As you probably can guess, any insect with a name like blister beetle is not an insect to approach with a casual attitude. Blister beetles contain a chemical called cantharidin. Instead of a painful bite or sting, the primary defense strategy used by blister beetles is to release canthardin on whomever is causing them trouble. In humans, canthardin can cause painful blisters within 24 to 48 hours. But the threat of canthardin continues in blister beetles, even if the blister beetle is dead.

One of the primary challenges associated with blister beetles is that they like alfalfa and can get baled with the harvested alfalfa. These dead blister beetles still contain canthardin and if ingested by the unsuspecting horse, these blister beetles can cause illness or even death in livestock. Horses that have ingested blister beetles in their alfalfa can exhibit symptoms of colic, restlessness, depression, congested mucous membranes or sweating. When these symptoms are present it’s time to contact a veterinarian.

The home gardener may encounter blister beetles feeding on peppers, radishes, or green beans and even sometimes on weeds. A large group of blister beetles can do quite a bit of damage in a short period of time.

One of the challenges in identifying blister beetles is that there are about 7500 different varieties but in Oklahoma the 4 more common varieties: margined, black, ashgray, and striped. Irrespective of the variety, blister beetles are usually about 1/3 to 2/3 inches in length with a long body, slender thorax (neck), and a larger head.

Adult blister beetles emerge in the spring and immediately begin to feed and mate. Females lay their eggs in the soil and these eggs will overwinter in the soil to emerge in the late spring or early summer the following year. Interestingly, when the eggs hatch, what emerges looks a little like a grub but it immediately begins to look for solitary bees or grasshopper eggs to feed upon.

Discovering blister beetles on your plants can be a little shocking because the like to congregate together, making them look like an invading insect army on your plants.

If you have a problem with blister beetles, there are some things you can do. First of all you can remove them manually and destroy them. However, when doing this, be sure to wear protective clothing because you don’t want any of that canthardin on you. By protective clothing I mean long sleeves, gloves, and long pants, all of which are perfect for this weather…

Organic pesticides containing pyrethrin work well on blister beetles for the home gardener. Neem oil is also a good choice, but in this heat, be sure to do a test spray of neem oil since horticultural oils and high outdoor temperatures don’t always get along well with plants

If you have horses and are concerned about blister beetles in your alfalfa feed, you can contact our diagnostic center to be connected with an agricultural educator. Good luck.

You can get answers to all your gardening questions by calling the Tulsa Master Gardeners Help Line at 918-746-3701, dropping by our Diagnostic Center at 4116 E. 15th Street, or by emailing us at mg@tulsamastergardeners.org. Photo: Clemson University