Blister beetles
Blister beetles arrive in clusters in mid to late summer and chew flowers and leaves. Tomatoes and potatoes can be damaged when numbers are large. None of the extension sources consulted offers a biological control. The case for doing much at all is mixed. Their larvae develop on grasshopper egg pods, and numbers rise after grasshopper years. The real hazards are to skin and to livestock. Their blood carries cantharidin. A beetle crushed on the skin raises a painful blister, and horses can die from eating hay with a few crushed beetles in it. If they are on your plants, remove them wearing gloves and without crushing them. If you keep horses, the precautions that matter are in the hay, not the garden.
Epicauta vittata feeding in a group on potato foliage in July. Photo: pending.
Identification
Blister beetles are narrow and elongate, with soft wing covers (UC IPM) and a body ¾ to 1¼ in long (Kentucky). Kentucky Extension names four common species in the eastern and central US: the striped blister beetle (Epicauta vittata), orange-yellow with black stripes, and the black (E. pennsylvanica), margined (E. pestifera) and ash-grey (E. fabricii) blister beetles. More than 200 species occur in the continental US, and Epicauta is the genus most often linked to poisoning (Merck Veterinary Manual).
The Missouri Department of Conservation gives a wider size range for the family, from under ¼ in to 2½ in, with most ½ to ¾ in, and notes that some are iridescent blue. The tell is the combination of a long, narrow, soft-winged body and the group habit. Kentucky says they congregate, so large numbers occur in concentrated clusters.
Easily confused with
- Soldier beetlesBeneficial — leave itAlso long and soft-winged, but flatter and parallel-sided. A blister beetle has a broad head on a narrow neck, the section behind the head is narrower than the head, and the wing covers curve round the body.
Damage symptoms
The adults eat flowers, pollen and nectar, and leaves. The Missouri Department of Conservation says they prefer the sunflower, bean and potato families, and that tomatoes and potatoes can be damaged when populations are large. The damage comes in patches, because the beetles feed in groups. UC IPM notes that each species has its own peak period of activity.
- Clusters of long, narrow beetles feeding together on one part of a planting
- Flowers and leaves chewed
- Tomato and potato foliage damaged when numbers are large
- Beetles on flowering plants and field edges in mid to late summer
- A painful blister on the skin after crushing one
Lifecycle & degree days
Blister beetles go through an unusual life cycle. Females lay egg clusters in the soil in late summer. The newly hatched larvae are active crawlers. They search the soil surface and its cracks for grasshopper egg pods, or in some species ride adult bees back to their nests and eat the bees' eggs and stores (UC IPM; Missouri DoC). Once on food, the larva turns into a sedentary, legless grub, passes the winter in a resting stage, pupates, and emerges as an adult the following summer. Kentucky notes that blister beetle numbers rise dramatically after high grasshopper populations, because more egg pods mean more larvae survive.
- Eggs
- Clusters in soil, late summer
- Larval food
- Grasshopper egg pods; some species, bee nests
- Overwinters
- As a resting larval stage in the soil
- Adults active
- Mid-July to early August (Kentucky)
- Cantharidin
- ~5 mg per striped beetle; ~0.5 mg in others
Host crops
The Missouri Department of Conservation says adults prefer the sunflower, bean and potato families, and names tomatoes, potatoes and clover among the crops damaged when numbers are large. In agriculture the crop that matters is alfalfa, not because of the feeding but because the beetles gather on blooming alfalfa and end up baled into horse hay (UC IPM).
Monitoring & action thresholds
UC IPM says no treatment thresholds have been established for blister beetles in alfalfa, and that insecticide applications are generally not needed. No garden threshold was found either. For horse owners the question is not a count of beetles but whether hay was cut and baled while beetles were in it.
Watch flowering plants and garden edges from early July, when the adults appear. Kentucky gives mid-July to early August, and advises monitoring hay fields from July to early September. Because the beetles cluster, check several spots rather than one. Look before you touch: gloves on, and no crushing.
Biological control
None of the extension sources consulted recommends a biological control for blister beetles. They are worth knowing about as part of the balance of a garden. The larvae of many species develop on grasshopper egg pods (Colorado State), and UC IPM lists blister beetles among the natural enemies of grasshoppers. Kentucky notes that blister beetle numbers increase dramatically following high grasshopper populations.
Cultural controls
In a garden the beetles are usually a passing problem. Protect your skin, remove them from the plants that matter, and if you keep horses, take hay seriously.
- Wear gloves to remove them, and do not crush them against bare skin. Kentucky says crushing one on the skin can raise a painful blister
- Cantharidin stays toxic in dead beetles for a long time (Kentucky), so dispose of collected beetles where pets and livestock cannot reach them
- Adults are especially attracted to flowers (Kentucky). In hay fields Kentucky advises good broadleaf weed management and harvest intervals that keep flowering of alfalfa or weeds to a minimum
- If you feed horses alfalfa, buy hay that has not been crimped and was cut before bloom or after a frost, when beetle activity is lowest (Merck Veterinary Manual). Kentucky also advises buying first-cutting hay, since the beetles are not active at first cutting there
- If you grow hay, cut before peak bloom (UC IPM) and avoid crimping (Kentucky). The Merck Veterinary Manual says crimping exacerbates contamination
- If a horse shows colic, restlessness or straining to urinate after eating alfalfa hay, treat it as an emergency and call a vet. Kentucky says animals can die within 72 hours
Chemical, last resort
Sources
Univ. of Kentucky ENTFACT-102, Blister Beetles (Townsend)
UC IPM Pest Management Guidelines: Alfalfa, Blister Beetles, UC ANR Pub. 3430 (updated 01/2017)
Merck Veterinary Manual, Cantharidin Toxicosis in Animals (Romano, updated 09/2024)
Missouri Department of Conservation Field Guide, Blister Beetles (Meloids)
Colorado State Univ. Extension, Grasshopper Control in Gardens and Small Acreages (Cranshaw & Hammon)
UC IPM Pest Notes: Grasshoppers, UC ANR Pub. 74103 (Flint, updated 06/2013)
Page status
Drafted from the sources below, then checked claim by claim against those sources on 23 September 2026.
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