Walking On Volume 3, Issue 2, February 2016 | Page 22
For the Health of It
Blister Beetles and Alfalfa:
A Potentially Lethal Mix
By Dr. Lee Townsend
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.
Reprinted with permission from Equine Disease Quarterly, Volume 25,
Number 1
22 • Walking On
Reducing the Potential for Blister Beetles in Hay
Tips for Hay Producers
• Learn to recognize blister beetles 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
fields. 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 preventative management practices.
• Develop a relationship with your hay producer or broker so that you know their production practices and hay
quality.