Bug of the Week

Blister Beetle (Family Meloidae)


Salutations, BugFans,

Blister Beetles have appeared as supporting cast members in a number of BOTWs over the past four years, most recently in connection with the cantharidin-eating Fire-colored Beetle. It’s time for them to have a BOTW of their own. And besides, it’s Insect Vocabulary Word Day.

BBs belong in the beetle family Meloidae, a family that contains about 400 species in North America and 3,000 species worldwide. Here in the Eastern side of the country, these mostly diurnal, medium-sized, wide-headed, long-legged, cylindrical beetles are often striped, spotted or drab in color. Their soft elytra (wing covers) are curved around the length of the abdomen but may not extend to its tip. BBs are beautifully diverse (diversely beautiful) in the Southwest.

Like all beetles, BBs practice Complete Metamorphosis. Their journey from egg to adulthood includes a larval stage and a stint in a pupal case where, like Clark Kent in his phone booth, the larva changes its appearance completely. Like the Wedge-shaped beetle, of recent BOTW fame, the BBs’ larval stage starts off with a bang.

BB eggs are laid on/in the soil or on stems, leaves or flowers. Hypermetamorphosis refers to a regimen in which the first larval instar out of the egg is a comparatively long-legged, active, un-grub-like critter called a triungulin (an instar is the feeding stage between two molting stages). Depending on the species, a triungulin uses its super-mobility to find food (grasshopper eggs) or to hitch a ride on a solitary bee that unwittingly carries the triungulin to her nest. There the larva eats food that was stored for the bee larvae and maybe eats the bee larvae, too (to facilitate matters, some BBs lay their eggs directly in the nests of solitary bees). After they’ve found a food source, triungulins molt into a typical short-legged grub and continue to eat. At the other end of their larval life, Meloids may also spend an inordinate amount of time – up to two years - in the penultimate larval stage (now they’re called pseudopupa ) before their final molt and the formation of the actual pupa. BBs overwinter as pseudopupa and pupate in spring.

While their young are mainly insect/egg predators or food predators (BB larvae are considered beneficial because of their work with short-horned grasshoppers), the adults are herbivores. They feed on nectar and pollen and sometimes on vegetation. Some adult BBs eat the leaves or flowers of agricultural crops and other cultivated plants, including potatoes, alfalfa, legumes and hollyhocks, and they’re considered pests.

The big story about BBs, of course, is their toxicity. The blood of Meloids contains an irritating substance called cantharidin that protects them from many (but not all) predators. In Secret Weapons, Thomas Eisner writes that “both spiders and ants have been shown to reject food that is laced with cantharidin at concentrations far below those at which the compound occurs in meloid blood” (some species are up to 10% cantharidin by weight).

He goes on to say that cantharidin is “stored in the blood and reproductive organs of the beetles, and the beetles commonly ‘reflex-bleed’ when disturbed. If seized by the leg, they emit a droplet of blood from the knee joint of that leg only. Held by the body instead, they may bleed from all six legs at once, and sometimes from the neck and elsewhere.” Males are the producers of the compound; females acquire it when they accept the male’s sperm packet, which contains enough to supply them and to protect their eggs.

Many BBs do not have aposematic (warning) coloration, but their behavior is suspicious. Their unhurried presence on flowers in broad daylight shouts “Hands Off!” to wary predators.

The internet is full of accounts of adult BBs being “harvested” and baled while feeding on alfalfa and then being consumed by hay-eating livestock (including horses) for whom the experience may be fatal. Cantharidin keeps its potency in bales for a long time. According to Eisner, “taken orally, the substance [cantharidin] induces severe, irreversible damage to the renal and reproductive systems, and is lethal (to both men and women) in very small quantities.”

Despite this, cantharidin has been employed medicinally and continues to be used to remove warts (see the “History” section of Wikipedia’s entry on "Spanish Fly"). The notorious “Spanish Fly” is a misnamed, metallic green Eurasian Meloid beetle whose cantharidin is harvested for an aphrodisiac of dubious merit. The tough skin on our fingers will often resist blistering, but the BugLady saw gruesome pictures of blisters on the back of someone’s neck. Medical intervention is not necessary unless exposure/blistering is extensive.

Representing the Blister beetle bunch in old fields locally (and in much of the U.S.) is the Black Blister Beetle (Epicauta pensylvanica), also known as the Black Aster Bug (“Pensylvania” was an alternate spelling of “Pennsylvania” back in 1775 when this species was described and named). It’s about 5/8” long with a velvety black exterior and antennae that resemble a string of beads. (Caveat – there are several BBB look-alikes in the genus Epicauta).

black blister beetle black-blister-beetle_300

The female BBB lays her eggs in the soil toward the end of summer, simultaneously with female grasshoppers (many grasshoppers overwinter in egg pods). When they hatch, the BBB triungulins search for and devour grasshopper eggs. Adult BBBs feed on goldenrod and a few other members of the Aster family. The BugLady has always found them on goldenrod in August, but she saw no BBBs in the Summer of 2012.

Cool BB story: Some triungulins in the genus Meloe produce pheromones (“perfumes”) that mimic the courtship pheromone of female solitary bees (each Meloe species targets a specific solitary bee species). In fact, triungulins of some species congregate in groups on flower tops so that their combined “Come hither” signal will be stronger. The lusty male bee flies in and lands, and when he doesn’t see a female, leaves. But not before a few super-larvae have hopped aboard. They’ll transfer to the next female he finds and ride with her to her nest.

One last word – that hitchhiking is called “phoresy.”

The Bug Lady