Blister Beetles, Family Meloidae

Understand the habits and traits of blister beetles

Black blister beetle
Flickr user David Hill ( CC license)

Few North American species of blister beetles will actually cause blisters, but it's still smart to be cautious when handling members of the beetle family Meloidae. There's some debate over whether blister beetles are pests (because the adults feed on many agricultural crops and can be hazardous to livestock), or beneficial predators (because the larvae ​consume the young of other crop-eating insects, like grasshoppers).

Description

Blister beetles look superficially similar to members of some other beetle families, such as the soldier beetles and darkling beetles. Blister beetles, however, do have some unique features that will help you identify them. Their elytra appear leathery and soft, rather than rigid, and the forewings wrap around the sides of the beetle's abdomen. The blister beetle's pronotum is usually cylindrical or rounded, and narrower than both the head and the base of the elytra.

Most adult blister beetles are medium in size, although the smallest species measures just a few millimeters in length and the largest can reach 7 centimeters long. Their bodies are generally elongate in shape, and their antennae will be either filiform or monofiliform. While many are dark or drab in color, particularly in the eastern U.S., some do come in bright, aposematic colors. Look for blister beetles on flowers or foliage.

Classification

Kingdom – Animalia
Phylum – Arthropoda
Class – Insecta
Order – Coleoptera
Family - Meloidae

Diet

Adult blister beetles feed on plants, particularly those in the legume, aster, and nightshade families. Although rarely considered a major crop pest, blister beetles do sometimes form large feeding aggregations in plants.

Many blister beetles consume the flowers of their host plants, while some feed on the foliage.

Blister beetle larvae have unusual feeding habits. Some species specialize in eating grasshopper eggs, and for this reason, are considered beneficial insects. Other blister beetle larvae eat the larvae and provisions of ground-nesting bees. In these species, the first instar larvae may hitch a ride on an adult bee as it flies back to its nest, and then settle in to eat the bee's offspring.

Life Cycle

Blister beetles undergo complete metamorphosis, like all beetles, but in a somewhat unusual way. The first instar larvae (called triungulins) usually have functional legs, well-developed antennae, and are quite active. These young larvae need to move because they are parasitoids and must find their hosts. Once they're settled in with their host (such as in a bee nest), each successive stage is typically less active, and the legs gradually diminish or even disappear. This larval development is referred to as hypermetamorphosis. The final instar is a pseudopupa stage, during which the beetle will overwinter. Depending on the species and environmental conditions, the blister beetle life cycle may last as long as three years.

Most species will complete a full life cycle within one year, however.

Special Behaviors and Defenses

Blister beetles are usually soft-bodied and may seem vulnerable to predators, but they aren't defenseless. Their bodies produce a caustic chemical called cantharidin, which they exude from their leg joints when threatened (a defensive strategy called "reflex bleeding"). Meloid species with high levels of cantharidin can cause skin blisters when handled, giving these beetles their common name. Cantharidin is an effective repellent for ants and other predators but can be extremely toxic if ingested by people or animals. Horses are particularly susceptible to cantharidin poisoning, which can occur if their hay feed is contaminated with blister beetle remains.

Range and Distribution

Blister beetles are most diverse in arid or semi-arid regions of the world, though widely distributed.

Globally, blister beetle species number close to 4,000. In the U.S. and Canada, there are just over 400 documented blister beetle species.

Sources:

  • Borror and DeLong's Introduction to the Study of Insects, 7th edition, by Charles A. Triplehorn and Norman F. Johnson.
  • Bugs Rule! An Introduction to the World of Insects, by Whitney Cranshaw and Richard Redak.
  • Beetles of Eastern North America, by Arthur V. Evans.
  • Family Meloidae – Blister Beetles, Bugguide.net. Accessed online January 14, 2016.
  • Blister beetle, Texas A&M University Department of Entomology website. Accessed online January 14, 2016.
  • Blister Beetles: Pest or Beneficial Predator?, Washington State University Fact Sheet (PDF). Accessed online January 14, 2016.