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Toxic blister beetles found in Montana, North Dakota
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Toxic blister beetles found in Montana, North Dakota

Blister beetle

Blister beetles were found in alfalfa hay this year, as they are many years, but in more numbers. Beetles can be deadly to horses and cause blisters in cattle. NDSU photo.

There have been reports of blister beetle attacks on hay in McKenzie County and elsewhere in North Dakota, along with parts of Montana this year, and Devan Leo, McKenzie County Extension agent, says blister beetles are toxic to livestock.

“Horses that eat alfalfa hay contaminated with blister beetles have a high risk for serious illness and death,” Leo said. “Cattle that eat as little as two beetles can get blisters and inflammation in their mouths.”

Blister beetles produce cantharidin, a toxin that causes inflammation and blistering of internal body tissues. Although all species produce the toxin, the striped blister beetle seems to have higher toxin concentrations.

In a recent study, scientists found the ingestion of from 25-300 beetles in hay could kill a horse. The beetles are protected by oil, so they stay on hay up to 2 years afterward.

Producers have waited for a better hay crop this summer, which makes economic sense.

“Our producers had a nice alfalfa hay crop at 20-40 percent bloom, but they were waiting for 80-95 percent bloom,” she said.

While they waited, the blister beetles moved in, attracted to the nice patches of bloom.

“They had a smorgasbord of blooms to choose from,” she added.

Leo has been out scouting alfalfa fields and told producers the hay can be harvested safely.

“When harvesting, it is important to open the mower conditioner all the way,” Leo said. “Then lay the hay down. While blister beetles can fly, they will lay down with the hay. Because they don’t like dry hay, they will eventually fly away when the hay dries down.”

Leo said blister beetles come in every year, but they usually are not as numerous as they were this year. Cattle and sheep are more tolerant, but studies have shown that two beetles in the mouth of a cow can cause blistering and ulcers, although it won’t kill the animal.

Gerald Stokka, NDSU Extension veterinarian, added, “Clinical signs of blister beetle poisoning usually occur 6-8 hours after ingestion, and affected horses often show signs of colic and depression, although urinary, cardiovascular, and nervous system problems also can occur. Horses consuming a toxic dose can die quickly (within 3-18 hours).”

Blister beetles tend to congregate in certain areas of a field. Thus, hay bales from those areas often contain high beetle numbers, whereas bales from other areas of the field may be beetle-free.

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