Blister Beetles and Alfalfa

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Seeing beetles on a hay crop is alarming, but the sight of blister beetles, which love to feed on alfalfa, is on another level. They’re most common in the eastern and southern U.S. but have been found throughout the country.

These insects were given their name because they literally cause blisters. Their body fluids contain cantharidin, which irritates the skin as well as the gastrointestinal and urinary membranes when eaten. When the beetle is crushed, it releases these fluids. Cantharidin is stable, so storing alfalfa after baling doesn’t reduce its potency. This means that blister beetle-infested alfalfa is a health threat to goats and other livestock.

What Does a Blister Beetle Look Like?

There are more than 300 different species in the U.S., with only a few being the most common. Most species have a long, narrow body (1/2 to 1-1/2 inches long). Their heads are broad, and the antennae are straight and make up a third of the body length. They can be a variety of colors, including black, gray, or striped.

What is Their Life Cycle?

Female blister beetles lay their eggs in the soil, in clusters of 100 to 200, during late summer. These eggs hatch into first-stage larvae, which forage for grasshopper eggs or hide in flowers until an unsuspecting bee carries them to the nest, where they get their nourishment from the eggs or immature bees. Here, they rest, molt, and grow into mature larvae that overwinter. They metamorphose into adult beetles by pupating in the spring and emerging in early summer. The date of emergence is temperature dependent, and they prefer drier weather when the grasshopper population is high.

Once they emerge from the soil, they seek plant materials, particularly flowers and foliage from alfalfa, pigweed, peanuts, soybeans, and other species. They eat the top leaves but prefer the nectar and pollen on the flowers.

The beetles tend to be found in clusters in fields and near grasshoppers. They’re most plentiful in August, so first — or second-cut alfalfa is less likely to be infested.

How do They Affect Goats?

Horses seem to be the most susceptible to blister beetles; cases of complications from blister beetles in goats are rare. This may be because goats determine what to eat based on their senses of sight, smell, and taste, among other behaviors. A 2017 study using webworms on leaves observed that goats shake, probe with their muzzle, spit out, and discard infested leaves.

As little as four to six grams of dried beetles can be lethal. The death rate is up to 50% of the animals affected.

In one documented case out of California, a male goat presented to the veterinary hospital with bloat, recumbency, and dehydration. He had elevated heart and respiratory rates, painful bloat, signs of kidney injury, and was in shock. Blood and urine were tested, and he was treated for a gastrointestinal issue and dehydration. He began to improve after three days and was well enough to go home after six days. Lab results showed Cantharidin intoxication.

After discovering the cause, the owners quickly removed the alfalfa, where they found a single blister beetle, which they submitted to the veterinary hospital.

Goat keepers who believe an animal has ingested a blister beetle should immediately contact their veterinarian. There is no cure, but the goat can be provided with supportive care, such as pain medication, IV fluids, and activated charcoal.

How Can Blister Beetles be Prevented or Controlled?

1. Learn to identify blister beetles so you don’t mistake them for other beetles that like alfalfa. Contact your local extension agent for identification. Sometimes, you can do this over email with photos you take.
2. From early spring through the growing season, look for them in your alfalfa field(s). If heavily infested, treat before and after cutting to ensure they’re gone. Avoid cutting in heavily infested areas. Most beetles in baled hay were probably killed as they went through the harvester. Some beetles will fall to the ground rather than be baled into the hay, but it’s important to realize that even dead, these beetles can be toxic.
3. If blister beetles are found before cutting, they can be treated with Sevin. Contact a local extension agent to determine how to treat.
4. Cut alfalfa at 10% or less bloom. This lessens the chances of field invasion by blister beetles, which are attracted to flowering plants. The first cutting is also safer because beetles aren’t yet established.
5. Control weeds in and around the field to reduce the chance of them migrating to the alfalfa.
6. Control grasshoppers and the weeds they prefer.
7. Take samples, especially around the perimeter and on the side of fields near range land, to avoid cutting infested areas.
8. Burn or bury baled alfalfa you find infested; don’t sell or feed to any livestock.

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Cheryl K. Smith has raised mini dairy goats in the Coast Range of Oregon since 1998. She is the owner of karmadillo Press, and author of Raising Goats for Dummies, Goat Health Care, Goat Midwifery, and the first two books in the Hidden Creek Farm cozy mystery series. www.GoatHealthCare.com

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References:

• Berman, TS, et al. 2017. “How goats avoid ingesting noxious insects while feeding.”Sci Rep 7: 14835. doi.org/10.1038/s41598-017-14940-6.
• Townsend, LH. 2011. “Blister Beetles.” Department of Entymology, University of Kentucky College of Agriculture. https://entomology.ca.uky.edu/ef102.
• UC Davis Veterinary Medicine. 2017. “UC Davis Veterinarians Discover Blister Beetle Toxicity in Goat.” www.vetmed.ucdavis.edu/news/uc-davis-veterinarians-discover-blister-beetle-toxicity-goat.
• University of Minnesota Extension. “Blister Beetles in Alfalfa Hay.” 2024. https://extension.umn.edu/horse-nutrition/blister-beetles-alfalfa-hay.
• Ward, Charles R. 1997. “Blister Beetles in Alfalfa.” New Mexico Cooperative Extension. http://animalrange.montana.edu/documents/courses/ANSC410/Blister%20beetles%20New%20Mexico.pdf.

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Originally published in the June 1,2025 digital issue of Goat Journal