An Offensive Chemical Defense

Jeff Mitton

Natural Selections (Appeared in the Daily Camera August 1, 2006)

A beautiful lavender wildflower, like other plants in the mint family, uses chemical defenses to ward off herbivore. But a small beetle has breached the defense, and uses the plant’s defensive chemicals in its own defense, in a most outrageous fashion.

Monarda fistulosa, also called wild bergamot, horse mint, and bee balm, is a an herbaceous native perennial native to most of North America. The plant is one to three feet tall, and each stem is topped with a purple inflorescence that is composed of many long, tubular, flowers.  During early summer, the inflorescence forms a globe, but toward the end of summer the drying inflorescences retain their flowers only around the periphery.

Bergamot, like other species in the mint family, is fragrant. But the fragrance comes not from nectar, but from monoterpenes or essential oils that are encased in trichomes, or tiny sacs. The trichomes are on the leaves and the flowers, and when something brushes against the plant, trichomes break open, releasing the volatile essential oils. Humans have used essential oils as fragrances and seasonings for thousands of years, but the plants have used them for millions of years to deter herbivorous animals.

Ken Keefover-Ring is conducting his Ph D thesis research on the geographical pattern of variation in the essential oils of bergamot. Bergamot has genetic variation that determines four chemotypes; each chemotype has a distinctive fragrance produced by a different monoterpene. The monoterpene geraniol has a sweet lemony fragrance; that chemotype is restricted to Alberta. Keefover-Ring recently discovered that a few plants in southern Colorado produce linalol, which has a sweet, woody fragrance. The vast majority of the bergamot in Colorado produce carvacrol or thymol, and their fragrances are similar to thyme or oregano.

Some populations consist only of the carvacrol chemotype, other populations have only the thymol chemotype, while most populations have mixtures of the two. It appears that thymol is more abundant in colder sites, such as deep canyons and at higher elevations. But an herbivore that specializes on bergamot might also influence the relative frequencies of the chemotypes. 345

The tortoise beetle, Physonota unipunctata, is a specialist on wild bergamot. Adults mate on and deposit eggs on bergamot, and the larvae pass through five instars feeding on the leaves and flowers. Although the larvae are not deterred by the essential oils, they have evolved a behavior to use the essential oils in their own defense.

They accumulate their feces into a large mass, nearly half their own weight, impale the mass on two projections at their hind end, and hold this fetid mass over themselves as a “fecal shield.” If a critter comes nearby and threatens them, they waive their shield in a threatening way, transforming the fecal shield into a club of crap. The fecal mass retains the essential oils, and the sight and smell of the shield or club effectively deters most predators.

The tortoise beetle has evolved tolerance of the essetial oils of bergamot to gain access to an uncontested resource, and it has usurped bergamot’s in its own offensive, outrageous, aromatic defense.

Photo legend:

The tortoise beetle accumulates its feces on two posterior appendages, and then holds the fetid mass over its body as a “fecal shield”

Photo by Ken Keefover-Ring