What are blister beetles, and why does this even matter?


Blister beetles are sort of cute, as far as bugs go.  They come in many colors and varieties, some have stripes, some have dots, some are mostly one color.  Usually not longer than an inch, they seem pretty harmless.  They can also kill your horse, and quickly.  This happens when your horse eats one (or more) of these bugs, which doesn’t seem likely, except that these bugs love alfalfa blossoms.  More on that later… let’s cover the gross stuff first.


Although the colors may change a bit, the body design is the same for blister beetles. 


Cantharidin is the name of the poison that resides in the living AND dead blister beetle. When a beetle is crushed, the toxin is released and causes blisters.  Here’s where it gets horrible - if your horse eats a few of these guys, his digestive system will blister as the beetles pass through.  From his mouth to his gut.  This causes inflammation, which might look like a colic due to the pain.  He’s likely to run a fever, start sweating like mad, have diarrhea, his TPR will elevate, and he will urinate frequently.  As thing progress, secondary infections take over, in addition to bleeding.  His electrolytes will be thrown out of whack, especially his calcium, which directly impacts his heart.  Damage to his heart is likely.  


Death is likely, usually within 24 to 72 hours.  This doesn’t give you much time to get the Vet out, so don’t wait.  It is possible for horses to recover from this, so get the Vet involved fast.  There is no cure or antidote, so your Vet will need support your horse with fluids, toxin absorption, correcting the electrolyte imbalance, and pain management. 


Blister beetles are also toxic to sheep and cattle, so be sure there are no other animals on your farm that are affected.  


One of the best things you can to do prevent blister beetles from invading your farm is to be knowledgeable about where your horse’s hay comes from.  Additionally, inspect your horse’s alfalfa hay closely before feeding.  Because blister beetles feed on pollen and alfalfa blossoms, it’s likely that other types of hay won’t contain blister beetles, but it’s still worth inspecting your hay.  


Inspect before you feed. 


Knowing a little about how hay is made helps, also.  For alfalfa, it’s likely that the first and last cuts are going to be blister beetle free.  The beetles like to congregate on alfalfa in the mid to late summer, well after most first cuttings.  The beetles are usually gone by the last cutting.  For more on the hay cutting and baling process, read this. 


You can also take some precautions by making sure the hay is cut and baled separately, without a crimper.  When hay is cut and baled at once, the blister beetles can’t escape.  If the hay is crimped, they can’t escape.  Talking with your hay farmer about his cutting times and methods can give you an idea of how safe the alfalfa is.  

Have you ever found a blister beetle in your horse's hay?