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Photo: Joseph Berger, Bugwood.org

When you think of beetles, an image like the photo to the left probably comes to mind first. This is a common ground beetle. These are only one type of many in the diverse order Coleoptera.  The beetles come in a wide array of sizes, colors and forms. In addition, they occupy diverse habits, have food preferences ranging from dead organic matter to plants and other animals and even fungi, and have a variety of both harmful and beneficial roles, depending on your perspective. The ground beetle pictured here is a beneficial predator of other insects and small prey. One of the very interesting groups of beetles are the blister beetles in the family Meloidae.

The common name blister beetle refers to the skin irritation resulting from contact with an exudate produced by these beetles when they are alarmed or injured. It contains the toxin cantharidin, an odorless chemical found only in this and one other beetle family, Oedermeridae (false blister beetles). Skin contact in humans can result in blisters but they are reported to be only minimally painful if at all and to clear up on their own in a reasonable amount of time. There is a much greater risk associated with consumption of beetles (and the toxin) in hay by livestock, especially horses. Some blister beetle species are attracted to alfalfa, especially during bloom, and when cut for hay during this time, beetles can be killed and inadvertently fed to animals. Different blister beetle species produce varying levels of toxin and therefore have different levels of severity when ingested. Reports indicate that if a horse ingests only 5-10 beetles (or their toxin) it may be fatal.

Striped blister beetle (Epicauta vittata)Photo: Clemson University – USDA Cooperative Extension Slide Series, Bugwood.org

What do blister beetles look like? They have a unique appearance. Wing covers (elytra) are generally shorter than the abdomen (note this in the photos featured in this blog). The neck (between the head and thorax) is very narrow and the thorax is wider at the abdomen than at the neck. Antennae are pretty long and look serrated or segmented. The striped blister beetle shown above is found in the eastern part of the US and southern Canada and the adults feed on some common vegetable plants and weeds, sometimes congregating in large numbers and causing damage. These and some other blister beetles are attracted to lights. As a group, the blister beetles are not nocturnal but are also not strictly diurnal.

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Meloe sp. by Joan Allen, UConn

So that’s the bad side of blister beetles (well, one of them anyway). An explanation of the life cycle of some of them in the genus Meloe (common name oil beetles) will shed some light on another somewhat negative impact. In blister beetles, the larval stages are typically predaceous while the adults feed on flowers or leaves. The earliest larval stage is called a triungulin. In many species, eggs are laid on or near the flowers of the host plant of the adult. After hatching, the triungulins attach to a male bee as it visits a flower and catch a ride to a female bee.  In some cases, large numbers of triungulins cluster together on flowers and emit a chemical attractant that mimics one emitted by the female of the target bee species to help attract males. Once transported to a female bee, the triungulins move from the male to her and accompany her to where she is building a nest, laying eggs and providing provisions for her young. There they leave the female and consume bee eggs, larvae and their provisions.  Adult Meloe sp. are easy to identify: their elytra are much shorter than their large abdomens as shown in the image above (possibly M. impressus or M. campanicollis).

As mentioned above, the adults are typically herbivores, feeding on plant material. Sometimes they will aggregate in large groups and cause significant but localized damage to food crops including those in the Brassicaceae, Amaranthaceae, Asteraceae, Fabaceae and Solanaceae. A couple of years ago there was a localized outbreak of Meloe campanicollis on Brassicas on farms in Connecticut (shown below).

Meloe campanicollis on Brassica leaves. Photo: Jude Boucher, UConn

Another type of blister beetle can be considered a bit more beneficial. Many in the genus Epicauta lay their eggs on or in the soil and the young feed on grasshopper eggs or even on the eggs of other Epicauta sp. The margined blister beetle (E. funebris) and the black blister beetle (E. pensylvanica) are examples of these beetles that occur in the northeast (and are widely distributed in the U.S. and southern Canada). Adult host plants preferred by the margined blister beetle include alfalfa, beet, eggplant, tomato, potato, and soybean. Black blister beetles are often found on goldenrod but will feed on many other plants too. See pictures below.

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Margined blister beetle (Epicauta funebris) by Pamm Cooper, UConn

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Black blister beetle (E. pensylvanica) on goldenrod by Whitney Cranshaw, Colorado State University, Bugwood.org