Tack n' Talk

Online Equestrian Resource

Small but deadly….Blister Beetles

To many horse owners, purchasing hay has become science:  Select the type of hay, grab a handful for the smell test, examine it for dust and mold and then feed it to the hungry horse awaiting for you at home.  Many hay producers will be, or have already been harvesting hay and have started selling their product to horse owner consumers. But as hay purchasers we must be aware of a tiny, toxic and potentially fatal tagalong in some perfectly healthy-looking alfalfa hay bales – the blister beetle.

 Blister beetles are from the Meloidae family so called for their defensive secretion of a blistering agent, cantharidin and come in many sizes, shapes and colours.  There are about 7,500 known species worldwide.  Cantharidin is a poisonous chemical that causes blistering of the skin and is highly toxic to horses.  A few beetles consumed in a single feeding of alfalfa can be lethal.

Blister beetles tend to swarm to feed on alfalfa flowers and simply touching a blister beetle, either dead or alive, is enough to cause inflammation and blistering of a horse’s skin within hours of contact.

If a horse ingests even a few beetles, the insects’ cantharidin can cause ulceration and inflammation of the mouth, stomach and intestines.  Clinical signs including decreased appetite, frequent drinking and urination, colic, and depression can be apparent with hours.  In the worst case scenario, ingesting these beetles can cause endotoxemia, shock and death within hours of ingestion.

If you suspect your horse has ingested blister beetles, immediately call your vet and most likely you will need to transport your horse to the nearest equine hospital for treatment.  There is no specific antidote for blister beetle poisoning.  Treatment is solely aimed at reducing absorption of the toxin by administering activated charcoal and mineral oil, intravenous fluids, gastrointestinal protectants and broad spectrum antibiotics.

So, before feeding your horse that bale of alfalfa hay, examine it carefully for the presence of blister beetles.

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3 Comments»

  cathy whitley wrote @

Clarissa-Good info! I’ve heard about these but fortunately never seen them. Do they generally survive in dried hay, and are they large enough that they’d be obvious if you looked?

  tackandtalk wrote @

Blister beetles are generally 1/2 to one inch in length and can be black, gray, brown, striped, or orange-striped. They have a narrow, cylindrical body with a distinct neck.

The beetles are found throughout the US, but are most common in the arid, western states. However, they are also found in some eastern states, especially in years following heavy grasshopper infestations.

Feed only first-cutting alfalfa for horses. Blister beetles don’t usually emerge until after the first cutting. Late fall cutting is also generally free of blister beetles because insect activity usually ceases in early fall, however changing weather patterns (particularly El Nino years) may affect when blister beetles are active in alfalfa fields, so buy your alfalfa hay from reliable producers.

Check your hay for blister beetles when you feed. Blister beetles retain their body shape and size even after death. Be sure to discard any hay that contains blister beetles as the toxin could have contaminated the surrounding hay!

  Debbie Solano wrote @

Tommy Scribner taught the Feeding and Nutrition class in the Horse Management program at Rogers State College in Claremore, Oklahoma. He used to teach us not to buy alfalfa that was grown in Oklahoma. That is consistent with your comment above. He also used to say that blister beetles didn’t use to be as big a problem in “the old days” when conventional methods were used for cutting hay. The new mower conditioners that cut the hay in one fell swoop squash and kill the blister beetles, thereby leaving them in the hay. The old way allowed the blister beetles to scurry away while the hay was being cut.


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