The larva of a thistle-feeding shield beetle. (Photo by Jeff Mitton)
By Jeff Mitton
A meadow on Flagstaff Mountain is home to two species of tortoise or shield beetles.
The names for this group of beetles come from their covering, which extends beyond the margins of the body to cover the legs, head and antennae and forms a seal with the substrate, usually a flat portion of a leaf. Adults protect themselves passively with a translucent shield, but larvae protect themselves more vigorously with a fecal shield.
Our local tortoise beetles are relatively easy to find, for they are restricted to specific host plants. The horse mint tortoise beetle,Physonata unipunctata, feeds on wild bergamot,Monarda fistulosa, which is also known colloquially as horse mint or bee balm. Bergamot is an herbaceous perennial native to North America. It grows 2 to 3 feet tall and is topped by a globe of tubular lavender flowers. Young larvae are black, older larvae are predominantly yellow with white and black spots, lines and spikes.
Thistle-feeding shield beetles,Cassida rubiginosa, were accidentally introduced from Europe to Canada in 1901, and were later purposefully introduced to Virginia to control Canada thistle. They feed on a variety of introduced and native thistles, including our local musk thistle and bull thistle. Larvae are mottled in tan, olive, green and white and they have many branched spines projecting from their sides. Their colors, texture and spines camouflage them on thistle leaves.
Larvae of both species consume the leaves of their host plants, ingesting defensive compounds toxic to most insect species. But unlike many species of butterflies, these larvae do not sequester the compounds in specific tissues, but they release them in tarry feces. The feces are not discarded; larvae spool them on their forked tails, and arch their tails to suspend the fetid mass over their bodies as a fecal shield.
The term "fecal shield" does not tell the whole story, for a shield is a defensive weapon. When I approach the larvae, the thistle-feeding larvae hold the fecal mass closer to their bodies in a defensive posture. But the larvae on bergamot lift the fecal mass high and brandish it as a weapon. The term "fecal shield" is used in the academic literature, but to recognize its offensive function I prefer the terms "turd truncheon" or "club of crap."
Experimental studies have demonstrated the efficacy of the shields and clubs. If the shield is removed, insect predators soon eat the unshielded larvae. If the shield is rinsed to remove the defensive chemicals but left otherwise intact, insect predators soon eat the deodorized larvae. Thus, the purloined plant defensive chemical compounds effectively ward off predators.
Other studies showed that bigger shields and clubs worked better, and thus older larvae, which are larger and carry disproportionately larger weapons, are more fully protected. The older larvae of our local tortoise and shield beetles carry shields or clubs that weigh 50 percent of their body weight. Divide your weight in half, and imagine carrying that weight around to discourage predators.
It seems that no defense system is perfect, and as aesthetically appealing as this system is, it has its shortcomings. Predators with long, piercing and sucking mouthparts, such as damsel bugs and some stink bugs, probe through the fecal club to pierce and drain the larvae.
It may not be a perfect defense, but trust me, when something brandishes a club of crap at you, they have your undivided attention.
Jeff Mitton (mitton@colorado.edu) is chair of the Department of Ecology and Evolutionary Biology at the University of Colorado. This column originally appeared in the Boulder Camera.
July 26, 2010