I found these oil beetles in the North Saskatchewan River valley in Edmonton on the 24 May. They were aggregating in a grassy area not far from Siberian Pea Shrub bushes (Caragana arborescens) which are the dominant shrub in this area. Caragana was first introduced to Canada in the 1930’s as a drought tolerant shrub for shelter belts (and is still recommended as such), and it has since been the scourge of many a natural area.
The Caragana Blister Beetle, Epicauta subglabra (Fall), feeds on Caragana and alfalfa as an adult, but the larvae are typical of oil beetles in that they consume insect eggs–in this case, the buried eggs of grasshoppers. Meloe niger, a larger oil beetle that I photographed earlier, feeds on the eggs of ground-nesting bees.
Why ‘blister‘ beetle? All beetles in the family Meloidae secrete cantharidin, a toxin which can raise blisters on the skin as well as cause inflammation in the digestive tract if ingested. Because the beetles aggregate to mate, and because they feed on alfalfa that is used for hay, dire consequences can result. An agricultural information paper issued by North Dakota State University (pdf) states that a lethal dose of cantharidin for horses would be about 1 mg per kg of horse, concluding that a typical horse would have to ingest about 200 blister beetles to reach a lethal dose. A poster (pdf) by Chris DiFonzo from the Michigan State University Field Crops Entomology program gives the cantharidin content and lethal dosage of common Michigan blister beetles.
Symptoms of cantharidin poisoning include,
“…inflammation, colic, straining, elevated temperature, depression, kidney failure, increased heart rate and respiration, dehydration, sweating and diarrhea. There is frequent urination during the first 24 hours after ingestion, accompanied by inflammation of the urinary tract. This irritation may also result in secondary infection and bleeding. In addition, calcium levels in horses may be drastically lowered, and heart muscle tissues may be destroyed.” (from Blister Beetle Toxicity in Horses (pdf), by Dr. Rebecca S. McConnico, DVM, PhD, School of Veterinary Medicine, Louisiana State University)
Humans are also occasionally struck by cantharidin poisoning, so we must have a Safety Data Sheet for it.
Of course, you can’t write an article on blister beetles without mentioning Spanish Fly…. or the woman who recently found a blister beetle in her organic lettuce… 🙂
Visit The Black Oil Beetle for more on the fascinating life cycle of these beetles, including video.
(Thanks to V. Belov of BugGuide for the ID)