Blister beetle, sometimes known as oil beetles (Spanish fly), secretes an irritating substance called cantharidin produced as a defensive secretion. The beetle does not have a stinger and cannot bite. So, Blister beetles discrete cantharidin to protect themselves from being eaten by predators. When collected, cantharidin is used medically as a topical skin treatment to remove warts. Cantharidin works by causing a blister to form under the wart. The blister cuts off the blood supply to the wart, causing the ward to die without scarring. In the past, inducing blisters was a common cure for many illnesses. It was also a major ingredient in so-called Spanish fly love potions. The best way to avoid contact with blister beetles is wearing long-sleeved shirts and pants when in areas where you are likely to encounter them, central interior of B.C. If a blister beetle is encountered, gently remove it and don’t crush it.
https://www.delta-optimist.com/opinion/blogs/blog-blister-beetles-and-warts-1.23427215