Notes From Nelson

It seems that every few years we see varying degrees of plant damage appearing in patches in crested wheatgrass, and this is one of them years. The insect causing the damage is called the “Black Grass Bug”.

The black grass bug can cause considerable damage to wheatgrasses and in some cases wheat. The primary host of the black grass bug in crested wheatgrass. As nymphs develop they will move out of crested wheatgrass and into adjoining areas of other wheatgrasses or wheat. Wheat fields are seldom severely infected except for borders that adjoin crested wheatgrass ditches or CRP acres with wheatgrass in it.

Black grass bugs have piercing, sucking mouth parts that they use to feed on plant juices. Feeding causes yellow to white, irregular spot damage and causes the foliage to dry as the plant juices are removed.

Damage may be quite severe, particularly in dry years. Grass may appear straw colored and upon closer examination you may notice yellow spots on the leaf blades from feeding. The insects may occur in populations above 100 per square foot.

The life cycle of black grass bugs allows several management strategies to be effective. The winter is spent as an egg laid in the stems of host grasses. The eggs hatch in the spring when the grasses begin to grow. One generation of black grass bugs is produced each year. Nymphs feed and molt five times before becoming adults. It usually takes four to five weeks from egg hatch to the appearance of adult insects. Bug numbers begin to decline about four weeks after adults appear as they lay eggs and then die. The time span of the population is regulated by temperature, with adults maturing more quickly at higher temperatures. Most females in a population do not have fully developed wings, which means they do not readily disperse to and colonize new areas.

It is important to identify an infestation of black grass bugs early in the spring so chemical control measures may be applied before extensive damage is done or egg laying begins. Insecticides will kill the adult bugs but will have no effect on eggs laid in grass stems. If enough eggs are present, a re-infestation may occur in the next growing season. One well timed application of insecticide may reduce bug numbers to a non-economic level for many years.

Timely grazing or burning dead grasses in the fall are management techniques that destroy black grass bug eggs and may limit the need for chemical control. Graze wheatgrasses heavily for a short time in the late fall or early spring of each year. Wheatgrass pastures that are not fully utilized by grazing or haying provide egg laying sites, winter protection, and habitat that favors black grass bug survival. Mowing and removal of hay may be a management technique for reducing the number of eggs in some circumstances.

Spraying at this point in the season may, for the most part, not be the best use of time and resources – the adult bugs are nearing the end of their life cycle and most of their eggs have already been laid.