Not Just Another Pest

The alfalfa-loving blister beetles are more than just a nuisance—they are potential killers when ingested by horses. Here's what you should know.

Blister beetle poisoning in horses was first identified in 1969 after a herd of horses in Oklahoma became ill and died after eating hay with beetles in it. Since, small numbers of cases have been reported across the continent.

The beetles generally feed on alfalfa because of the blooms, in addition to other blooming plants found in the field. Once ingested, however, these insects can be a horse’s worst nightmare.

The beetle’s defense mechanism is a substance called cantharidin, which is deadly to many animals. When a horse eats a beetle, the lining of the digestive tract becomes blistered. These blisters rupture causing sores and ulcers, and a great deal of tissue damage. The pain in the gut then causes colic so the blister beetle poisoning is generally not suspected until several horses get sick from eating the same hay.

And it doesn’t take much. Research at Texas A & M has shown that half a milligram of cantharidin per kilogram of the horse’s weight is enough to be fatal. It takes only 3 to 10 beetles to kill a horse, depending on the type of beetle. A fatal dose can also accumulate over several days if the horse eats a beetle or two with each feeding. Only about 20% of horses that show signs of blister beetle poisoning recover with treatment.

The poison is deadly whether the horse eats hay containing live beetles or shredded remains of dead ones in last year’s hay; cantharidin remains deadly for a long time.

Other signs to watch out for are: As cantharidin is absorbed by the horse’s system, his temperature rises due to massive inflammation throughout the body, and his gums become purple due to circulatory failure. He may have a foul body odor, caused by failure of the kidneys to remove the offending substance from the bloodstream.

Frequent urination is another sign, caused by inflammation of the urinary ducts. He also becomes dehydrated. Body temperature may rise to 104 degrees F and pulse up to 72 or higher, sometimes up to 100. Respiration increases, sometimes with jerking of the abdomen. The horse may tremble or be stiff in his movements.

Treatment for blister beetle poisoning can sometimes save a horse if begun immediately—and if the horse has not eaten a large number of beetles. The best treatment seems to be replenishing fluids intravenously to help keep the horse from going into shock, minimizing pain with drugs, and giving activated charcoal to absorb the poison in the gut time.

Blister beetle deaths are reported infrequently, but Sue Blodgett, University of California pest management specialist, says poisoning cases are more common than reported. A cow or inexpensive horse is usually not examined for cause of death.

And, poisoning is becoming a bigger problem than in the past because alfalfa hay is now shipped more widely across the country.

Blister beetle contamination is unpredictable. Some years are worse for infestation than others. When there’s a large population of grasshoppers, there is an increase in beetles the following year, since the beetle larvae feed on grasshopper eggs laid the season before.

The newer haying methods have also increased problems with blister beetles. Older ways of harvesting—cutting hay in a flat swath, letting it dry, then raking into windrows—gave beetles a chance to leave the hay, and dead or injured beetles tend to fall out as hay was raked. By contrast, the new methods crush and retain beetles.

Even insecticides are not a foolproof solution because beetles killed by spraying remain in the field and can still contaminate the hay. And new swarms can come in even after the field is sprayed.

The time of cutting is important. The first cutting of alfalfa is often safest for horses, because adults of most blister beetle species do not emerge from the ground until summer. They feed primarily on alfalfa flowers, eating the pollen, so hay cut in late July or August, especially if cut after starting to bloom, is much more susceptible to blister beetle contamination than early hay.

Whether cutting your own hay or buying from somewhere else, a few precautions, and a few questions, may prevent a tragedy at your farm.






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