Blister Beetles: Life Cycle & Identification

Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

Blister beetles are a common insect in the U.S. that have an interesting defense mechanism. In this lesson, we'll discuss what blister beetles are, how you can identify them, and learn about their life cycle.

What Are Blister Beetles?

My son and I were out walking the other day and came upon a really cool-looking insect. I tend to respect wildlife, insects included, but my toddler likes to poke and prod just about everything. So he picked up this beetle and squeezed it. Poor bug! But really I should be saying poor me…when my son squeezed the insect, it ejected something that landed on my skin and later caused a really painful blister. I was now very curious what on Earth this bug we met was, so I looked it up. Turns out it was an aptly named blister beetle!

I know I hadn't ever heard of a blister beetle, and I'm guessing you may not have either. These guys are members of the family Meloidae, which contains about 2,500 species. And that stuff that caused my skin to blister is actually a chemical defense mechanism. The beetles contain a toxin called cantharidin, which irritates our skin and causes it to blister. And that's precisely how these guys got their name!

A toxin in blister beetles causes blisters when it comes in contact with your skin
blister on skin

Where Are Blister Beetles?

Blister beetles are common throughout the eastern and central United States. They are not limited to the U.S. though. They are also found in the West Indies, South America, and Central America.

Blister beetles are vegetarians and like to feed on the flowers of plants like sunflowers, legumes, and plants in the nightshade family (tomatoes, peppers, eggplants). Occasionally they will also eat the leaves, but really the flowering parts are their favorite.

Blister beetles may also be found on hay. For people with farm animals, it's important to be able to recognize blister beetles because that chemical they contain can be toxic (in large amounts) to animals like horses, cows, and sheep. This type of poisoning can occur if the beetles are crushed when the animals eat hay with them in it.

Blister Beetle Identification

So how do you identify a blister beetle? These insects come medium- to large-sized, and they have an elongated body that is sometimes referred to as cylindrical. They have a broad head that looks rectangular from above. They have soft, leathery bodies, and their antennae are beaded and thread-like.

The wings of blister beetles fold back on their body and look like body armor, but the front wings are soft and flexible, unlike other beetles that have hard front wings. In the U.S. they often appear somewhat dull in browns, grays, and blacks. In other places though, they may be very brightly colored.

Blister beetles are generally dull in color in the U.S., can be brightly colored in other places, have long bodies, and rectangular heads.
blister beetle

Blister Beetle Life Cycle

Just like other bugs, blister beetles have a life cycle that involves different stages (called metamorphosis). But what's different about these guys is that they have a hypermetamorphic life cycle. This means that instead of having larval stages that look and function similarly, they have larval stages that are visually and functionally different from each other.

The life cycle of a blister beetle is typically about one year, though it can be as short as a month. To start, a female will lay her eggs in a cluster in the soil. This happens in late summer. These initial larvae that hatch from the eggs have well-developed legs and antennae, and are parasitic. They crawl across the soil and look for a host, usually something like grasshopper eggs.

After they find a host, they continue to develop, but now they are mostly immobile and legless! They'll continue to develop through later stages with reduced legs and minimal activity. The larvae will then move on to a pseudopupal or resting stage for the winter. This is an inactive intermediate stage between the larval and pupal phases. The beetles will pupate the next summer and present themselves to the world as adult blister beetles.

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