Surprising Information About the Black Blister Beetle

September 13, 2022

On the Mitigation Lands this time of the year the flowers that are blooming can be full of many types of pollinators. The variety of butterflies, bees, and beetles keep a person engaged in close observation of all the late summer flowers. The more common sky-blue, aster-like flowers are visited by a dark black beetle that eats the blue petals. Upon further research and review of the photos I took, these insects were revealed to be blister beetles, and this one is the punctate blister beetle, Epicauta puncticollis. This is a beetle aptly named as it contains a chemical defensive fluid they can secrete, called cantharidin. If the beetle is crushed against our skin, it can cause severe blisters. As I watch the soft-bodied, dark black (with a blue sheen) beetles with many small dents, eating the Chicory petals, I have a newfound respect for them. They are soft bodied with “particularly soft leathery wing covers with a head that is turned down,” according to the California Field Guide to Insects 2nd Edition, published in 2020. The punctate blister beetle is mostly seen in late spring or summer and is found all over the state.

Blister beetles deposit their multitude of eggs on the ground. Their larvae have complicated and interesting life stages, the first being a silverfish-looking triungulin that moves rapidly around looking for hosts. The host can be grasshopper eggs or ground dwelling bee larvae, and when they find one, they molt into a larger maggot-like form that eats this host. The authors state that the maggot larvae then molt into a “pupalike larvae and then into further feeding stages that complete the larval growth by feasting on the host.” The adult beetles eat plant material and flowers, not other insects. On the east coast there have been infestations of agricultural crops, such as alfalfa. These infestations do not damage the crop too much but if the beetles are harvested and then dried with the hay, the cattle or horse feed can cause illness and sometimes death to livestock. This is again because of the chemical, cantharidin.

The chicory plant, Cichorium intybus, which is frequently visited by the blister beetles, is a native of the Mediterranean region and was brought here by European settlers in the 1700’s as a cheap coffee substitute. It is still used all over the world as a coffee additive or as a stimulant beverage made from the dried and roasted root. This is a common roadside weed in the Willits Valley that is visited by different pollinating butterflies and bees throughout its long blooming season June into fall.