Blister Beetles: Skin Blisters, Horses and Hay
Blister beetles (family Meloidae) are soft-bodied, long-legged beetles whose body fluid contains cantharidin, a chemical that raises blisters on human skin and can kill horses that eat beetles crushed into alfalfa hay. They do not bite or sting; the blister comes from pressing or crushing one against your skin. In the garden they strip potatoes, tomatoes and beets, while their larvae eat grasshopper eggs. For most homeowners they are a nuisance; for horse owners buying hay, they are a real risk worth managing.
The US has more than 300 species, according to New Mexico State University, and Missouri’s Department of Conservation puts the North American count above 400. Most of the trouble comes from one genus, Epicauta, the striped, grey and black beetles that swarm into alfalfa and vegetable plots in summer.
What do blister beetles look like?
New Mexico State lists two features that hold for most species. The segment behind the head (the pronotum) is narrower than the head and the wing covers, which gives a distinct “neck”, and the wing covers are soft and flexible rather than hard. In Epicauta, New Mexico State notes, the rounded tips of the wing covers form a sideways “3” where they meet. The legs are long, and with a hand lens each claw is split into two blades.
| Beetle | Length | What it looks like | Cantharidin per beetle |
|---|---|---|---|
| Striped blister beetle (Epicauta vittata) | 12 to 18 mm (about half to three-quarters of an inch) | Long, slender, brown and yellowish grey with yellowish stripes | Highest: about 4.5 to 5.2 mg |
| Ash grey blister beetle (E. fabricii) | 10 to 15 mm | Grey from a thick covering of tiny hairs | About 0.5 to 1.25 mg |
| Black blister beetle (E. pensylvanica) | 7 to 15 mm | Solid black, more robust | Low: about 0.08 to 0.40 mg |
| Margined blister beetle (E. funebris) | Not given by our sources | Black with white-edged wing covers | Not measured in our sources |
| Oil beetles (Meloe) | Varies | Black or bluish black, very short wing covers | Not measured in our sources |
Compiled by The Pest Control Brief from University of Nebraska Extension G1645 (sizes, cantharidin ranges from female to male averages), University of Arkansas, New Mexico State University and Missouri Department of Conservation descriptions.
How big are blister beetles?
Most are about half to three-quarters of an inch, the Missouri Department of Conservation says, though the full range runs from under a quarter of an inch to two and a half inches. Southwestern species run larger: New Mexico State gives half an inch to an inch and a half.
Look-alikes
New Mexico State warns that blister beetles are often confused with tamarisk leaf beetles, southern corn rootworms and caterpillar hunters, and Arizona extension adds checkered, soldier, darkling and ground beetles. The University of Arkansas gives the cleanest test against false blister beetles (Oedemeridae): their pronotum tapers towards the rear, not the front, and they lack split claws. Do not handle false blister beetles either. A dermatology review in Cutis notes they also produce cantharidin, with smaller blisters.
Why is it called a blister beetle?
Because of what its blood does to skin. UF/IFAS explains that the beetle’s hemolymph raises blisters on contact, and that adults release it by reflex bleeding from the leg joints when pressed or rubbed. The active chemical is cantharidin. The European species Lytta vesicatoria is the “Spanish fly” of folklore, and cantharidin is still used in medicine to remove warts. The scientific name for the family is Meloidae; Epicauta is the genus behind most US hay and garden problems.
Are blister beetles dangerous to humans?
On skin, they cause a chemical burn rather than a bite. The Cutis review describes burning and tingling within minutes of contact, followed later by fluid-filled blisters, often in a line where the beetle was dragged or crushed, and “kissing” lesions where skin folds touch, such as the inside of an elbow. UF/IFAS says blisters are most common on the neck and arms, from beetles attracted to outdoor lights at night, and that ordinary handling seldom blisters unless the fluid reaches the thin skin between the fingers. A line of blisters with no beetle seen is hard to pin down; our bite identification guide explains why marks on skin are weak evidence.
Swallowing them is a different matter. New Mexico State notes that human deaths from cantharidin have been reported, and UF/IFAS says it is highly toxic to mammals taken internally. Children are the group to watch: the Cutis authors note many blister beetles are brightly coloured and attractive to small children. If a child or pet may have eaten one, call Poison Help on 1 800 222 1222 or a vet. For skin blisters, especially near the eyes, see a doctor; we do not give treatment advice.
