The Blister Beetle

Richard Haynes

“Bugs are eating our autumn clematis and have nearly destroyed it," complained my wife one afternoon last fall. This was my introduction to a very interesting insect, the blister beetle.

Peering into the damaged ornamental (see Fig. 1), I could see small black insects on leaves and stems (Figs. 2 and 3). I casually picked a few, thinking they must be a species of firefly. Looking at them more closely as they roamed over my hand, I knew they were not our usual red-capped fireflies (Photinus ignites). What were they?

After consulting several books, especially "Beetles" by Richard E. White (Peterson Field Guide series, 1983), I learned that the insects were black blister beetles (Epicauta pennsylvanica). Blister beetles are classified in the family Meliodae, order Coleoptera. Epicauta, with over 100 species, is the largest genus.

There are more than 2,500 species of blister beetles around the world and more than 335 in the United States. The greatest populations of blister beetles in the United States occur west of the Mississippi River.

Blister beetles of the Meliodae family vary in size and shape and often can be quite beautiful. Their coloration includes metallic blues, greens, copper and rose. Some may be striped yellow or spotted yellow and black. These types of beetles are found in the West and Southwest U.S. Those in the East U.S. tend to be duller in appearance.

After mating (tail to tail), the female blister beetle lays clusters of eggs in the soil in late summer. The larvae then crawl over the soil to enter cracks in search of food. Grasshopper egg pods are an important part of their diet. During years of heavy grasshopper infestation, there will almost certainly be a large “crop” of blister beetles.

The larvae spend the winter in a pseudo pupa stage and go through a spring molt. There are ten stages (instars). A short period of activity follows before the larvae enter a true pupal stage.

Adults emerge in mid- to late-summer, typically around the same time, and gather in groups. They move in swarms, and their dietary preferences can cause considerable damage to vegetables, flowers, ornamental foliage and field crops, such as sugar beets and alfalfa.

Alfalfa damage in particular is worrying, not only to growers but also to livestock owners who buy the dried crop for large animal feed. When the alfalfa is cut, beetles that are feeding on the plants are harvested with the crop and may be caught within the vegetation and be unable to escape. They die and can be subsequently eaten by horses or cattle feeding on the dried crop. If enough of these beetles are consumed, the feeding animal can die. Cases of such livestock deaths are reported each year in the U.S. In the 1980s two valuable racehorses died after eating blister beetle infested feed, and this prompted much activity on new ways to harvest these field crops.

While researching these facts, I discovered I had been fortunate when handling blister beetles. For when they are agitated, roughly handled, squeezed or crushed, these small beetles bleed yellow-orange blood, or hemolymph, from their leg joints, primarily the knee. This viscous fluid causes severe, painful burns that form blisters on the skin that may require days or even weeks to heal. By gently handling them, I wasn't harmed.

What causes the blistering and the animal deaths? The culprit is cantharidin (Fig. 5), a compound biosynthesized in the seminal vesicles of the male meloid beetle, which may produce it from the mevalonic acid molecule. The complete biosynthesis mechanism remains unknown. Cantharidin is a powerful vesicant (blistering agent), and its oral toxicity is comparable to that of strychnine and cyanide. Only 100 mg may be lethal to humans. Large animals such as cattle or horses need only ingest as little 4 to 6 g of dried beetles for a fatal reaction.

In meloid beetles, only the males seem to produce cantharidin, and they transfer it with their semen during mating, possibly a kind of “love gift.” The females coat their fertilized eggs with it when they lay them. In addition to protecting the eggs from predators, both male and female blister beetles use cantharidin as an effective defense against vertebrates and invertebrates as shown in studies by Thomas Eisner and James Carrel at Cornell University .

Cantharidin is also the active agent in Spanish fly, a so-called aphrodisiac from the 19th and early 20th centuries. It has had other applications as well. Its toxicity, uses and history are sufficiently interesting to have a look at it in more detail in a future article.