Thyme is of the Essence

Thyme is native to this typical Mediterranean landscape near Montpellier, in southern France

Photo by Ken Keefover-Ring

A single large thyme, Thymus vulgaris, can bear 2,000 flowers

Photo by Ken Keefover-Ring

Oil glands on the surface of the leaves contain the various monoterpenes that confer the six basic fragrances of thyme. The oil glands rupture when they are disturbed by the touch of an herbivore, smearing the leaf surface and the herbivore with a monoterpene. Depending on the monoterpene and the animal, the monoterpenes may act as attractants or deterrents

Photo by Ken Keefover-Ring

Lycaenid butterflies are commonly seen on thyme, but honeybees are the most common pollinator of thyme

Photo by Ken Keefover-Ring

Aphids and other insects can cause considerable damage on thyme. Crab spiders feast on them all

Photo by Ken Keefover-Ring

Thyme is of the Essence

Jeff Mitton

In the south of France, the summer evening air is fragrant with the aroma of thyme, rosemary, oregano, and lavender. Humans have learned to cook with thyme, rosemary and oregano, and to make perfume with thyme and lavender. But plants synthesize those aromatic compounds not for our benefit, but to defend against a horde of hungry herbivores, and perhaps to attract pollinators.

CU Professor Yan Linhart and his doctoral students Ken Keefover-Ring, Bianca Breland and Kailen Mooney have developed thyme as a model system to study the ecology of plant/animal interactions and the evolution of plant defenses. They conduct their experiments in gardens and pastures near Montpellier, in southern France.

Thyme is an ideal plant for studying chemical defenses, for the aromatic chemicals are monoterpenes that can be identified by a discriminating nose. Thyme comes in six essences, or biochemical phenotypes, all produced by a single biochemical pathway. The genes controlling the biochemical pathway are variable, and that variation causes each plant to produce just one of the six monoterpenes: geraniol, linalol, terpineol, thuyanol, carvacrol, or thymol. The structural differences among these monoterpenes determine not only the essence or fragrance of each plant, but also the efficacy of both defense and attraction.

Monoterpenes are packaged in the oil glands on the surfaces of leaves. When a rabbit nuzzles a thyme plant, or a beetle crawls across a thyme leaf, the oil glands rupture, smearing the monoterpene across the leaf surface and onto the animal.

Linhart and his crew have recently discovered that pollinators respond to the defensive chemicals in thyme. Lycaenid butterflies are commonly seen in thyme populations, but honeybees are the most common pollinators. Honeybees produce geraniol and use it as a pheromone, or chemical signal. Conversely, terpineol is mildly toxic to honeybees. Careful behavioral observations have revealed that honeybees most prefer to visit thyme plants defended by geraniol, and avoid thyme plants defended by terpineol. Thyme produces geraniol to deter herbivores, but it also attracts honeybees.

We use thyme plants that produce thymol or carvacrol or thuyanol for cooking, but none of the others are suitable. Just as we find some essences of thyme attractive, and others repugnant, a species of herbivore typically finds some plants to be invitingly edible, and other distasteful, if not toxic.

Thyme is munched by a host of herbivores: 5 mollusks, over 80 insects, and rabbits, goats and sheep. The thyme crew has found that herbivores differ in their tastes, or alternatively, are repelled by different fragrances. For example, garden snails are most effectively repelled by carvacrol, goats by thuyanol, sheep by linalol. As a consequence, each plant, by virtue of its chemical defense, is defended against some herbivores, but susceptible to others. Thyme is harried by nearly a hundred herbivores, and there is no perfect defense. In response to a host of herbivores with diverse likes and dislikes, thyme has evolved a diversity of chemical deterrents.

Consider a population of thyme being ravaged by a single species of herbivore. The herbivore might consume all of the plants producing a particular monoterpene, but avoid others. As long as the population is not besieged by too many species of herbivores in one season, it will persist. Across a broad geographic region, the relative abundance of herbivores from place to place alters the types of thyme in the various populations, producing the mosaic of fragrances smelled in the south of France today.

It is curious but satisfying to appreciate that the slugs, snails, beetles, rabbits, goats and honeybees help fashion the delicate fragrances of Mediterranean landscapes. Furthermore, it is ironic that the tastes that we use in cooking and the fragrances that we employ as perfumes initially evolved in chemical warfare.