Blister Beetle

Blister beetles are long, soft-bodied beetles in the family Meloidae. They are famous for a chemical called cantharidin, which can cause painful blisters on skin. There are about 7,500 species around the world, and some can be very dangerous to horses and other farm animals.

Blister beetle Quick Facts

Common name	Blister beetle
Scientific name	Meloidae
Kingdom	Animalia
Phylum	Arthropoda
Class	Insecta
Order	Coleoptera
Type	Insect (blister beetle)
Diet	Adults eat leaves, flowers, nectar and pollen; larvae eat grasshopper or bee eggs
Habitat	Grasslands, deserts, fields and gardens
Native range	Every continent except Antarctica
Size	Most about 1-2.5 cm long
Colours	Black, gray, striped, orange or metallic
Special feature	Contains cantharidin, which causes skin blisters

What Blister Beetles Look Like

Most blister beetles are 1 to 2.5 cm long, with a narrow, cylinder-shaped body and soft, flexible wing covers. The head is wide and joined to the body by a narrow “neck,” and the legs and antennae are fairly long.

Their colors vary a lot. Some are black or gray, some are striped, and others are brown, orange or shiny metallic green or blue. Many have bright warning colors that tell predators to stay away.

The Blister Beetle’s Poison

Blister Beetle Eating Flower

Cantharidin is found in the blood and body tissues of blister beetles. When a blister beetle is disturbed, it can squeeze out drops of yellowish liquid from its leg joints, a trick called reflex bleeding. If this liquid touches your skin, it can cause a burning feeling and a blister a few hours later.

In many species, the males make the cantharidin and pass some to females during mating, and the females put it on their eggs to protect them.

Cantharidin is a strong poison if swallowed. It damages the lining of the stomach, intestines and urinary system. Horses are especially sensitive, and eating hay with dead blister beetles in it can cause colic, diarrhea, sweating, depression and even death. Cattle, sheep and goats can also be poisoned.

Strangely, some insects are not harmed by cantharidin. Certain beetles and flies are actually attracted to it.

People have used cantharidin as a medicine for a long time. Today, doctors use it in tiny, carefully controlled amounts on the skin to remove warts and similar skin growths.

Where Blister Beetles Live

Blister beetles are found on every continent except Antarctica, mostly in warm, dry places such as grasslands, deserts and farm fields. In North America, they are especially common in the Midwest and Southwest of the United States, and they also live in parts of Canada.

Scientists divide the family into four groups called subfamilies: Eleticinae, Meloinae, Nemognathinae and Tetraonychinae. The Meloinae is the largest and most widespread group.

Some common species in the United States are the striped blister beetle (Epicauta vittata), the ash-gray blister beetle (Epicauta fabricii) and the black blister beetle (Epicauta pennsylvanica).

What Blister Beetles Eat

Adult blister beetles eat leaves and flowers, and many also feed on nectar and pollen. Some feed on garden and farm crops, including potatoes, tomatoes, beans, peas, beets, carrots, melons, cabbage and eggplants. They often arrive in large groups and can strip plants quickly.

The larvae have a very different diet. Depending on the species, they eat grasshopper eggs or the eggs, larvae and food stores of wild bees.

A Strange Life Cycle

Blister beetles have one of the most unusual life cycles of any insect, called hypermetamorphosis. This means their larvae change shape a lot as they grow.

Females lay batches of eggs in the soil, under stones or on plants, depending on the species. Some species lay thousands of eggs in their lifetime, because very few of the young survive.

The first larva to hatch is tiny, with long legs, and it is very active. Its job is to find food. In many Epicauta species, it hunts through the soil for a grasshopper egg pod. In other species, it climbs onto a flower and waits to grab onto a visiting bee, then rides back to the bee’s nest.

Once it has found food, the larva molts into a fat, slow, grub-like form that does little but eat. Later it becomes a resting stage that can survive the winter. Finally, it pupates and becomes an adult. In many species, this whole cycle takes about a year.

Because their larvae eat grasshopper eggs, some blister beetles actually help farmers by keeping grasshopper numbers down.

Nemognatha Blister Beetles

Blister Beetle (nemognatha sparsa)

Blister beetles in the genus Nemognatha do not have well-known common names, but they are often seen on flowers. Many have very long mouthparts that form a tube for sipping nectar, a little like a butterfly’s. They are usually orange, rusty or black, and they visit thistles, sunflowers and other plants in the aster family.

Females lay their eggs on or near flowers. The newly hatched larvae climb onto solitary bees that visit and ride back to the bee’s nest, where they eat the stored pollen and nectar.

Iron Cross Blister Beetles

Iron Cross Blister Beetles – Photo by: Michael J. Plagens

Iron cross blister beetles belong to the genus Tegrodera. They have a bright orange-red head and black wing covers with yellow or orange markings shaped a little like a cross. They live in the dry Southwest of the United States and nearby Mexico.

In spring, large numbers of these beetles appear, often walking together in groups across the ground. They feed on flowers and leaves, and they can damage alfalfa fields. Their bold colors warn predators that they contain cantharidin.

Blister Beetles as Pests

Blister beetles cause the most worry in alfalfa, which is often made into hay for horses. When alfalfa is cut and crushed during haymaking, beetles can be killed and trapped in the hay, and their cantharidin stays poisonous even after drying. A small number of beetles can make a horse very sick.

To stay safe:

• Never pick up blister beetles with bare hands. If you touch one, wash your skin with soap and water.
• If you get a blister, keep it clean and see a doctor if it is large or near your eyes.
• In gardens, an adult wearing gloves can knock beetles into a bucket of soapy water, and floating row covers can protect crops.
• Horse owners should buy hay from trusted suppliers who check fields for blister beetles, and call a vet right away if a horse shows signs of poisoning.

Related: grasshoppers · apple trees

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