Terpene-based selective herbivory by Helix aspersa (Mollusca) on Thymus vulgaris (Labiatae)

In natural populations of Thymus vulgaris in the south of France, six genetically different chemical "chemotypes" occur, each differentiated by a single dominant monoterpene that gives individual plants a characteristic smell and taste. Individual populations may contain all six chemotypes, but are often dominated by one to three chemotypes. We tested the hypothesis that this chemical polymorphism was associated with selective herbivory by the snail Helix aspersa, which feeds upon T. vulgaris in nature. The relative preference of juvenile and adult molluscs was investigated by presenting individual animals with a choice of six chemotypes in three experimental trials: (1) whole plants, (2) nutritive gels containing crushed thyme leaves, and (3) nutritive gels containing a distilled monoterpene as an additive. We found significant and consistent patterns of preference. Adult and immature molluscs preferred the linalol chemotype, and ate carvacrol and thymol-containing foods the least. This preference was more marked when using the gels than the real plants, particularly the monoterpene-based gels, demonstrating the importance of the monoterpene for the choice. Furthermore, molluscs fed exclusively on linalol-containing food gained weight whereas those fed on carvacrol-containing food lost weight. After being fed these exclusive diets both groups showed a significant preference for linalol. These results (1) demonstrate that food choice is based upon the presence of specific monoterpenes and is not learned, (2) suggest that phenolic monoterpenes such as carvacrol are repellent to the animals, and (3) indicate that selective herbivory may play a significant role in the maintenance of the chemical polymorphism of T. vulgaris.