Watch Out For The Blister Beetle

This lightning bug lookalike can cause a painful welt—hence its name—if it gets too close, not to mention defoliate your garden.

by Kevin Fogle
PHOTO: Don Burkett/Flickr

Adult blister beetles are sometimes confused with the common lightning bug—though, this is a mistake that gardeners and landscapers only make once. While lightning bugs create an amazing summer evening spectacle, the blister beetle may leave behind an angry blistery reminder of its presence: a bite that can leave you with a painful welt. With several hundred species of blister beetles found across North America, it’s important that gardeners be aware of them. Aside from their ability to irritate the skin, several of these beetle species are confirmed garden pests that tend to gather in sizable numbers to defoliate host plants.

Definitely Not A Lightning Bug

Part of the confusion between blister beetles and lightning bugs is the fact that both insects are soft-bodied beetles with leathery elytra (wing covers), which are in stark contrast to the hard elytra found on most beetle species. Adults grow between 1/2 to 1 inch long, with a broad vertical head connected to its cylindrical body by a narrow “neck.” The neck-like appearance is an important clue to distinguishing blister beetles from lightning bugs.

With so many individual species of blisters beetles in North America, it is challenging to describe their exact coloration, though many species tend to be dark gray or black, with some exhibiting longitudinal stripes down the body. Larval forms of the beetles are predatory, and the species that consume grasshopper eggs are even considered beneficial, but most species of blister beetles are confirmed pests, attacking the nests of some native bee populations that serve as important pollinators.

Blister Beetle Damage

All blisters beetles have the ability to exude cantharidin, an oily substance that irritates the skin and causes large fluid filled blisters to form on human skin. While the concentration of cantharidin varies among species, the release of this caustic substance is increased when beetles are crushed or injured next to the skin. The blisters generally aren’t considered a serious human threat, though if they get infected, they can cause additional issues. Livestock, such as horse and sheep, on the other hand, may become quite sick or even die if they accidentally consume blister beetles in the field or if their dried feed is contaminated with dead blister beetles or cantharidin from beetles randomly crushed in the harvest.

In the garden, several blister beetle species, such as the striped blister beetle or the ash gray blister beetle, can congregate and defoliate several important economic crops: alfalfa, beans, beet, carrot, cabbage, corn, eggplant, melon, pea, pepper, potato, squash and sweet potato, all of which are common hosts. If blister beetles are an issue in your garden, the best treatment for small populations is handpicking (with gloves), or for large populations, use an organic spray containing the ingredient spinosad.

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