Ecologia mediterranea 1999-25(2)

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Linhart et al. Selective herbivory ofthyme chemotypes by a mollusk and a grasshopper At first contact, the primary detectable differences among thyme plants are those associated with the monoterpene contents of their glands. But there are also other, more subtle differences among them: these include for example pubescence, leaf size, thickness, toughness and disposition, and all may affect the plants' palatability to various herbivores (Strong et aI., 1984). For this reason, an artificial diet which reduces these differences among food items is an important confirmation of the specific role of terpenes in herbi vore choice. We were only able to use artificial diets using ground leaves for the slugs as no artificial diets were available for the grasshoppers. The slugs ate somewhat larger amounts of geraniol containing cakes, just as they ate more plants of the G chemotype than the others. This observation provides support for the role of the specific monoterpenes synthesised by the plants as determinants of level of her bivory. This role was further supported in other experiments which involved the snail Helix aspersa (Linhart & Thompson, 1995) goats (Capra hircus) and sheep (Ovis aries) (Linhart & Thompson, 1999). In all three cases, patterns of relative preference of live plants characterised by specific chemotypes were the same as choices made when feeding on synthetic diets supplemented with pure distilled monoterpenes (Lin hart & Thompson 1995, 1999). The difference in patterns of preference between slugs and grasshoppers is notable as it shows that the two animals react very differently to the smell and/or the taste of the two molecules. These interspecific dif ferences are also interesting in combination with another comparison made between patterns of herbivory by the grasshopper Omocestus viridulus, and the mollusk Deroceras reticulatum feeding upon mixtures of the graminoid Dactylis glomerata and the legume Trifolium repens. The two animals behaved very dif ferently, with the grasshopper feeding disproportion ately on the common species, with a periodic preference for the grass, while the slug fed dispropor tionately on the rarer species, with a consistent prefer ence for the legume (Cottam, 1985). Such results further document the existence of important inter specific differences in herbivore behaviour, and sug gest that the presence of multiple species of herbivores can contribute to small-scale heterogeneity of botani cal and biochemical composition within plant communities. 150 The patterns of herbivory shown by the slugs in our experiment differ markedly from those of Gouyon et al. (l986a) who found the animals in their experi ments showed preference patterns of A=C>T>U. We presume the differences are associated with the fact that their slugs had no experience feeding on thyme, and also were not exposed to G or L. In any case, the differences illustrate how animal origin and experi mental design can influence the outcome of a feeding trial. The inter-specific differences we saw are consis tent with other patterns of herbivory of T. vulgaris. For example Helix aspersa and the Chrysomelid beetle Arima marginata preferred the L chemotype, while sheep (Ovis aries) preferred U, while goats although less specific in their choices, clearly preferred A, L or C to the other three chemotypes either in the form of whole plants or in synthetic diets (Linhart & Thomp son, 1999). Other plants that show genetically-based intra-species variability in chemical defenses have also been studied experimentally by exposing diverse chemical phenotypes to various herbivores and para sites. In the great majority of cases, the herbivores show species-specific host selection; that is, there are consistent, inter-specific differences in the chemical phenotypes they select as food or host plants. This is not surprising, as one may well expect that these dif ferences reflect the interspecific differences in physi ology, metabolism and behaviour that are to be expected among organisms as different as molluscs, insects, fungi, birds, fish and mammals. These pat terns have been observed repeatedly in many plant species as diverse as Labiatae, Leguminosae, Graminae, Fucoidae, Compositae, and various conifers (Simms, 1990; Linhart, 1991; Linhart & Thompson, 1999; Iwao & Rausher, 1997; Van Der Meijden, 1996; Snyder & Linhart, 1998). The consistent nature of these observations implies that such species-specific herbivory and parasitism can contribute to the mainte nance of genetic variability in host defenses via com plex patterns of selection. The results also indicate that for a given plant species, no one molecule can be counted on as a dependable defense against diverse herbivores. Acknowledgements We thank the staff of the Centre d'Ecologie Fonc tionnelle et Evolutive (CNRS) in Montpellier, France, ecologia mediterranea 25 (2) - 1999
