This is especially true in the relatively arid states of the west, central and northern Great Plains. All species of blister beetles produce a toxic substance called cantharidin. The amount of toxin produced varies considerably between species. The ash-gray, black, striped, and margined blister beetles are a few of the more common species which produce levels of cantharidin capable of poisoning livestock. Livestock come in contact with blister beetles when they consume infested alfalfa hay.
Horses are most susceptible to the toxin, while sheep and cattle are more tolerant.
Researchers have determined the lethal dose of cantharidin to be approximately 1 milligram per kilogram of horse body weight. This means that about 200 blister beetles could have levels of toxin sufficient to kill an adult horse. In addition, an average of 5.0 mg of cantharidin has been found in striped blister beetles, which indicates that 30 to 50 adults could be potentially lethal. However, even a few beetles may cause colic in horses. Cantharidin can also be lethal to cattle and sheep.