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Blister Beetles get their common name from the irritating reaction of their body fluids with animal skin or delicate membranes. Although some species of blister beetles are considered a beneficial insect because the larvae eat grasshopper eggs, the adults are considered an important pest because, in addition to eating foliage, they cause health problems to livestock when ingested in contaminated hay.

Blister Beetles are found throughout the entire United States; however, heaviest populations occur west of the Mississippi River. Blister Beetles vary in size and shape but can be recognized by the first portion of the thorax, just behind the head, which is narrower than either the head or first pair of wings. Blister beetles contain the blistering agent "cantharidin" in their hemolymph (the blood of insects). Cantharidin is highly toxic when ingested by horses or other livestock and may cause illness and even death in these animals. It is a very stable compound that retains its toxicity to livestock even when dried remains of beetles, that have been killed in the harvesting process, are fed along with forage. Cantharidin is produced only by male blister beetles and is stored until mating. Mating status determines whether females contain the toxin.

Adults emerge in late May and June and feed on leaves in the tops of plants, including alfalfa. Emergence of adult blister beetles typically occurs after the first cutting of alfalfa is completed. Blister Beetles normally do not migrate long distances. In alfalfa, they are often found within 50 years of the field margin. Scouting for blister beetles should be concentrated along field borders.

Measures that alfalfa producers can take to avoid blister beetles in the hay:

Spray border areas to reduce chances of blister beetle migration.
Cutting without using crimpers and avoiding wheel traffic on freshly cut alfalfa.
Spray entire fields with a short residual insecticide just before harvesting. Spray fence rows and field borders as well.
Inspect fields shortly before cutting and spray only the infested areas if any are found.
The insecticide Sevin is registered for Blister Beetle control in alfalfa.

Things an alfalfa buyer should do:

Know your alfalfa supplier.
Ask producers what precautions were taken to avoid presence of blister beetles in forage.
Inspect hay before feeding if presence of blister beetles is suspected.
If feeding small amounts of alfalfa, examine each flake for concentrations of dead blister beetles.
Purchased hay harvested before May or after September. This still does not guarantee a lack of problems with blister beetles but will reduce the risks significantly.
If symptoms appear, call your veterinarian immediately.

Symptoms in horses are dependent on the amount of cantharidin eaten. If a large amount of cantharidin is consumed, a horse may die within 6 hours. If a small amount is consumed, only depression or mild colic (pawing, looking to the side, stretching) may occur. A frequent symptom of cantharidin illness is placing the muzzle in water and playing in the water with the lips and tongue. Severe poisoning may result in low blood calcium and magnesium. These low electrolyte levels may cause stiffness or an exaggerated "goose-stepping" gait. An exaggerated contraction of the diaphragm may also occur. Horses that survive at least 24 hours may strain frequently and void small amounts of darkened urine (blood in urine). Any horse showing these symptoms should be promptly examined by a veterinarian and the forage inspected for presence of blister beetles.

Article information by: Oklahoma Cooperative Extension Service. Phillip Mulder - Area Extension Entomologist, Richard Shawley - Professor, Department of Veterinary Medicine and Surgery, John Caddel - Forage Agronomist. Article by: Margie Turner