Blister Beetle, Alfalfa and Horses
By Eleanor Blazer
      Blister beetles kill horses.

      If you feed alfalfa, your horse could be in danger!

      Being able to identify blister beetles is a requirement for all horse owners who feed alfalfa hay. 

      There are more than 2,500 species of blister beetles worldwide.

      Blister beetles are most common in southern and western US states, but when conditions are right they can appear in the east. 

      The blister beetle larvae (baby) feed on grasshopper eggs, so if an outbreak of grasshoppers occurs, blister beetles may also be present. Alfalfa attracts grasshoppers; grasshoppers attract blister beetles.  Some varieties of adult blister beetles feed on alfalfa pollen and the flowers, and it's the adult beetle that can poison horses.

      It's not the bite of a blister beetle that is the problem - it's the chemical they secrete when startled or killed.  Females also coat their eggs with it for protection. The chemical is called cantharidin.  It is a blistering agent.  

      When a small amount of the chemical gets on the skin it causes blisters.  The blisters are uncomfortable, but not exceedingly painful.  The blisters will heal and simple first aid is usually all that is necessary.

      Horses have problems with cantharidin when they eat dead beetles present in alfalfa.  The caustic chemical blisters the digestive tract and creates lesions on other internal organs.  It is very painful and can be a slow agonizing death.

      Each species of blister beetle can have different concentrations of cantharidin. The male contains higher amounts of the chemical.

      Research at Texas A & M showed a half of a milligram of cantharidin per kilogram (2.2 pounds) of body weight can kill a horse.  Using this formula proves that it would only take three beetles of some of the more toxic species to kill a horse.

      Because blister beetles tend to swarm if one beetle is in the alfalfa hay, there will be more.

       Symptoms of blister beetle poisoning are colic, frequent urination, ulcers, dehydration, sores in the mouth and abnormal vital sign readings.  These are symptoms of many other equine illnesses, so if a horse is eating alfalfa hay the presence of blister beetles must be considered- especially if other horses are showing the same symptoms.  Examining the mouth for blisters and sores may help determine the cause of the symptoms.

       If you buy alfalfa hay that is imported from other states you must be able to identify the insect by anatomy.

       Do not rely on color or markings. Stripes, solid black, brown, gray, spots, bright green or turquoise coloration are examples of possible colors and markings.

       The length of the beetle can be a half inch to 1 ˝ inches.  The head is fully exposed.  The neck is very narrow.  The pronotum (the first segment of the body where the first pair of legs attach) is narrow at the neck and wider where it intersects with the main part of the beetle (the abdomen). The wings can cover the entire main section of the body (the abdomen) or be short, depending on the species.  Blister beetles have six legs (three pairs). The antennae are about one third of the body's length.

       For a picture click here.

       The sooner blister beetle poisoning is recognized the better the prognosis for the horse.  The veterinarian may administer mineral oil to help protect and comfort the damaged lining of the digestive tract. The mineral oil may also help move the beetles through the digestive system quicker. Fluids will be given to off-set dehydration and shock. Pain killers will also have to be used. Charcoal may also be administered to try to absorb the toxin.

      If the horse survives, long term kidney and other organ damage may permanently impair the horse.

      In addition to being able to identify blister beetles, the horse owner who feeds alfalfa can take precautions: 

       1. Try to buy first cutting as the adult blister beetle usually does not appear until summer.  Buy it before blossoms have formed, as this is what some blister beetles species feed upon. 

       2. Avoid hay that has been crimped.  Crimping kills the beetles.  If the hay producer does not use a crimper, the live beetles may abandon the field during harvest.

       3. Purchase alfalfa hay from a producer who is aware of blister beetles and tries to prevent them.  Using chemicals to kill grasshoppers, controlling weeds and timing harvests can deter blister beetle infestations.

        While all of these steps can decrease the risk of blister beetles in your alfalfa hay, inspection of each bale and flake before feeding is a must.

         Observation and awareness is the key.

        Proper nutrition and management practices can prevent many problems associated with caring for horses.  You can learn how to provide your horse with a better life-style by taking the online course "How to Feed for Maximum Performance" taught by Eleanor Richards Blazer.

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