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Blister Beetles and Horses

Many horse owners may have heard of blister beetles and fear their toxic properties. Their main concern revolves around horse consumption of beetles. Blister beetles often occur in hay-related crops such as alfalfa, which often is a food source for domesticated animals like horses. The problem for the horses and their owners arises from the interesting way the beetles defend themselves. They generate a chemical known as cantharidin. Normally, cantharidin only is inflammatory to the skin and may cause blisters to form. If swallowed, however, it can cause severe digestive distress and in large enough doses can cause death.

When alfalfa and other related crops are grown, the beetles can infest the crop for a couple of reasons. First, the larvae feed on grasshopper eggs that may be in the fields. Outbreaks of blister beetles often can be linked to grasshopper outbreaks. Second, adults are attracted to the flowers of the alfalfa or nearby weeds. A particular species of blister beetle called the striped blister beetle is known to gather in large clusters along weedy field edges.

Regardless, the beetles are harvested and unintentionally ground up along with the crop. When ground, they release the cantharidin into the hay. Depending on the concentrations of the cantharidin, horses and livestock can suffer gastric distress or even death. If a horse may have consumed hay contaminated with cantharidin, the owner should consult their veterinarian.

Growers can help manage blister beetle populations by using timely pesticide applications specifically targeting weedy areas. If horse owners particularly are concerned about blister beetle poisoning, they can grow their own hay or make sure to buy hay from growers effectively managing the beetles. The batch of hay produced first also appears less likely to be contaminated since the beetles do not become largely active until later in the season.

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