Know Your Friends

Blister Beetle Larvae

Can insects have a split personality? Blister beetles (family Meloidae), which seem to be quite common in many parts of the Midwest this year, may qualify. Although blister beetles are probably best known as pests of many crops in their adult stage, the larvae of most species are actually good guys that feed on grasshopper eggs. Some other species are parasitic in the cells of various soil-nesting wild bees.

The common name "blister beetle" comes from the body fluids of adults of some of the common species which contain cantharadin, a compound that may cause blisters when applied to the skin. This toxic compound can also cause livestock deaths when ingested by susceptible animals in contaminated alfalfa hay. Horses are particularly sensitive to this toxin.

The adults are generally -1 inch long, narrow and elongate and the body is often soft and flexible unlike many beetles. There are many different species of blister beetles which are usually dark colored, often very beautiful metallic blue, green, copper or rose. They may be striped or solid-colored, with the elytra either smooth or reticulated. The clumsy beetles often feed together in groups, and tend to move around a lot. They can live 4-6 weeks as they feed on foliage and flowers of vegetable and ornamental crops.

The life history of blister beetles is quite complex, with additional life stages not found in other insects ("hypermetamorphosis"). They have several distinct larval stages with very different appearances. The females lay their elongate, cylindrical yellow eggs in clusters of 100 to 200 in shallow burrows in the hard, dry soil of the breeding grounds of the host. The eggs hatch in about 2 weeks to the triungulin stage. This first instar is an active, strong-jawed, long-legged form that burrows through the soil until it finds a grasshopper egg mass. It gnaws into the egg pod and feeds on one or two eggs before molting. During the next four weeks the larva continues to feed on the egg mass and molts three times, changing considerably in form and appearance. Its legs, mouth parts and other appendages grow progressively smaller with each molt. The margined blister beetle (Epicauta pestifera) consumes about 43 eggs of the differential grasshopper during its development. After the fourth instar, the following larval stages are inactive and do not feed. Depending on the species (and environmental conditions for some), the beetle will pass through 5-7 instars before it pupates in the soil. Most species overwinter as later instar larvae and pupate to complete their development in the spring. The adult beetles emerge rather suddenly, often in great numbers, in June and July. A few species, such as the clematis blister beetle (E. cinerea), have two generations per year, but most species have only one, correlated with the cycle of their host.

A survey by the USDA's Grasshopper Control Project in 1938-40 showed that 15% of the grasshopper egg pods in the western and midwestern states were destroyed annually-6.9% by bee flies, 5.6% by blister beetles and 2.5% by ground beetles. In some counties the average predation was much higher at 50%. Even when blister beetles are abundant, they do not destroy more than 25% of the grasshopper egg masses, but still contribute to natural control that prevents explosive outbreaks.

- Susan Mahr, University of Wisconsin - Madison


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