesperius.  As with all true bugs, they have piercing-sucking mouthparts and go through simple metamorphosis with an egg, nymph, and adult stage. (In simple metamorphosis, young insects look like adults, but without wings.)  Adults range in size (1/6-1/3-inch long) and have dark bodies.  Black grass bugs appear to have bulging eyes on the sides of their head (but should not be confused with the beneficial insect, minute pirate bug, which also has bulging eyes).  Some black grass bugs also have dark “kneecaps.”  The forewings are dark and relatively slender compared to other true bugs.  In some cases the fully developed female has shortened or reduced wings and readily disperses to new areas.

Black grass bugs have a wide host range and will feed on a variety of range grasses (e.g., wheatgrass, brome grass, orchardgrass and bluegrass), and field crops (e.g., barley, wheat, rye, oats).  In general, Great Basin wildrye and wheatgrasses are preferred, especially blue bunch wheatgrass, crested wheatgrass and intermediate wheatgrass.  Nymphs and adults use a piercing stylet to feed on the mesophyll cells within leaf blades.  Black grass bugs damage the chloroplasts (where photosynthesis occurs) while feeding; white spots and chlorotic blotches often appear near feeding sites.  Normally, they begin feeding at the leaf tip and progress down the leaf.  Heavily infested plants will be covered in black frass, or excrement, and look straw colored or frosted.

The life cycle of black grass bugs allows for several management options.  All species have one generation per year and overwinter as eggs protected in grass stems.  Egg hatch begins in late April, depending on the temperature and elevation.  Young grass bugs (nymphs) feed for about 4-5 weeks and go through five developmental stages (instars) before becoming adults.  Adults feed and mate for another 4 weeks. Identifying early infestations before extensive damage occurs is important.  Treating nymphs before they become adults can provide long term control.  Timely grazing and burning in the fall and spring can also greatly reduce egg hatch the following growing season.

For more information, go to the UTAH PESTS Web site and search for the black grass bugs fact sheet at www.utahpests.usu.edu/insects/htm/factsheets.

-Erin Hodgson, Extension Entomologist (No longer at USU)