Blister Beetles

Blister beetles may look harmless, but they use chemical warfare to defend themselves!

Epicauta puncticollis (Judy Gallagher) CC BY 2.0

Blister beetle identification

Blister beetles tend to be long and narrow. Their wing covers are soft and flexible. Adults are usually 1.2 an inch long. Depending on the species, these pests are usually black, gray, or green, and some have orange or yellow stripes. Some species are very colorful, warning would-be predators to think twice. Blister beetle species found elsewhere in the world can be metallic green, blue, reddish, or copper, with spots or stripes.

Lytta vesicatoria (Stefanie Hamm) Public Domain

Blister beetle families
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There are three major groups of blister beetles that I know of: Epicauta spp., Lytta spp., and Tegrodera spp., and hundreds of species. [When reading Latin names, the “spp.” seen after the word is an abbreviation of “species.” It refers to all the species within that genus.]

Tegrodera latecincta, or iron cross blister beetle (Mike) CC BY-SA 3.0

Blister beetle lifecycle

​As far as insects go, blister beetles have a pretty interesting life. Female start the cycle by depositing fertilized eggs in low spots in the soil. The eggs hatch into larvae (called triungulin) , who hunt for underground cricket and grasshopper, and bee eggs. As they feed, they go through three growth stages before turning into sedentary pseudo pupae. This is their overwintering stage. As temperatures rise in spring, the pseudo pupae enter a true pupal stage before reaching adulthood. Adults feed on flowers and vegetable plants and breed throughout summer, depositing eggs in the soil, and the cycle continues.

Bee parasitized by blister beetles (Pjt56) CC BY-SA 4.0

Parasitic blister beetles
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Some blister beetle species larva parasitize solitary bees. They do this by congregating and mimicking the pheromones used by female solitary bees to attract males. When the males appear, blister beetle larva climb aboard for a free ride (phoresy). When the male bees mate with a female, the blister beetle larva transfer to the females, which takes them directly to the nest, where they will consume nest provisions and eggs.

Toxic chemical
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Blister beetles use a chemical called cantharidin to defend themselves. When they feel threatened, they automatically spray this chemical onto their would-be attacker from their leg joints and mouth. This chemical is so strong that consuming six beetles can kill a horse! While horses don’t generally eat beetles, the dead insects can be found in baled alfalfa (UCANR). While I normally feed any pests I find to my hens, these bugs go in the trash! Only males produce this chemical, but they transfer it to females during mating. Cantharidin can cause painful blisters, so you probably don’t want to touch them bare handed. Medical attention is generally not needed for these blisters, but I can guarantee that you don’t want the experience!

​Medical uses of blister beetles
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Historically, dead blister beetles were dried and crushed and used on a variety of medical conditions, including gout, carbuncles, rheumatism, and impotence. The (ineffective) Spanishfly of legend is actually ground up blister beetles. The only safe and effective medical use for blister beetles is to remove warts. Even then, I would leave it to the pros. 

If you see blister beetles in your garden or landscape, stomp on ‘em and then use gloves or some other barrier to pick them up and put them in the trash.