A Say's blister beetle on a flower. Credit: Courtesy of Griffin Dill

There are thousands of different kinds of beetles in Maine. But only one — the blister beetle — produces a toxic chemical that causes skin blisters if touched, is fatal to livestock, was once a common treatment for warts and gave us the term “Spanish Fly” for aphrodisiacs.

Maine is home to five species of blister beetles — the black blister beetle, the gray blister beetle, the ash gray blister beetle, Say’s blister beetle and the short-winged blister beetle. All five feed on flowers and can quickly decimate a plant of its blooms or petals. Lupine, asters and legumes are particular favorites of blister beetles.

Sometimes blister beetles are mistaken for potato beetles if they are found munching on potato plants. But unlike potato beetles, which tend to spread out in a field, the blister beetles will concentrate in a small area.

“Adults are the pests,” according to Jim Dill, pest management specialist with University of Maine Cooperative Extension. “The big story with them is they do release a fatty material from the joints of their legs that is very toxic and causes severe blistering.”

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That fatty material is the chemical cantharidin. In addition to causing irritating skin blisters, it can be fatal if swallowed as it will form blisters inside the body. This is particularly concerning when it comes to livestock.

Blister beetles are toxic to sheep and cattle, but especially deadly to horses, according to The American Association of Equine Practitioners. All it takes is four to six grams of blister beetles to quickly kill an 1,100-pound horse, even if the horse only eats parts of the blister beetles.

“You can imagine if a horse is grazing on feed that has lupine growing in it,” Dill said. “All of a sudden there is a lupine with 10 or 15 blister beetles on it and the horse munches it down — it can be really bad.”

It was that same toxic property in cantharidin that made it a popular ingredient in topical treatments for warts. The chemical causes a blister to form on the wart that lifts the wart off the skin. When the blister dries up, the wart falls off. In 1962 the United States Food and Drug Administration withdrew its approval of cantharidin as an ingredient in over-the-counter skin medications due to a lack of data supporting its effectiveness.

A black blister beetle on goldenrod. Credit: Courtesy of Griffin Dill

Blister beetles are also called Spanish fly, the same term given to aphrodisiacs made from cantharidin that have been around for centuries. Studies have shown that it’s not a true aphrodisiac, but as recently as 1996 four young men were treated in Pennsylvania after drinking Kool-Aid they had laced with cantharidin. They had planned to give the concoction to a woman one of them was dating.

Blister beetle larvae do not release cantharidin, but they do have a different chemical they use in an elaborate scheme to trick bees.

The larvae of blister beetles will climb to the tops of flowers or plants and clump together in a solid mass that’s the same shape and size of a female solitary ground bee. They then exude a chemical that mimics the female bee’s sex pheramones to attract a male bee.

When the male ground bee comes along and attempts to mate with what it believes is a female bee, the larvae hop on and hitch a ride. Then, when the male bee mates with an actual bee those larvae jump on to her for a ride back to her nest. Once there, the blister beetle larvae take up residence and eat any bee eggs and larvae, or the honey and pollen the adult bee has stored for its own young. The blister beetle larvae remain in the nest until they reach adulthood.

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Unlike the toxic hairs of the brown tail moth caterpillar, which can be shed and find their way onto your skin, you have to actually touch and squish a blister beetle for it to release its toxin.

“They are common in Maine, but you really don’t see them that much,” Dill said. “If you do see the beetle and want to get rid of it, you can put on rubber gloves, hand pick them off the plants, dump them in a bucket of soapy water and when they are dead, dump them in a hole and bury them.”

Burying them is key because even in death, blister beetles retain the toxin in their corpses.

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Julia Bayly

Julia Bayly is a reporter at the Bangor Daily News with a regular bi-weekly column. Julia has been a freelance travel writer/photographer since 2000.