Blister Beetles 

by Haley Ruffner
13 July 2022 | [email protected] | 307.205.6628


Anyone who feeds Western alfalfa is familiar with the dreaded signs of cantharidin toxicity from blister beetles: colic symptoms, frequent urination, elevated heart and respiration rate, fever, mouth sores, kidney damage, and digestive issues. In humans, contact with the toxin raises blisters on the skin, hence the insect’s name. As the only treatments currently available are supportive rather than curative, a horse’s chance of surviving the ingestion of blister beetles depends largely on how much cantharidin is in their system and how early supportive treatment began. 

The stable nature of the toxin that blister beetles excrete means that even if no beetles are visible in the end product, the hay could still be contaminated if beetles were killed during cutting. Knowing how to recognize blister beetles—a half inch to an inch in length with a narrow body, colors ranging from black to brown (sometimes with orange stripes), and a soft body—is essential for any horse owner who feeds alfalfa. Different species of blister beetles exist across the US, all with different characteristics and tendencies; for example, striped blister beetles tend to congregate in tight clusters. They all, however, share the toxin cantharidin and are just as dangerous.  

  Although buying alfalfa only from a trusted supplier and knowing how to identify the insects can mitigate the chances of encountering an infestation, the only surefire way to avoid blister beetles is to not feed alfalfa. Farmers can take measures to lessen the chances of an infestation, but ultimately these are not a guarantee. Baling hay without a crimper can lessen the chances of cantharidin toxicity because it allows beetles to move out of cut alfalfa during the drying process. According to South Dakota State University entomology field specialist Patrick Wagner, blister beetles will move out of cut hay within three to six hours. Baling with a crimper or hay conditioner results in high blister beetle mortality rates, leaving their crushed bodies in the windrows to be baled into the hay. Raking hay before baling can also help to dislodge any dead beetles remaining in the hay. 

According to researchers at the University of Missouri, insecticides like Sevin can be used to kill the beetles, which will then fall to the ground and should not be picked up during harvest. Alfalfa treated with an insecticide must not be harvested for one week after application, after which the fields should be inspected for signs of reinfestation. Spot treatment insecticides are also common, as is the practice of leaving cluster infestation areas unharvested. A sweep net is an efficient way to monitor for the beetles’ presence and determine the necessity of spraying fields. Most sources agree that monitoring should be done consistently from June through September.  

Other factors that impact the likelihood of blister beetle infestations are maturity at harvest, time of year, and presence of grasshopper larvae. Blister beetles are attracted to flowering alfalfa and weeds, so managing harvest time, controlling weeds, and cutting pre-bloom can lessen the chances of blister beetles being present in the hay. Blister beetles usually begin to emerge after the first cutting in the Western states, so feeding a boot stage first cutting alfalfa (5-10% bloom) is the safest bet to avoid cantharidin toxicity in horses. Once the beetles have begun to emerge in midsummer, they are attracted to areas with a large grasshopper population because they feed on grasshopper eggs. Controlling the grasshopper population, or at least keeping tabs on it, can give farmers a good idea of the likelihood of potential blister beetle infestations. While blister beetles are overwhelmingly a Western hay issue, they can occasionally be observed in fields of Eastern-grown in fields that experienced a large grasshopper infestation the previous year.  

While some sources tout buying only local hay as a way to prevent getting blister beetle-infested hay, this isn’t always the case—trust is a stronger foundation than proximity alone when sourcing hay. A farmer or broker you trust and who has high quality standards and strict monitoring practices can be a safer option than the guy down the road selling alfalfa. Despite the lack of a tried-and-true way to prevent all possibility of blister beetles in hay, all good farmers monitor their product closely and will never knowingly sell infested hay—after all, the profit from selling an infested load to an unknowing customer will never outweigh the irreparable damage to their relationship with customers and their reputation. 

Other options to increase safety in feeding Western alfalfa to horses include stocking up on first cutting hay to avoid having to buy potentially more dangerous late-season cuttings or switching to a grass mix hay towards the end of summer if that fits within your horse’s nutritional needs. If you live in the central United States, you could also consider switching from Western alfalfa to Eastern alfalfa for your later-cutting load purchases if shipping costs are comparable. As with all other toxins and issues horses are susceptible to, all we can do as horse owners is to monitor our animals closely, do our due diligence in sourcing hay, and stay educated on where our products come from and best practices in agriculture.  



Haley Ruffner is a Sales Broker for Aden Brook and an accomplished writer on the topic of horses.



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