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Blister beetles

by Wyoming Livestock Roundup

Published on Jan. 25, 2020

According to Kansas State University (KSU), blister beetles are a type of beetle that secrete an irritating substance called cantharidin. The technical description for blister beetle toxicity is canthanridin intoxication. There are thousands of species of blister beetles. 

            KSU continues, “There are several developmental stages of blister beetles, the larvae may ingest other insects or even grasshopper eggs, while adult blister beetles may feed on flowers or leaves of plants such as alfalfa, particularly when in substantial bloom, substantial being at over 20 percent.”

            “Blister beetles often swarm, and if they are feeding on alfalfa in the field, they may be harvested with the hay in a localized area due to this swarming nature of the insects,” says KSU. “Most commonly, beetles are abundant during the mid-summer months, so the second cutting of alfalfa is the stage that is most likely to be consumed by beetles and a potential threat to horses.”

            “In general, early and later cuttings of hay are less likely to have beetles present. The first and last are considered safer cuttings,” according to KSU. “It is important that the individual purchasing the hay speak with the farmer regarding the cutting of hay and the method used for harvesting.”

            KSU continues, “When the hay is cut and baled at the same time, generally with a crimper, this makes it harder for the beetles to escape the harvesting process. Therefore, it is ideal to purchase alfalfa where a crimper was not used.”

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Horse health 

Blister beetles can be toxic to horses 

            Even after years of being dead, the blister beetle and their toxic remains can still spell trouble for horses who may ingest them through forages such as alfalfa. 

            Wyoming State Veterinarian Jim Logan explains the beetles have a toxic principle known as cantharidin. 

            “Cantharidin is a contact irritant that basically burns mucus membranes such as the tongue and mouth and intestinal tract,” says Logan. “If the toxin gets down into the gut, it can absolutely fry the intestinal tract before the animal dies.” 

            Logan notes he has noticed issues with blister beetles disproportionally affect horses in comparison to ruminants such as sheep and cattle. However, ruminants are also susceptible to the toxin.

            “Anything that eats hay could be affected, but ruminants don’t seem to be as affected as horses in my experience,” says Logan. 

Signs and treatment 

            “Blister beetle poisoning is difficult to treat as there isn’t an antidote for the toxin and no drug we can give them to counteract the symptoms,” says Logan. “One thing we can do is administer a palliative treatment such as giving them oil to combat absorption  of the toxin.” 

            Logan notes colic is a primary symptom of the toxin, and vets can treat the condition as they normally would. 

            The toxin can also cause damage in the kidneys, bladder and liver. 

            “Another sign of the toxin is excessive salivation or going off of feed due to pain caused by ulcers in the mouth,” says Logan. “This is often the first sign horse owners will notice.” 

            According to the American Association of Equine Practitioners (AAEP), other clinical signs of the of the poisoning include endotoxic shock, salivation and anorexia, watery feces, bloody stool, cardiac arrhythmias and hematuria. 

            AAEP also notes depending on the species of beetle, as few as four to six grams of blister beetle can be deadly to a 1,100 pound horse.

Testing and prevention 

            “Testing after the fact is often a moot point because the beetles are often deadly to horses,” says Logan. 

            He continues, “As far as prevention goes, a lot of it has to do with how and when producers harvest hay.”

            Logan notes crimping hay, which is a popular process to aid in quicker drying of the forage, can be a cause of beetles being killed and stuck in the bales. 

            “This is often when the beetle gets killed in the haying process,” says Logan. “If the beetle gets killed during crimping and stuck in the bale, this is when we run into danger for horses.” 

            “In the past we have had issues with beetles in Wyoming,” says Logan. “It has been relayed to me through agronomists that the beetles like a standing crop and won’t stick around too long once it’s knocked down, which is why skipping the crimping can be effective.”

            University of Wyoming Extension Forage Specialist Anowar Islam notes an earlier cutting can also prevent the beetles from being an issue, as they often emerge from their over wintering following the first cutting of alfalfa. 

            “Using first cutting hay can be beneficial for horses and a safe way to avoid the bettle, although the hay may not be as nutritious as a second or third cutting would be,” says Islam. 

            Islam notes spraying for the beetle should be a last resort if nothing else works, as spraying can also harm beneficial insects in the field. 

            “Producers need to talk to their neighbors and raise awareness of the beetle and formulate a plan of attack,” says Islam. 

            Similarly to Logan, Islam recommends cutting earlier to avoid issues with the beetle.

Callie Hanson is the managing editor of the Wyoming Livestock Roundup. Send comments on this article to

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