Horses and hay: the serious risk
Live blister beetles rarely hurt horses. Arizona extension points out that horses do not want to eat live beetles and that poisoning is rare unless dead beetles are in the hay. The danger is the hay itself. Beetles gather in swarms in flowering alfalfa, the mower or crimper crushes them, and the bodies, or just their fluid, end up in the bale. Cantharidin is stable: New Mexico State says storing infested hay does not significantly reduce it, and dead beetles stay toxic.
The lethal dose is small. New Mexico State reports that doses used to kill horses experimentally range from 0.45 to 1 mg of cantharidin per kilogram of body weight, and Nebraska uses 1 mg per 2.2 lb as an estimated minimum lethal dose. How many beetles that takes depends almost entirely on which beetles.
The gap between species is the most useful fact a horse owner can learn. On Nebraska’s averages and the 1 mg per kg threshold, our arithmetic puts a lethal dose for a 1,000 lb horse at roughly 87 to 100 striped beetles, but more than 1,100 male black blister beetles. New Mexico State’s own calculation for striped beetles is 81 to 128. Reports of lethal doses range from a few beetles to as many as 200, partly because cantharidin varies within a species, with males carrying more than females.
Signs in horses
Kansas State and Nebraska list mouth sores and ulcers, sweating, diarrhoea, blood in the faeces, frequent attempts to urinate with little output, and blood in the urine; Nebraska adds that poisoned horses may put their muzzle in water without drinking. Nebraska notes animals can die within 72 hours, and New Mexico State reports death 24 hours after a heavy dose. Kansas State notes cattle can get mouth sores too, and the University of Arkansas lists sheep, alpacas and poultry as sensitive. If you suspect poisoning, switch hay and call a vet at once. Unlike horse flies, which bite horses in the same summer months, blister beetles do harm only through feed.
Buying and cutting safer hay
The advice from state to state is consistent in principle and differs in dates, so we put it side by side.
| Source | Lowest-risk cuttings | Bloom stage at cutting | Equipment and handling |
|---|---|---|---|
| New Mexico State (CR536) | Before mid-May and after late August | 10 per cent bloom or less | Crimpers crush beetles beyond recognition; raking before baling lets more dead beetles fall out |
| University of Nebraska (G1645) | Before mid-May or after early September; first cutting usually precedes adults | Before 5 per cent bloom | Uncrimped hay cut with a sickle bar or rotary mower lets live beetles leave; scout two to three days before harvest |
| Kansas State | Beetles most common in second, third and fourth cuttings | Not specified | Avoid crimpers from mid-June through September; avoid hay from field ends where tyres cross windrows |
Compiled by The Pest Control Brief from New Mexico State University Circular 536, University of Nebraska Extension G1645 and Kansas State University Extension Entomology.
For buyers, New Mexico State advises buying from a grower who scouts fields, inspecting hay as it comes off the bale, and never feeding infested material to any livestock, however long it has been stored. Kansas State is honest that no practice guarantees beetle-free hay. Florida shows how far the risk travels: UF/IFAS says most Florida poisonings come from alfalfa imported from western states with grasshopper outbreaks.
Where do blister beetles live, and when are they out?
They are found in every region, but sightings concentrate in the Great Plains and the Southwest, the same country where grasshoppers boom. Our analysis of iNaturalist records for Epicauta puts Oklahoma far ahead, with 27.85 records per 10,000 observations, more than double any other state.
Nebraska notes most Epicauta larvae feed on grasshopper eggs and that blister beetle numbers may be higher the year after high grasshopper numbers. That makes a bad grasshopper summer, which our grasshoppers guide covers, a warning sign for next year’s hay. The University of Arkansas describes one to two generations a year, with adults present from late May to late October and peak emergence in July.
Our seasonality index shows Epicauta records climbing from 51 in May to 242 in July and a peak of 318 in August, then falling back by November. Spur-throated grasshoppers peak a month later, at 343 in September. The blister beetle family as a whole shows a separate early peak in March (211) and April (175), months when Epicauta is almost absent (10 and 23). The early-spring records are other blister beetle groups; the summer swarms in hay and gardens are Epicauta.
