Blister Beetles

Scouting alfalfa fields, taking precautions while harvesting, spraying fields or infested areas, and inspecting baled hay all help prevent toxicity in horses from consumption of alfalfa containing blister beetles.  

Blister beetle adults emerge from the soil in late spring or early summer, and large numbers normally will congregate in small areas of fields, usually within 50 yards of a field margin. The predominant species found in Oklahoma is the yellow and black striped blister beetle.  

The first alfalfa cutting of the year normally is free of blister beetles problems because they are not normally in alfalfa fields in large numbers that early.  

Blister beetles contain a toxic blistering agent, cantharidin, which can cause irritation or serious damage to the stomach lining and other organs of the digestive and urinary tracts of horses.  

According to some reports, ingestion of as few as 15 beetles can cause illness, and consumption of 50 to 100 can cause death.  

Because of their congregating nature, remains of beetles killed during the alfalfa harvesting process are likely to be concentrated in a few hay bales or small portions of bales. Cantharidin is a stable compound that retains its toxicity to livestock even when dried remains of the beetles are fed along with the forage.  

When infesting alfalfa, blister beetles prefer to feed on blossoms. Swarms of beetles often are crushed by crimper rollers of the swather and are trapped in the windrowed hay.  

If beetles remain alive in the windrows, most of them will crawl out. They also can be trapped and killed in hay if windrows are driven on before beetles have had time to escape.  

Two of the best ways to avoid blister beetle problems in hay from an infested field are swathing without crimpers and avoiding wheel traffic on freshly cut windrows.  

Application of a short-residual insecticide to infested areas, entire fields, fence rows, and borders can kill beetles just before harvest. Sevin has given good results under Oklahoma conditions. Methoxychlor also is registered for blister beetle control. Both products have a seven-day waiting period before harvest.  

Parathion also can be effective, but it has a 15-day waiting period before harvest. Malathion and the permethrin products have no waiting period, but their labels don't list blister beetles as a target pest, and effectiveness of their blister beetle kill in Oklahoma hasn't been tested thoroughly.  

Livestock owners buying alfalfa should know their supplier well and find out what precautions were taken to avoid blister beetles in hay. If presence of beetles is suspected, hay should be inspected thoroughly before feeding.  

Purchase of alfalfa harvested before May or after September reduces blister beetle risks significantly.  

If toxicity symptoms are seen in horses, a veterinarian should be called immediately. If a large amount of cantharidin is consumed, a horse may die within six hours. Small amounts consumed cause mild colic. A frequent symptom of cantharidin illness of a horse 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, which can cause stiffness and an exaggerated goose-stepping gait. Horses that survive 24 hours may strain frequently, and small amounts of blood may be seen in the urine.  

In addition to having a veterinarian examine a horse showing symptoms of toxicity, remaining hay also should be inspected for presence of blister beetles.  

O.S.U. Extension Facts No. 2072 has photos and additional information about blister beetles.