Blister beetles are sometimes baled into alfalfa and contain a toxin, cantharidin, that causes extensive intestinal, urinary bladder and heart damage in horses. The toxin from as few as 10 (sometimes even fewer) beetles can severely injure or kill a horse.
When feeding alfalfa be alert for early symptoms of oral irritation, such as drooling, playing in the water and head shaking. Horses treated aggressively at this stage may be saved. Later signs include colic, bloody urine, stretching out to urinate frequently and eventual collapse and death.
Blister beetles are a bigger problem in the Southwest than in northern states but can occur anywhere. They are ?? to 1 inch long, cylindrical with a distinct neck and range in color from black or brown to gray, some with orange stripes. Larvae feed on grasshopper eggs and heavy infestations follow years with high grasshopper numbers. Only the mature beetles are toxic, so hay harvested before July (i.e. first cutting) is not likely to be infested. Flowering weeds also attract the beetles, making hay with obvious weed content more dangerous.
Even the dried juice from beetles crushed by harvesting equipment (hay crimpers/conditioners or even the tires of sicklebar mowers) is toxic. When harvesting hay for horses, it is recommended that only self-propelled harvesters with wide set wheels be used to minimize the chance of killing beetles. Live beetles will leave the cut hay when it is curing in the field.
It is virtually impossible to certify any alfalfa hay as being free of blister beetle. The beetles tend to concentrate themselves in scattered small areas throughout the field resulting in large numbers of bales being free of beetles while only isolated bales, or even sections within a single bale, are contaminated. The absence of beetles doesn’t guarantee the hay is safe anyway, as crushed beetle parts, or the juices from them, are equally toxic.
A monitoring system for alfalfa hay that could chemically test for cantharidin as the hay is being harvested has been proposed as the only potentially feasible way to guarantee safe hay, but such as system is not yet available.