Blister beetle

It might be hard to imagine that an essential part of the horse’s diet could contain potentially deadly hidden toxins. But it’s a hard truth that horse owners must be aware of: Alfalfa hay can harbor blister beetles (Epicauta spp), which can contain a harmful toxic substance called cantharidin.

A member of the Meloidae family, blister beetles live throughout the United States and Canada. Their average body length is about 0.3 to 1.3 inches. A blister beetle’s diet is mainly composed of pollen, blossoms, and leaves of flowering plants, making alfalfa the perfect meal for them. Most alfalfa infestation occurs during late summer and early fall, when the adult blister beetle population also peaks.

Male blister beetles produce a natural defense toxin called cantharidin. This irritant can cause blisters on skin (of both horses and humans) within a few hours of contact, hence the insect’s name. When ingested, cantharidin is lethal in horses with as little as half a milligram per kilogram of body weight, equivalent to consumption of around 125 beetles for an average-sized horse. And it’s not just the live insects that are harmful: The toxin is still effective long after the beetle dies.

Cantharidin negatively impacts horses’ urinary and digestive systems, and signs of toxicity can include colic; diarrhea; elevated temperature, heart rate, and respiration rate; and frequent urination.

There is no specific treatment for blister beetle poisoning, and therapy typically focuses on supportive care, reducing toxin absorption, and administering intravenous fluids, gastrointestinal protectants, and broad-spectrum antibiotics. If treatment is unsuccessful, or if a horse us untreated, death usually occurs within 72 hours of blister beetle ingestion.

Fortunately, there are steps you can take to reduce the risk of inadvertently feeding blister beetles, including:

  • If practical and possible, and your horse requires alfalfa hay in his diet, grow your own so you can use proper preventive management practices;
  • Buy from local sources, develop a relationship your hay producer, and learn about their production practices and hay quality;
  • Buy first cutting hay, since blister beetles are not active early in the season;
  • Because blister beetles are attracted to alfalfa’s flowers, purchase hay cut prior to flowering;
  • Inspect hay closely for blister beetles when purchasing or before feeding alfalfa hay. Do not feed any hay that you think could contain blister beetles; and
  • Consider eliminating alfalfa hay from your horse’s diet if he doesn’t need it and switching to a different type of forage.

Blister beetles can be deadly to horses, but you can reduce that risk by checking alfalfa hay closely before feeding and choosing cuts that are less likely to contain these insects and the toxins they contain.