Blister beetles in the garden
UF/IFAS says Epicauta, especially the margined and striped blister beetles, often defoliate alfalfa, beet, potato, tomato and other crops, and that because they arrive in swarms their attacks can be locally catastrophic. The University of Arkansas adds peppers, eggplant and chard, and describes beetles that stay spread out at low density and then suddenly gather in large feeding and mating swarms. Arizona notes adults can leave nothing but leaf stalks and stems.
There is a trade-off. The same beetles’ larvae eat grasshopper eggs, and Arizona extension notes some experts regard garden damage as minor and the beetles as useful grasshopper control. Our garden pest control guide covers how to judge when a pest is worth acting on.
What kills a blister beetle?
In a home garden, hand removal is usually enough. UF/IFAS says that in small gardens it may be sufficient simply to pick the beetles off. Arizona extension’s method is the standard one:
- Wear disposable gloves, not cloth or leather, to keep cantharidin off your skin.
- Pick or knock the beetles into a container of soapy water, which Arizona says will kill them. A shop vacuum also works.
- Cover vulnerable crops with floating row cover as a physical barrier when swarms arrive.
- Tolerate feeding on woody plants, which Arizona notes usually withstand occasional defoliation.
- Turn off or shield outdoor lights near doors and seating areas if beetles gather there at night, since UF/IFAS links most skin blisters to beetles drawn to lights.
In hayfields the logic reverses: killing beetles in the field is the problem, because dead beetles are toxic. Nebraska says spraying insecticide within the field is not recommended and that producers should wait until beetles move out before cutting. Any insecticide use belongs to the grower, following the label. Arizona adds that beetles in a water trough should be scooped out with a net.
Frequently asked questions
What happens if you get bit by a blister beetle?
You almost certainly were not bitten: the Cutis dermatology review states that blister beetles do not bite or sting. What people call a bite is contact with cantharidin, released when a beetle is pressed against or crushed on the skin. Burning and tingling start within minutes, and blisters form later, often in a line. Blisters near the eyes, large areas, or any sign of infection are reasons to see a doctor.
Are blister beetles harmful?
To people, mainly as a skin irritant, though swallowing them is dangerous and New Mexico State notes human deaths have been reported. To horses, beetles crushed into alfalfa hay can be fatal at doses as low as 0.45 to 1 mg of cantharidin per kilogram of body weight. To gardens, swarms can strip potatoes, tomatoes and beets quickly. Their larvae, meanwhile, eat grasshopper eggs.
Do blister beetles bite?
No. Blister beetles have chewing mouthparts for eating flowers and leaves, but a dermatology review in Cutis is explicit that they do not bite or sting. Their defence is chemical: they release cantharidin-laden fluid from the leg joints when disturbed. UF/IFAS notes that simply handling one seldom causes blisters unless the fluid reaches thin skin such as the webbing between your fingers.
Is it safe to feed hay if I find one dead blister beetle?
Extension sources err on the side of caution. New Mexico State advises against feeding blister beetle infested hay to any livestock, however long it has been stored, and Nebraska says hay from infested parts of a field should not go to horses. How dangerous one beetle is depends on species and the horse’s size, and a crushed beetle can contaminate hay around it. Ask your vet.
Sources
- New Mexico State University Cooperative Extension Service, Circular 536, Blister Beetles in Alfalfa, revised by J. B. Pierce (Table 1 adapted from Capinera et al. 1985).
- University of Nebraska Extension, G1645, Blister Beetles in Alfalfa, 2022.
- Kansas State University Extension Entomology, Blister Beetles (alfalfa crop protection).
- UF/IFAS Extension, EENY166, Blister Beetles (Insecta: Coleoptera: Meloidae), Selander and Fasulo, revised 2023.
- University of Arkansas System Division of Agriculture, What’s That Bug? Blister Beetle.
- University of Arizona Cooperative Extension, Backyard Gardener #21, Blister Beetles, 2022.
- Missouri Department of Conservation, Field Guide: Blister Beetles (Meloids).
- Hodge, B. D., Elgart, G. W. and Elston, D. M. (2019), What’s Eating You? Blister Beetles Revisited, Cutis 104: 317 to 319.
- Capinera, J. L., Gardner, D. R. and Stermitz, F. R. (1985), Cantharidin levels in blister beetles associated with alfalfa in Colorado, Journal of Economic Entomology 78: 1052 to 1055 (as cited by New Mexico State).
- The Pest Control Brief analysis of iNaturalist research-grade records, pulled October 2026.
