Blister Beetle

Epicauta pensylvanica

Blister Beetle

Epicauta pensylvanica

Take care

Handle with care

It does not bite. It carries cantharidin, a chemical that raises a blister on skin it is crushed against, so the injury comes from swatting one rather than from the beetle attacking. Brush it off instead.

We identify, we don't diagnose. If you've been bitten or stung and symptoms develop, or you're worried, seek medical attention.

Size10–15 mm

MarkingsLong, narrow, soft-bodied — wing cases look slightly too small and often gape · Distinct narrow neck: the plate behind the head is narrower than both the head and the body

WhereFlowering plants in fields, gardens and field margins — alfalfa, goldenrod, aster and many garden flowers.

SeasonAdults late summer into autumn

How to tell it apart

Long, narrow, soft-bodied — wing cases look slightly too small and often gape
Distinct narrow neck: the plate behind the head is narrower than both the head and the body
The black blister beetle is uniformly matte black, 10–15 mm
Often in numbers on flowers, especially goldenrod and alfalfa in late summer
Walks rather than flies when disturbed, and is easy to knock off a plant

Where you’ll find it

Across most of the United States, with dozens of Epicauta species between them covering effectively the whole country.

Flowering plants in fields, gardens and field margins — alfalfa, goldenrod, aster and many garden flowers.

Adults late summer into autumn.

How it behaves

Adults feed on flowers and foliage, often gathering in numbers on one plant. The defence is chemical rather than physical: cantharidin is released when the beetle is squashed or pressed, which is why the blister appears where somebody slapped at one on their arm. The larvae are predators of grasshopper eggs, so a good grasshopper year is often followed by a good blister beetle year.

You’ve found one indoors

Do not squash it, on your skin or anywhere you will touch afterwards. Flick or brush it off, or lift it on a leaf. If you have been exposed, wash the area with soap and water promptly. A blister that forms is usually superficial and heals without scarring, but see a doctor if it is large, near the eyes, or if a child is affected.

Common questions

Did a blister beetle bite me?

Almost certainly not — they do not bite. What raises the blister is cantharidin released when the beetle is crushed against skin, so the usual sequence is that someone brushed at one without noticing.

How bad is the blister?

Usually superficial. It can take a few days to settle and does not normally scar. It is worth medical attention if it is extensive, close to the eyes, or on a child.

Are they dangerous to horses?

This is the serious end of it. Beetles crushed into alfalfa during cutting can contaminate hay, and cantharidin poisoning in horses can be fatal. It is a real agricultural concern rather than a garden one.

How do I tell one from a soldier beetle?

Look at the neck. A blister beetle has a distinctly narrow plate behind the head, noticeably slimmer than both head and body. Soldier beetles are broader through that join and are completely harmless.

Commonly confused with

Soldier Beetle Harmless

Keep identifying

Identify another animal from a photo or return to the Beetles guide.

Previous: Ladybug · Next: Soldier Beetle

Where this was checked

iNaturalist — Epicauta pensylvanica checked 2026-07-30
University of California IPM — Blister Beetle (Alfalfa) checked 2026-07-30

Last reviewed 2026-07-30. We identify, we don’t diagnose — what we can and can’t tell you.