There are over 2,500 species of blister beetles found throughout the World and approximately 250 species in the United States. The most common in our region are the ash-gray blister beetle, black blister beetle, striped blister beetle and the black and gray blister beetle. All have elongated body shapes with a very narrow thorax. This cylindrical shape seems to be characteristic of blister beetles. They also possess a chemical held within their legs called cantharidin. This chemical can be potentially harmful or even deadly to horses. It is believed that as few as 550 beetles, if ingested while grazing, or eating hay, can kill a foal or small horse (275 pounds or less). This is also dependent upon the species of blister beetle present and how sensitive your horse is to the chemical. For those pasturing your horses you need to be diligent and apply control measures to eliminate these beetles from your pastures, or hay fields. This same chemical is also used to create a controversial drug called “Spanish Fly.” It is this chemical that earned the beetle their common name of blister beetle. When skin comes in contact with the chemical a reaction occurs and can create painful blisters. I myself have experienced this. A few years ago, one landed on my neck, I brushed it away, but not before it released a healthy dose of cantharidin and left blisters on my neck that were painful for days. This is chemical warfare at its best in a bug-sized package. It is believed that many species may have as much as 50% more concentrations of cantharidin than other species. The striped blister beetle is one such species with higher levels of this chemical.
I don’t want to sound all doom-and-gloom when it comes to these insects, like nearly all bugs there is a good side. With the blister beetle it comes in the form of pest control, and in the creation of various drugs. One of which treats pox viruses like chicken pox.
This year, according to several individuals I spoke to, is a boom year for blister beetles. What causes insects to have boom or bust populations from year-to-year? There are often numerous reasons, and in the case of the blister beetle, it could be the past few years of excessive grasshopper numbers. With so much food available to the blister beetle, their survival rate would be greater as would their population numbers. We also had a milder winter, and plenty of moisture early on in the spring. All of these conditions combined could have contributed to the explosion of blister beetles this year.Love them or hate them, there is both good benefits and downright bad effects caused by the blister beetle. If you have horses and notice blister beetles in your pastures, then control may be warranted. If you have garden produce being destroyed by the adults feeding habits control may be needed. However, if the feeding is not causing issues, leaving them may be the best choice, thus allowing them to do their best work in the form of controlling those damaging grasshoppers.