Approximately 35 species of blister beetles occur in Colorado.  While all are thought to develop as predators of grasshopper eggs, most with the exception of the black blister beetle, also feed on leaves and flowers of plants.  Blister beetles common name is derived from the production within their blood of a highly toxic compound called cantharidin.  The compound is capable of causing irritation and even blisters in high concentrations.  The level of toxicity varies with the species.  Black blister beetles produce a very low level of cantharidin while the threestriped blister beetle has very high levels in its blood.  The threestriped blister beetle has the potential for harming livestock, especially horses, when accidentally baled into hay which is then provided to animals as feed.  Being known for periodically massing in blooming alfalfa fields, the threestriped blister beetle is most commonly a problem in contaminating the first cutting of alfalfa early in the season.

Black Blister Beetles –

The black blister beetle is common and has statewide distribution in Colorado.  Nearly uniform in their black color, the black blister beetle varies in size from 9-12 mm.  The body is straight sided with slight bulging at the abdomen.  Wing covers are soft, as is typical of blister beetles.  Adult blister beetles lay their eggs in soil that is utilized by grasshoppers as egg beds.  Hatching larvae burrow into the soil to seek out the grasshopper eggs on which they will feed.  Multiple molts are required prior to reaching the sixth instar in which the grub-like larvae becomes dormant in order to overwinter.  Development resumes in the spring with pupation and adults emerge in the early to mid summer.  Adult black blister beetles feed on pollen and nectar and are most commonly found on yellow-flowered plants such as goldenrod and rabbitbrush.  Unlike other blister beetle species, black blister beetles do not damage foliage.  When suitable flowers remain present, black blister beetles can be present into October.