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UW Extension Education

by Wyoming Livestock Roundup

By Amy Smith, UW Extension Educator

What are Those Pesky Grass Bugs?

Due to the above normal amount of moisture Wyoming received this spring, many may have noticed wheatgrass varieties in and around the pastures turning a blotchy yellow and sometimes straw color.  

This is due to a little insect known as the black grass bug, of which there are several species within the U.S. 


Considered a native of Western grassland, the adult black grass bug (Labops hesperius Uhler) is approximately one-quarter of an inch long and mostly black in color with straw coloration along the outside edge of the wing covers. 

Immatures are smaller in size with similar coloration.  

In native rangelands, these bugs have historically been found in relatively low populations and do not appear to affect overall growth of native grasses. 

Plant damage was first noticed around 1938, when an initiative began to reseed damaged rangeland with introduced wheatgrasses after prolonged drought, and the Dust Bowl had caused a degradation of native rangelands.  

Large plots of wheatgrass monocultures provided suitable habitat for the black grass bug leading to a population explosion.  


The black grass bug has a simple lifecycle with only one generation per year.  

The eggs are laid inside dried grass stems for protection over the winter and hatch in the spring when the grass begins to come out of dormancy. Newly hatched nymphs feed at night and drop to the ground during the day or when disturbed.  

As the nymphs mature, they will feed during the day with adults. Nymphs feed and molt five times before becoming mature adults. The process from egg hatch to mature adults is completed within four to five weeks.  

Adult bugs are active for roughly five to six weeks. Females, however, begin laying eggs in dry stems approximately two weeks after adult emergence.  

Temperature and length of daylight

All bug activity is dependent on temperature and length of daylight after sunset. Black grass bug feeding has been observed when ambient air temperatures are 22 degrees Fahrenheit and the temperature within the microclimate found at the crown of the grasses is 45 degrees Fahrenheit.  

Although black grass bugs will feed on a variety of grasses, introduced wheatgrasses are preferred when available, specifically crested wheatgrass and intermediate wheatgrass. 

There is usually a delay of four to seven years between reseeding and an infestation of black grass bugs, indicating the infestation is caused by a population explosion of bugs present, rather than dispersal from neighboring populations. 

Low winter temperatures may reduce the number of overwintering eggs, while snowpack may insulate and protect overwintering eggs.  

Pest infestations 

Black grass bugs cause damage to plants by piercing the leaves and sucking out the sugary substance found within leaf cells. The bugs typically feed on the upper surface of the leaf, beginning at the tip of the leaves and moving towards the leaf base. 

Once the insects start feeding, whitish spots appear because the bugs are removing chlorophyll from the leaves. This results in heavily infested patches looking frosted, yellowish, or in extreme cases, straw colored.  

Heavy infestations may prevent seed formation and reduce palatability for cattle.  Damaged plants will most likely recover with adequate moisture, although drouthy conditions and severe outbreaks may lead to some plant mortality.

While research based economic thresholds have not been determined, infestations have been observed from 100 black grass bugs per square foot to over 1,000 bugs per plant. 

Control methods

An integrated approach to strategically control black grass bugs include cultural controls such as heavy early spring or late fall grazing by multiple animals, when possible, to utilize the nutrients found in the young plants before it is removed by the insects.  

Mowing or control burning dead grass may also reduce egg hatch the following year. 

The use of chemicals to control persistent infestations based on the life stage of black grass bugs is a viable option as well.  An insecticidal treatment is most effective during nymphal stages or before adult females lay eggs.  

Additionally, when remediating a pasture, consider using a variety of grass species, which will also reduce the likelihood of a black grass bug infestation.  

It is important to keep in mind black grass bugs may not need to be controlled every year, as their populations fluctuate depending on winter temperatures, snowpack, amounts of moisture, spring temperatures and any number of other events which disrupts their lifecycle.  

The key to effective control of the black grass bug, as with any pest, is correct identification, awareness of the lifecycle and early detection.  

Amy Smith is an agriculture and natural resources educator with University of Wyoming Goshen County Extension. She can be reached at or 307-532-2436.

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