Blister beetles belong to a family of plant-feeding insects (Meloidae) that produce cantharidin, a toxic defensive chemical. Contact with it in the blood of live or dead beetles causes blistering of the skin or mucous membranes of sensitive mammals, especially horses. Cantharidin is stable and remains toxic in dead beetles for a long time, so animals may be poisoned by eating crushed beetles in cured hay. The severity of the reaction depends upon the amount of cantharidin ingested and the size and health of the animal. The lethal dose for livestock is estimated to be 0.45 to 1.0 mg of the chemical per kilogram of body weight.

Clinical signs associated with poisoning usually appear within hours and include irritation and inflammation of the digestive and urinary tracts, colic, urinary straining, and frequent urination. This irritation may also result in secondary infection and bleeding. In addition, calcium levels in horses may be drastically lowered and the heart can be damaged. Since animals can die within 72 hours, it is imperative to contact a veterinarian as soon as blister beetle poisoning is suspected.

Meloids in the genus Epicauta, especially the striped blister beetle group (E. occidentalis, E. temexia, and E. vittata) are most commonly associated with poisoning of equids. Blister beetles are attracted to flowering alfalfa, or other blooming plants, and may be trapped and crushed in hay during harvest. Blister beetles can be found in the Central and Eastern United States (see Figure 1). In addition to their high cantharidin content (approximately 4 mg/beetle), striped blister beetles tend to congregate in large clusters along field margins. This can result in high concentrations of beetles in baled hay. Additional blister beetle species have been identified in poisonings in other areas of the United States.

Reducing the Potential for Blister Beetles in Hay

Tips for Hay Producers:

• Learn to recognize blister beeltes and understand their behavior. An effective preventive program will reduce potential problems. There is no efficient way to inspect baled hay carefully enough to ensure that it is free of blister beetles or cantharidin.
• Blister Beetles usually are not active when the first cutting of alfalfa hay is made; harvest at the late bud stage or when the first flowers open for high-quality horse hay.
• Blister beetles are attracted to blooms. Manage harvest intervals to minimize flowering of alfalfa or weeds in hay feidls. Practice good broadleaf weed management.
• Check hay fields for blister beetles before cutting from July through early September. They prefer blooming plants and tend to cluster in masses near field edges. Avoid harvesting areas where beetles are present.
• Avoid crimping hay during harvest. Straddle cut swaths to avoid crushing beetles with tractor tires.

Tips for Horse Owners:

Reduce the risk of feeding blister beetles to horses by understanding blister beetle basics, and by taking the following precautions:

• If practical, grow your own alfalfa to ensure proper preventive management practices.
• Develop a relationship with your hay producer or broker so that you know their production practices and hay quality.

This article was written by Dr. Lee Townsend, Department of Entomology, University of Kentucky. It is reprinted from the Equine Disease Quarterly, published by the University of Kentucky College of Agriculture, Food and Environment Department of Veterinary Science and sponsored by Underwriters at Lloyd’s, London. You may subscribe to this publication for free.