Unlike other bug bite blisters, blister beetle bites are not caused by actual bites but rather by contact with the beetle. They’re rarely a cause for concern but can be painful.

Blister beetles are long, narrow plant-feeding insects (Meloida) that vary in color from yellow to gray. They live in flower beds and grassy fields, and congregate around outdoor lights in the evenings.

While blister beetles are common throughout eastern and central states, they’re an insect you probably don’t think much about. That is, until you develop a blister or welt that matches the description of blister beetle dermatitis.

Read on to see pictures and learn more about these beetles, including how you get blister beetle dermatitis, how to treat it, and how to protect yourself.

Blister beetle dermatitis results from contact with a blister beetle, not from an actual insect bite.

Unlike some insects, blister beetles don’t have stingers, nor are their jaws strong enough to break human skin.

The welts or blisters on your skin are a reaction to cantharidin, an odorless, colorless chemical the beetle releases to protect itself against its enemies.

Although cantharidin is highly toxic and dangerous to a blister beetle’s enemies, it’s not toxic to human skin. Contact with the substance, however, can cause a local reaction.

Blisters caused by exposure to cantharidin can form on any exposed skin, such as the face, neck, arms, and legs. You may develop a blister or welt after a blister beetle crawls on your skin, or if you crush a blister beetle on your skin.

Blister beetle dermatitis causes a localized blister or welt. The welt may look like a raised, red patch of skin, whereas the blister produces a pocket of fluid and pus.

The reaction develops on areas of skin exposed to the beetle. Pain, burning, redness, and swelling often accompany these lesions.

This type of dermatitis appears within 24 to 48 hours after contact with a blister beetle. Some people initially notice the blister after waking up in the morning.

Blisters are temporary, with symptoms improving within a week. There’s a low risk of scarring, but some people do have post-inflammatory hyperpigmentation after a blister disappears.

Blister beetle welts and blisters can be painful, but the skin reaction isn’t life-threatening to humans, and it doesn’t typically cause permanent damage to the skin.

However, while these blisters aren’t dangerous to your skin, it’s important to use care to avoid spreading cantharidin to your eyes. This can happen if you touch a blister or welt and then rub your eyes. You may develop a type of conjunctivitis called Nairobi eye.

Washing your eye with soap and water may ease irritation from Nairobi eye, but you should also see an eye doctor for treatment.

Beneficial uses of cantharidin

Interestingly, the cantharidin in blister beetles has some beneficial uses in humans. For example, cantharidin, when combined with salicylic acid and podophyllin, can treat viral skin infections like warts.

When applied to warts, the cantharidin in these medicines cause a blister to form underneath the wart, cutting off its blood supply. As a result, the wart gradually disappears without damaging the skin.

Topical cantharidin may also treat molluscum contagiosum, an infection caused by the poxvirus.

Cantharidin is also an active ingredient in Spanish fly, a popular aphrodisiac. Blister beetles don’t only use cantharidin to fight off enemies, the males also use it to arouse the female beetles, which explains why the substance is used as a sexual stimulant.

Keep in mind, however, that Spanish fly contains only a safe amount of cantharidin. Ingesting too much cantharidin may cause severe poisoning in humans. Symptoms of poisoning include mouth burning, nausea, hemorrhaging in the gastrointestinal tract, and renal dysfunction.

Cantharidin is also poisonous to sheep, cattle, and horses that eat alfalfa hay. Blister beetles are sometimes found in alfalfa fields, making their way into the hay bales.;

Ingesting 4 to 6 grams of blister beetles is enough to be fatal to a 1,110-pound horse, according to the American Association of Equine Practitioners (AAEP).

Welts and blisters will disappear in about a week. To treat symptoms of a local reaction, wash the blister with warm, soapy water each day, and then apply a topical steroid or antibiotic. This can prevent a secondary infection and ease redness, swelling, and pain.

Applying a cold compress to the lesion several times a day can also ease swelling and pain. You don’t need to see a doctor, but you should seek medical attention if cantharidin gets in your eyes.

If you’re working, playing, or relaxing outdoors, be mindful of areas that might have blister beetles. These include grassy fields, flower beds, and light fixtures. Wear long-sleeved shirts and long pants if you’re likely to encounter these beetles.

If a blister beetle lands on your skin, don’t crush it. Gently remove the beetle by blowing it off of your skin. After skin contact with the beetle, wash the exposed area with soap and water.

Remove and wash any clothing that comes in contact with blister beetles, too.

Blister beetle welts and blisters aren’t dangerous, and they don’t typically cause scarring. But they can be painful and uncomfortable.

To protect yourself, learn how to recognize blister beetles, and then take precautions to prevent skin contact with these insects.