Aphid presence and abundance, more than variation in leaf terpenoid profiles at the plant and plot-level, drive ant behaviour on the perennial forb Tanacetum vulgare

1) Differences in specialized metabolites are common both within and between plant species and are often thought to have regulating functions in ecological interactions, including herbivores like aphids. Although ants commonly rely on chemical cues in their behaviour and resource finding, very little is known about whether specialized plant chemistry regulates ant behaviour to foster successful ant-aphid mutualisms on plants and in the surrounding vegetation. 2) Using a chemodiversity experiment containing 84 plots with 6 chemotypes of Tansy (Tanacetum vulgare L. Asteraceae) planted in different proportions, i.e., plot-level chemotype richness, we tested the effects of plot-level chemotype richness, plot-level chemodiversity metrics, and individual chemotype presence on black garden ant (Lasius niger Linnaeus) nesting, patrolling and recruitment behaviour to plants and plots. Furthermore, we assessed the influence of plant chemotype on the pink tansy aphid (Metopeurum fuscoviride H.L.G. Stroyan) presence and abundance, as well as ant occurrence on Tansy plants. 3) We found that Tansy plot-level chemodiversity only minimally affected most of the observed ant behaviour, except for nesting, which was marginally positively impacted by plot-level chemotype richness. Clear effects of individual terpenoid chemotypes were observed on ant visitation rates, as well as on aphid presence and abundance. Strongly significant relationships between the probability of ant occurrence and aphid abundance and occurrence observed in our study suggest that ants and aphids are most strongly guided by the presence and abundance of their mutualist partners, rather than by specialized chemistry alone in the Tansy system.