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Copyright 2013 Rocky Mountain Rider. ALL RIGHTS RESERVED. Reproduction of any editorial material, artwork and photos is strictly forbidden without express written permission of the publisher. For information about reprint rights, please contact the editor;


Toxic Blister Beetles in Alfalfa Hay


By Dorinda Troutman, RMR Staff Writer


September 2013 issue  


      A year ago, the State of Idaho warned livestock owners about a blister beetle outbreak (they called it a “blister beetle year”) that occurred north of Boise .

      Tony McCammon, Idaho Extension Educator in Ada County , warned in a press release in July 2012, that horse owners might “consider switching from feeding alfalfa to some type of grass hay — blister beetles do not feed on grass hay.”

      McCammon also advised to “know your alfalfa supplier. Talk with them to be sure they are aware of the issues…” and that they have taken steps to ensure that blister beetles are not in their crops.

      Many species of blister beetles are native to the Northwest U.S., but Idaho had not had a problem with them for many years. However, even a few dead blister beetles have the potential to harm or kill a horse.


Black blister beetles are the common species in the Northwest and are not extremely toxic. Blister beetles can be solid tan to black or spotted or striped. They range from 1/4-inch to 1.5 inches in length, have soft wing covers, and fly.


About the blister beetle

      There are more than 355 species of blister beetle in the U.S. , and they all have the toxin “cantharidin.” Cantharidin is as deadly as cyanide or strychnine.

      The toxic properties of each species of blister beetle, individual beetles and even of male or female beetles vary widely. In some particularly toxic species that breed in the Southwest U.S. , the amount of cantharidin in three to ten beetles can kill a horse. So far, the most toxic species do not breed in the Northwest.

      It also helps that northern horse owners feed more grass hay than southern horse owners, who rely much more on alfalfa.

      Blister beetles are actually considered beneficial insects as their larvae eat grasshopper eggs. A “good year” for grasshoppers can mean a good next year for beetles.

      Adult beetles usually swarm to feed on leafy or flowering host plants at the edges of planted fields next to weedy areas. Because the beetles swarm to eat, one bale of hay can have hundreds of beetles in it, and the next bale none. Cultivated host plants include alfalfa, potatoes, tomatoes and soybeans.


Touching a blister beetle can cause painful blisters on a person’s skin.


       When a beetle is touched or crushed, cantharidin raises painful blisters on the skin, and is medicinally used as a wart and tattoo remover. However, if beetles are killed and crushed in hay-making, the toxin pollutes the hay (even if the insect body is gone) and even a small amount may be lethal to a horse.


Recognition of Poisoning

      If a horse is eating forage in a field, the beetles will drop or fly out of the way, but if the beetles are caught up in harvested and baled forage, the toxin can survive for years.

      Researchers have found that the newer methods of harvesting alfalfa — which include crushing and windrowing hay — immediately increase the risk of blister beetle toxin. The older methods of cutting hay with a flat mower, letting it dry and then raking it into windrows didn’t kill beetles and gave them time to leave the hay.

      If you suspect your horse has eaten hay with blister beetles in it, contact your veterinarian immediately. Check for blisters in the horse’s mouth. When a horse eats blister beetles, cantharidin causes sores and ulcers in the entire digestive tract, from the mouth and lips to the anus, leading to shock, dehydration and death. The horse seeks water but cannot swallow.


The striped blister beetle is highly toxic and breeds in the southern U.S.

      Symptoms are usually colic-like and few cases are reported unless a large number of horses or a particularly valuable horse dies and an autopsy is performed. If the ingestion of the toxin is not high enough to kill, an owner often assumes that their horse simply had colic and recovered.


Reported poisoning cases in RMR distribution area

      Blister beetle poisoning is not required to be reported to a state veterinarian, and so there may be poisonings in states that have no official reports.

      State Veterinarians in Washington , Oregon , Nevada , Utah , Montana , and Idaho have all stated to RMR that there have been no reported cases of blister beetle poisoning of horses in recent years.


Blister beetles often swarm together to feed.


      Wyoming State Veterinarian Jim Logan says that while blister beetles have not been a problem in 2013, they have been in the past. “Four years ago, there was a fairly extensive outbreak in a few counties.”

      Logan says that people should always watch for blister beetle evidence in alfalfa hay and for acute colic in their horses after eating alfalfa hay. He also cautioned that people should be aware of where their hay comes from.

            Logan’s advice is probably good for all horse owners who feed alfalfa hay.

Copyright 2013 Rocky Mountain Rider. ALL RIGHTS RESERVED. Reproduction of any editorial material, artwork and photos is strictly forbidden without express written permission of the publisher. For information about reprint rights, please contact the editor;

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