Effect of plant chemical diversity on structuring herbivore populations across different spatial scales

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In an ecological community, species interactions structure local populations and influence species coexistence. Two different, yet complementary, approaches to studying species interactions are community genetics, which recognises that within-species genetic variation can alter the interactions between different species; and, metacommunity ecology, which emphasizes the role dispersal plays in structuring local communities. We investigate how community genetic interactions influence metacommunity dynamics, using a model tansy (Tanacetum vulgare) plant – aphid system. Tansy exhibits high intraspecific variation in volatile organic chemicals (VOCs), which can influence the preference of aphids and their natural enemies. In Phase 1 of this project, we characterized the chemotypic diversity of tansy within a single field site and studied in detail how small-scale variation in the chemical composition of the host plant determines colonisation patterns of the insect community on tansy plants. We also studied the relationship between volatile (terpenoids) and metabolomic (mostly phenolic compounds) profiles of tansy, and the role of the metabolic phenotype (‘metabotype’) for aphid colonisation. We developed microsatellites for aphids and found, at the small spatial scale of our field site, an association between the volatile profile of plants and the aphid genotypes. Using laboratory set-ups, we characterized the response of different tansy chemotypes to attack by aphids and a chewing herbivore, and aphid and ant host choice. Overall, Phase 1 provided evidence for the importance of plant chemotypes for structuring the insect metacommunity on tansy plants. Here we propose to extend our analysis to 1) a larger spatial scale and 2) to belowground plant-insect interactions by characterizing the root chemotype. In a laboratory setting, we will analyze the chemotypical composition of the plant roots, to understand if there a metabolic and genotypic relationship between the terpenoid patterns in the aboveground part of the plant and those of the roots. We will then use root-feeding aphids to investigate if there is an interaction between aphid species feeding on the belowground and aboveground parts of the plant, and what roles root and shoot chemotypes play for aphid metacommunity structure? Using samples from across Germany and beyond, we will study the geographical distribution of root and shoot terpenoid chemotypes of tansy and analyze the role of dominant and minor compounds for structuring the insect communities on tansy. By developing and applying SNP genetic markers, we will also link the chemotypical differences between plants to the population genetics of the dominant aphid herbivore.Overall the project will shed light on fundamental mechanisms structuring ecological communities in the field.