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Blister Beetles

Experts offer advice on keeping livestock safe from a toxic pest.

By Becky Mills | Photos By Vladimir Blinov

The blister beetle

The blister beetle

Those two words strike fear into the hearts of hay producers and horse owners. The toxic blister beetle makes itself at home all over the U.S. and in the agricultural regions of Canada. Its toxin is cantharidin, and horses are particularly sensitive to it, although cattle and sheep also can be affected. Here’s advice from the experts on how to control these pests.

Mark Muegge, Texas AgriLife Extension entomologist, explains that killing the beetles isn’t necessarily the answer. “Dead beetles are still toxic. The trick here is to keep them happy, healthy and alive so they’ll move out of the alfalfa and take their toxin with them.”

If you’re looking for the offending pests, Muegge says blister beetles vary in size and shape, but most adults have distinctly narrow necks and large heads. Adults range in size from 5/8 to 1 inch long.

Blister beetles do have a bright side though. First, Muegge says they love to munch on grasshopper eggs. Second, they are generally fairly predictable, which makes it a bit easier to avoid them.

How Much Is Too Much? When it comes to horses and the toxic cantharidin in blister beetles, it is hard to say how much it takes to cause serious damage. Muegge says research shows a lethal dose in horses is 0.5 to 1 milligram of cantharidin per pound of body weight. However, he also says, “Because toxins vary considerably among the blister beetle species, it is hard to figure how many beetles must be eaten to kill a horse.”

In West Texas, he says the most commonly found blister beetles average from about 0.4 to 5.2 milligrams of cantharidin per beetle. So a healthy, 1,200-pound horse would have to eat about 115 of the most toxic beetles in a single feeding to die, “remembering,” continues Muegge, “that many factors, including the horse’s age, general health, weight and breed, and the species of blister beetles consumed, determine the number of beetles [that could] cause mortality.”

Even if they don’t die, cantharidin poisoning is terribly rough for animals. It can cause blistering of the mouth, esophagus, stomach and bladder. Colic or diarrhea containing blood and/or mucosal linings may be signs of poisoning. Horses may strain to urinate with little success. If they are able to urinate a small amount, it often contains blood.

Here is a list of blister beetle habits and what you can do to work around them:

• Don’t send flowers. “Blister beetles are attracted to blooming alfalfa,” says Muegge. “So cut hay at 5% bloom or less.” The beetles’ affinity for blooms goes for weeds too. Muegge says, “Keep the fields free of weeds, and keep field margins clean. Silverleaf nightshade and Russian thistle are attractants to blister beetles.”

• Keep the edge. Bruce Anderson, University of Nebraska-Lincoln Extension forage specialist, says, “Typically, they do not move very far away from the edge of the field.” If you do cut hay from the field’s edge, he recommends playing it safe and keeping it separate, so you can feed it to animals that aren’t as sensitive to the toxin as horses.

• Don’t crimp their style. “Avoid using a mechanical crimper that can kill and crush the beetles,” says Anderson. Don’t forget, their toxin is still there, dead or alive.

• Double-check your timing. Check with your local county agent or forage specialist on this one. In Nebraska, Anderson says blister beetles usually aren’t a problem before the first cutting in mid-June. And in Texas, Muegge says they are predominately a summer pest. So keep in mind it may vary in your area.

• Look before you cut. “Check for them on a fairly regular basis, especially before baling,” Muegge recommends. With any luck, he says, they’ll move on.

• When you can’t wait for relocation. “An insecticide treatment may be necessary,” says Muegge, “keeping in mind that [the producer] may need to raise the cutterhead a bit to help avoid picking up dead blister beetles. Also, he says, be sure to “inspect the baled alfalfa collected from the infested area for blister beetle contamination.”

The blister beetle isn’t the only hazard that can find its way into a bale of hay. Certain varieties of weeds are also toxic. For tips on identifying and controlling them, see “Tackling Toxic Weeds.”

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