Plants of the same species often strongly differ in morphological traits, as well as in the abundance and composition of specialized metabolite profiles. Specialized metabolites can act as mediators of interactions on plants, and partially explain insect presence and abundance in the field. However, how specialized chemistry shapes plant attractiveness to herbivorous insects is not fully understood. Here we used common tansy (Tanacetum vulgare L., Asteraceae) - a perennial plant that is highly diverse in terpenoid composition and is known to have variable chemotypes - to test whether 1) plants with different chemotype profiles differ in attractiveness to two specialized aphids, Macrosiphoniella tanacetaria and Uroleucon tanaceti, in pairwise choice assays. Furthermore, we tested whether 2) the diversity of the terpenoid blend affects aphid attractiveness. Lastly, we tested how 3) plant chemical traits relate to plant morphological traits, and which best explain aphid preference. We found that M. tanacetaria preferred two out of five chemotypes, dominated by alpha]-thujone/beta-thujone and beta-trans-chrysanthenyl acetate, respectively, while avoiding a chemotype dominated by alpha-pinene/sabinene. U. tanaceti showed no clear preference towards chemotypes, but when given the choice between chemotypes dominated by alpha-thujone/beta-thujone and by alpha-pinene/sabinene, they preferred the former. Importantly, plant attractiveness to aphids tended to be negatively correlated chemodiversity, i.e. the number of terpenoid compounds, in M. tanacetaria, but not in U. tanaceti. Interestingly, the approximate concentration and number of terpenoid compounds was generally higher in larger and bushier plants. Hence, we did not observe a trade-off between plant growth and defence. We conclude that plant chemical composition affects plant attractiveness to aphids and hence may contribute to variation in natural aphid colonization patterns on plants of the same species.

Plants produce a great number of phytochemical compounds mediating a variety of different functions. Recently, phytochemical diversity (chemodiversity), a way which to quantify the complex phenotype formed by sets of phytochemicals, has been suggested to be important for function. However, no study has systematically examined the potential (in)direct functional importance of chemodiversity on a general level, partly due to a lack of an agreement on how to quantify this aspect of the plant phenotype. This paper has four aims: 1) We discuss how chemodiversity (deconstructed into components of richness, evenness and disparity) may quantify different aspects of the phenotype that are ecologically relevant. 2) We systematically review the literature on chemodiversity to examine methodological practices, explore ecological patterns of variability in diversity across different levels of biological organization, and investigate the functional role of this diversity in interactions between plants and other organisms. 3) We provide a framework facilitating decisions on which measure of chemodiversity is best used in different contexts. 4) We outline open questions and avenues for future research in this area. A more thorough understanding of phytochemical diversity will increase our knowledge on the functional role phytochemical compounds, and how they shape ecological interactions between plants and their environment.

Aim: Intraspecific variations of specialized metabolites in plants, such as terpenoids, are used to determine chemotypes. Tansy (Tanacetum vulgare L.) exhibits highly diverse terpenoid composition profiles in leaf tissues, particularly of mono- and sesquiterpenoids. The substantial chemotypic variation in tansy plants influences their associated insect communities in the field. However, it is not fully known whether and/or how patterns of their chemical composition and associated insects vary on a large scale. In this study, we investigated the geographic distribution of mono- and sesquiterpenoid chemotypes in tansy leaves and the effects of these chemotypes on colonization by insect communities across Germany. Location: Germany Year of data collection: 2014 Major taxa studied: Tanacetum vulgare L. (Asteraceae), Metopeurum fuscoviride Stroyan (Hemiptera: Aphididae), Lasius niger L. (Formicidae), Formica rufa L. (Formicidae), Myrmica rubra L. (Formicidae) Methods: We sampled tansy leaves from 26 sites along a north-south and west-east transect in Germany. Leaf tissue from ten plants with and five plants without aphids was collected from each site. Hexane-extracted metabolites from leaf tissues were analysed by gas chromatography-mass spectrometry (GC-MS). Plant morphological traits, aphid occurrence and abundance, and occurrence of ants were recorded. The effect of plant chemotype, plant morphological parameters, and site parameters such as temperature and precipitation on insect occurrences were analysed. Results: Tansy plants clustered into four monoterpenoid and four sesquiterpenoid chemotype classes. Monoterpene classes differed in their latitudinal distribution, whereas sesquiterpene classes were more evenly distributed across the transect. Aphid and ant occurrence were influenced by monoterpenoid class and specific plant morphological traits. Specifically, we found that plants of monoterpenoid class 1 were colonized by Metopeurum fuscoviride and ants significantly more often than expected by chance compared to plants from monoterpenoid class 4. Moreover, aphid abundance was negatively affected by host plant height, and increasing average annual temperature positively influenced the occurrence of ants. Conclusion: We found significant geographic differences in the chemical diversity of tansy and show that monoterpenoids affect aphid and ant occurrence, while host plant height can influence aphid abundance. Our work shows that geographic variation in plant chemistry and morphology influences insect communities' assemblage on tansy plants.

Intraspecific plant chemodiversity plays a fundamental role in interactions between plants and their interaction partners. Individuals of a plant species can be clustered into chemotypes by dominant chemical compounds or their chemical composition. Intraspecific stands of plant communities can vary in the number and type of plant chemotypes that grow in them (i.e., chemotype richness). Chemotypic diversity at the stand level is a special case of intraspecific diversity that is predicted to increase stand-level ecosystem functioning. Here we describe a biodiversity experiment in which we manipulated intraspecific plant chemodiversity at the plot level using six different chemotypes of common tansy (Tanacetum vulgare L., Asteraceae). We tested the effects of chemotype identity and plot-level chemotype richness (1-6) on plant growth and reproductive traits under field conditions. We found that plant chemotypes differed in growth and reproductive traits, both at the plant level, and at the plot level, and that reproductive plant traits and plot-level trait means were affected by tansy chemodiversity. The plot-level trait means were influenced by the presence or absence of certain chemotypes in a plot. The community's headspace terpenoid blend minimally reflected plot-level leaf terpenoid compound blends. Although tradeoffs between chemodiversity and growth and reproductive traits were observed, the links between chemodiversity and traits expressed themselves in the early establishment but dissolved over time, suggesting that different chemotypes adopt different growth strategies, which may facilitate their establishment in nature. This long-term field experiment will allow for further investigation of the consequences of intraspecific chemodiversity for plant-insect interactions and insect community assembly.