Blister beetles found in Reeves County alfalfa

Writer, Steve Byrns, 325-653-4576,
Contact: Dr. Mark Muegge, 432-336-8585,

PECOS – A Texas AgriLife Extension Service entomologist at Fort Stockton warns alfalfa producers and those buying the forage to be on the lookout for blister beetles.

Blister beetles on silverleaf nightshade. (Texas AgriLife Extension Service photo by Dr. Mark Muegge)

“Recently, I’ve been seeing a fairly large outbreak of blister beetles in Reeves County,” said Dr. Mark Muegge. “The extent of this outbreak is unknown, but alfalfa producers need to keep an eye on it. The outbreak could lead to statewide implications since alfalfa from West Texas is shipped all over.”

Muegge said serious problems occur when livestock, especially horses, accidentally eat the insects.

“Blister beetles are attracted to blooming alfalfa and a variety of weed hosts, especially silverleaf nightshade and Russian thistle, where they can gather in very large numbers,” he said. “Beetles feeding on alfalfa can be crushed into the hay during the cutting and baling operations where animals may later eat them along with the hay.”

Muegge said the problem isn’t the beetles eating alfalfa, it’s the extremely toxic compound called cantharidin they contain. Cantharidin is a blistering agent that when eaten causes severe health issues and risk of death in livestock, especially horses. Dead blister beetles are just as toxic as live ones and remain so for months, thus ruining any hay they are in.

Cantharidin poisoning symptoms, called cantharidiasis in horses, are many and varied, according to Muegge, but often include blistering of the mouth, esophagus, stomach and bladder. Colic or diarrhea containing blood and/or mucosal linings may be signs of poisoning. Horses that strain to urinate with little success is another. If they are able to urinate a small amount, he said the urine produced often contains blood.

“Research has shown a lethal dose in horses to be 0.5-1 milligram of cantharidin per pound of body weight,” Muegge said. “Because the toxin varies considerably among the blister beetle species, it’s hard to figure how many beetles must be eaten to kill a horse. Cantharidin concentrations average from about 0.4-5.2 milligram per beetle in some of the blister beetles commonly found in West Texas alfalfa. So a healthy 1,200 pound horse would have to eat about 115 of the most toxic beetles for death to occur.”

Muegge said the beetles vary in size and shape, but nearly all adults have distinctly narrow necks and large heads. The most common species found in West Texas alfalfa are solid velvety gray or rust-colored, or entirely dull black. Adults range from 5/8 to an inch long.

Muegge said when outbreaks occur, they are worst from early June through August.

“Since dead beetles are still lethal, killing them isn’t the answer,” he said. “Besides, the immature stage of the beetle we’re most concerned about loves to eat grasshopper eggs, which most would agree is a good thing, especially this year when most of the state is being overrun with hoppers.

“No, the trick here is to keep the beetles happy, healthy and alive, so they’ll move on out of the alfalfa and take their toxin with them.”

Muegge said successful blister beetle control is a perfect example of integrated pest management where producer awareness far outweighs any benefit from applying pesticide.

“Since blister beetles are a summer pest, those producing horse hay should cut hay early before the pests arrive or late when they are gone,” Muegge said.

“It’s also a good idea to keep alfalfa fields and areas around the fields weed-free. Cut and bale alfalfa when 5 percent or less of the plants are blooming, because the flowers are what attract the beetles. And consider using a self-propelled mower/windrower without crimper or conditioning rollers that crush the hay. Studies have shown these kill far fewer beetles, thus keeping them out of the hay.”

Muegge said spraying pesticides around field margins can sometimes be effective, but strongly cautions against spraying the crop itself.

“Chemical control will sure kill the beetles, but since the dead ones are just as dangerous as the live one, if those killed don’t drop to the ground, there is a real risk of baling the dead, but lethal blister beetles into the hay.”

For more information on blister beetles see


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