Blister beetles still active in alfalfa

August 22, 2023 7:45 AM
Blog Post

I received several questions about blister beetles in alfalfa hay over the weekend. I thought it might be helpful to review what they look like and provide a few management tactics to minimize the negative effects for animals.

There are at least four species in Iowa, including a) black, b) ashgray, c) margined, and d) striped. Photos by a) Joseph Berger, b) Whitney Cranshaw, c) Clemson University, and d) Clemson University at Adults are typically active May - October but the highest numbers are observed in July and August. These beetles tend to aggregate in alfalfa, forming small swarms. They are also more likely found along field edges. People notice them when harvesting because they get disturbed and attempt to fly away.  

Various blister beetles in Iowa.

Male blister beetles secrete an odorless and colorless liquid that contains cantharidin.* When droplets of cantharidin touch the skin, it is absorbed by the lipid layers, leading to cell breakdown and blistering. If ingested, cantharidin can quickly cause burning and blistering of the mouth, throat and stomach; and cramping, vomiting, colic, and frequent urination. In severe causes, it can cause death to humans and other animals.

It is important to know that beetles don’t have to be alive to cause problems. Dead, especially crushed, beetles still have cantharidin and can remain toxic for a long time. The severity of the reaction depends on the type of animal, plus the size and health of the individual. Horses are most sensitive to consuming blister beetles. A general threshold is that eating 25 to 300 beetles can kill a mature horse. Cattle and sheep are less susceptible, but blister beetles will reduce milk production and digestibility of hay. If you suspect blister beetle poisoning, contact a veterinarian immediately for treatment (e.g., activated charcoal, fluids, etc.).

Considerations for feeding alfalfa hay to animals:

  1. Save the first cutting for the horses. Beetle activity is lowest in the spring and this would likely decrease the chances of having beetles in the bales.
  2. Cut alfalfa well before flowering. Beetles are attracted to pollen for food. Reduce flowering weeds within and around alfalfa fields, too.  
  3. Avoid crimping or conditioning plants because equipment can crush beetles. Wind-rowed hay will allow beetles to leave and avoid being baled.
  4. Scout for beetles with a sweep net before cutting.
  5. Inspect underneath windrows for the presence of beetles before baling.
  6. Insecticides will likely kill the beetles but they can still cause issues if the dead adults are included with the bales.
  7. If people buy hay from others, it would be important to know what cutting the bales came from (first cutting preferred).
  8. Inspecting hay before feeding is recommended, although very time consuming.

* Maybe TMI (too much information) but male blister beetles secrete cantharidin for defense and also give to female beetles as a copulatory gift. In humans, it is purposefully used as a topical application for warts; and orally ingested for ulcers, rabies, and anticancer treatments. Cantharidin is also known as “Spanish fly” and still used as an aphrodisiac around the world. Read more here.


Erin Hodgson Professor

Dr. Erin Hodgson started working in the Department of Entomology, now the Department of Plant Pathology, Entomology, and Microbiology, at Iowa State University in 2009. She is a professor with extension and research responsibilities in corn and soybeans. She has a general background in integrated...