Blister beetles vary widely in colors and shapes and contain an irritant called cantharidin and I found one in Mission Trails Regional Park. Thankfully, I didn’t touch it or pick it up or try to bring it to show to others.

Blister beetles, of the family Meloidae, go through what’s called hyper-metamorphosis, which is essentially complete insect metamorphosis, but with some larval in-stars being distinct from each other.

They start as eggs. Once they hatch, the larvae are insectivorous and mainly eat bees and other insect eggs.

These are typically the solitary bees and the adult female beetle lays eggs near the holes of the solitary bees. Once they hatch, they climb either into the hole of the bee or onto a flowering plant and wait for the pollinating bee. The baby instar beetle hooks itself onto the bee and the bee brings it back home. The larvae can be considered both a predator and parasite as it feasts on the bee, the bee larvae, and the stored pollen.

The beetle will continue to metamorphosize and will emerge as an adult from the bee’s former home to now feed on flowers and leaves of plants. A lot of those plants are in the Asteraceae, Fabaceae, Solanaceae, and Amaranthaceae families, which are all over the park.

The blistering agent is used by humans to remove warts. It is mainly collected from what’s called the Spanish fly, which is an emerald-green beetle of the blister beetle family. The beetles will secrete a milky fluid, which contains the cantharidin component, causing irritation, blistering, bleeding, and discomfort. This is also known as Blister Beetle Dermatitis. If you encounter it on the trail, washing with soap and water is the first step. If severe, see a doctor.

George Washington was supposedly treated with Spanish Fly for epiglottitis. Cantharidin was used as an aphrodisiac, as part of the first recorded stink bomb, and as a topical medication to remove warts and tattoos. It is also a controlled toxin and as little as 10mg can be lethal to humans.

The founder of osteopathy recommended inhaling a tincture of cantharidin (remember: this stuff is deadly!) as a treatment for smallpox and was vocally against the smallpox vaccine. Back on the positive side, there are studies for cantharidin use in fighting cancer cells.

There is a slew of information about blister beetles on the internet, including this preview of a BBC documentary at: youtu.be/ZQ8h1YBTSvE.

If you encounter a blister beetle out on the trail, just be sure to employ the “look, don’t touch” method.

Editor’s note: Philip Hwang is a trail guide at Mission Trails Regional Park.