Chemodiversity affects preference for Tanacetum vulgare chemotypes in two aphid species

Plants of the same species can strongly differ in their specialized metabolite profiles, which can affect 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, 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 specialist aphid species, Macrosiphoniella tanacetaria and Uroleucon tanaceti, in pairwise choice assays; 2) the diversity of the terpenoid blend affects plant attractiveness to aphids; 3) how plant chemical traits relate to plant morphological traits and which traits 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, while avoiding a chemotype dominated by alpha-pinene/sabinene. Uroleucon tanaceti showed no clear preference towards chemotypes, but when given a choice between chemotypes dominated by alpha-thujone/beta-thujone and by alpha-pinene/sabinene, they preferred the former. Importantly, plant attractiveness to aphids was marginally negatively correlated with chemodiversity, i.e. the number of terpenoid compounds, in M. tanacetaria, but not in U. tanaceti. Interestingly, the relative concentration and number of terpenoids were generally higher in larger and bushier plants. Hence, we did not observe a tradeoff 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.