Linhart et al. Selective herbivary afthyme chematypes by a mallusk and a grasshopper and especially C. Collin, D. Couvet, M. Debussche, and B. Dommee. Very useful help and advice were also provided by J. Lamy and the Chambre d'Agri culture de la Drome. Technical assistance was provided by L. Bowden, N.B. Linhart, and B. Stojanovic Meunier. Financial aid was provided by the U.S. Na tional Science Foundation, and the University of Colo rado. REFERENCES Adzet T., Granger R., Passet 1. & San Martin R., 1977. Le polymorphisme chimique dans le genre Thymus: sa signification taxonomique. Biochem. Syst. Evol., 5: 269 272. Cottam O.A., 1985. Frequency-dependent grazing by slugs and grasshoppers. J. Ecol., 73: 925-933. Gleizes M., 1976. Biologie des terpenes vegetaux: etude des composes monoterpeniques et sesquiterpeniques. Annee BioI., 15: 7-127. Gouyon P.H., Fort P. & Caraux G., 1986a. Selection of seedlings of Thymus vulgaris by grazing slugs. 1. Ecol., 71: 299-306. Gouyon P.H., Vernet P., Guillerm J.L. & Valdeyron G., 1986b. Polymorphisms and environment. The adaptive value of the oil polymorphisms in Thymus vulgaris. L. Heredity, 57: 59-66. Iwao K. & Rausher M., 1997. Evolution of plant resistance to multiple herbivores: quantifying diffuse coevolution. Am. Nat., 149: 316-335. Langenheim J.H., 1994. Higher plant terpenoids: a phytocentric view of their ecological roles. J. Chem. Ecol., 20: 1223-1280. Letchamo W., Saez F. & Stahl-Biskup E., (in press). The genus Thymus. Linhart Y.B., 1991. Disease, parasitism and herbivory: multi dimensional challenger in plant evolution. Trends Ecol. Evol., 6: 392-396. Linhart Y.B. & Thompson J.O., 1995. Terpene based selective herbivory by Helix aspersa (Mollusca) on Thymus vulgaris (Labiatae). Oecologia, 102: 126-132. ecologia mediterranea 25 (2) - 1999 Linhart Y.B. & Thompson J.O., 1999. Thyme is of the essence: biochemical polymorphism and multi-species deterrence. Evol. Ecol. Research, 1: 151-171. Passet J., 1971. Thymus vulgaris L. : chemotaxonomie et biogenese monoterpenique. These Ooct. d'Etat. Universite de Montpellier, Montpellier. Ross J.O. & Sombrero c., 1991. Environmental control of essential oil production in Mediterranean plants. In: 1.B. Harborne & F.A. Tomas-Barberan (eds.). Ecological chemistry and biochemistry ofplant terpenoids. Clarendon, Oxford, U.K., 83-94. Simms E.L., 1990. Examining selection on the multi-variate phenotype: plant resistance to herbivores. Evolution, 44: 1173-1188. Sokal R.R. & Rohlf F.J., 1981. Biometry. 2nd ed., Freeman, San Francisco, CA. Strong O.P., Lawton J.H. & Southwood T.R.E., 1984. Insects on Plants. Community Patterns and Mechanisms. Harvard Univ. Press Cambridge, MA. Thompson J.O., (in press). Population structure, and the spatial dynamics of genetic polymorphism in thyme. In: Letchamo, W, Saez, F. & Stahl-Biskup, E. (Eds.), in press: The genus Thymus. Thompson 1.0., Manicacci O. & Tarayre M., 1998. Thirty five years of thyme: a tale of two polymorphisms. Bioscience, 48: 805-815. Snyder M.A. & Linhart Y.B., 1998. Subspecific selectivity by a mammalian herbivore: geographic differentiation of interactions between two taxa of Sciurus aberti and Pinus ponderosa. Evol. Ecol., 12: 755-765. Van Oer Meijden E., 1996. Plant defense, an evolutionary dilemma: contrasting effects of (specialist and generalist) herbivores and natural enemies. Ent. Exp. Appl., 80: 307-310. Vernet P., Gouyon P.K. & Valdeyron G., 1977. Genetic control of the oil content in Thymus vulgaris L.: a case of polymorphism in a biosynthetic chain. Genetica, 69: 227-231. Whelan R.J., 1982. An artificial medium for feeding choice experiments with slugs. J. Appl. Ecol., 19: 89-94. 151
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