Case Studies

Pests We Treat Case Studies: Dangerous American Oil Beetles in Windsor

Tuesday, May 2nd, 2017 by Todd Mosa


I was dispatched to a residence in Windsor after the homeowner contacted Cowleys after finding his yard overrun with “bugs.” During my inspection, it turned out that these were not your usual garden bugs. These intruders were American oil beetles. These particular bugs are mostly found in the eastern United States and parts of Canada. During my inspection, I found these beetles throughout this homeowner’s yard. When this particular beetle infests your property, they make themselves known!

Oil beetles are a type of blister beetle, a group of beetles that secrete oily droplets (hence their name) containing a caustic, poisonous chemical, cantharidin, from their leg joints as a defense mechanism whenever it feels threatened. Adult females cannot synthesize this substance, and receive it as a “gift” from a male oil beetle when mating. 

Cantharidin is so toxic, that even in small amounts, it will irritate human skin and cause the formation of painful blisters. This blistering agent is so powerful that it’s compounded in an ointment used by dermatologists to treat benign skin lesions. There are many things in nature, including insects and plants, that can cause rashes, burns, or other skin irritations just from touching them. It’s a good lesson that if you don’t know what you are dealing with, it’s much safer to look than to touch.  

Cantharidin is also an extremely toxic poison. In fact, it’s so toxic that it can kill horses if they are in their feed and accidentally swallow a few. If humans accidentally ingest this substance, as little as 10 mg (.0004 oz) can be fatal. The female oil beetle will coat their eggs with this substance to protect the eggs from predators. Needless to say, with this substance floating in their bodies, these beetles have few natural enemies. At one time, it was believed that minute doses of cantharidin was an aphrodisiac, and the dried remains of one type of blister beetle, spanish fly, was used to make a primitive “love potion.”  This concoction was probably more effective in making people sick than in making them amorous. 

These beetles are unusual not only because they are one of the few species of beetles that carry a poison. They have one of the strangest life cycles that based on deceiving the poor solitary bee! The oil beetle larvae will sit on flowers in clusters, forming a shape that resembles a female bee. They even emit a pheromone that smells like the female bee. When the male comes in to mate, the larvae latch onto the bee. When the male bee finds a real female bee to mate with, the larvae then climb on board the female who then carries them back her burrow. Once there, the beetle larvae feed on the bee larvae and the honey stores provided by the female bee until they emerge as adults.  

American oil beetles have a unique appearance — they are cumbersome-looking and appear as though they are “out of shape” and have trouble moving around. These beetles, especially egg-carrying females, have an soft overly enlarged abdomen. They have a shell covering that looks like overlapping plates. Since they are flightless, they have shortened wing-pads. They are often black, but they also come in other colors as well.The adult beetles can be seen on the plants that they like to eat, such as buttercups. 

I proceeded to do a full treatment to the outside perimeter and all vegetation on the property to kill these pests.


I proceeded to do a full treatment to the outside perimeter and all vegetation on the property to kill these pests.