Hand-checking for dead blister beetles in any hay that includes alfalfa is the only way to know for sure it isn’t contaminated with the toxic insects, warns Sonja Swiger, Texas AgriLife Extension Service entomologist.

Due to this year’s drought, many Texas livestock producers are getting hay from out of state, which puts cattle and horses in danger of blister beetle toxicity, says Swiger. Blister beetles are feared by horse owners because they can cause colic, diarrhea, bloody feces, body tremors, fever or death if ingested. But the toxin also causes cattle to go off their feed.

The lethal dose of cantharidin is approximately 1 milligram per kilogram of horse body weight, according to a North Dakota State University publication. That means about 200 blister beetles could have levels of toxin sufficient to kill an adult horse, she says. In addition, an average of 5 mg of cantharidin has been found in striped blister beetles, which indicates that 30-50 adults could be potentially lethal. Even a few beetles may cause colic in horses.

Discard any hay containing blister beetles; removing dead beetles from the hay won’t make it safe for use as the toxin can still be present, Swiger advises. Cantharidin is odorless and colorless, so the only way to determine if the toxin is in alfalfa hay is the presence of blister beetles.

Blister beetles range from ¾ to 1¼” in length with long, narrow bodies, broad heads, and "neck-like" appearance. They vary in color from black to gray to brown, and some species are spotted or striped, she reports.