Named for the effect it has on human skin, the blister beetle is both a gardening pest as well as a friend, depending on its stage. As a larva, it feeds on grasshopper eggs but as an adult, it feeds on alfalfa and a variety of vegetable crops, depending on the species. More than 300 species of blister beetles are found in the U.S. — many (but not all) have long and narrow bodies; in nearly all, however, the head is larger than the body segment next to it. Color ranges from solid black or gray to striped or metallic.
Blister beetles contain a toxic substance, cantharadin, that can cause blisters to form on the skin. The toxin is especially problematic for livestock that eat feed contaminated by the beetles. Horses are especiallly susceptible; horses that eat sufficient quantities of the feed can become extremely ill and die. Blister beetles vary in the amount of cantharadin they contain. It's been estimated that 80 three-striped blister beetles or 1,100 black blister beetles would provide a lethal dose of cantharadin for a 550-lb. horse.
Applying an insecticide is usually not an effective control for the blister beetle. Hay growers typically use mechanical means for getting rid of the pest. If you find blister beetles in your veggie garden, pick them off by hand — and be sure to wear gloves!