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 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 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 α-thujone/β-thujone and β-trans-chrysanthenyl acetate, while avoiding a chemotype dominated by α-pinene/sabinene. Uroleucon tanaceti showed no clear preference towards chemotypes, but when given a choice between chemotypes dominated by α-thujone/β-thujone and by α-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.

Methods
Leaf samples of tanacetum vulgare were collected and plants were chemotyped based on their terpenoid profiles, using hexane extractions and comparing retention indices and mass spectra to available synthetic reference compounds from the library entries of the National Institute of Standards and Technology (NIST) 2014, Pherobase and mass spectra reported in Adams (2007; Adams, R. P. (2007). Identification of Essential Oil Composition by Gas Chromatography/Mass Spectrometry, 4th Edn. Carol Stream, IL: Allured Publishing Corporation). Plant terpenoid profiles were then clustered using unsupervised hierarchical k-means clustering.

Plants from five chemotypes were propagated. In choice assays one aphid was placed into a petri dish containing a leaf from two different chemtoypes each. After two and five hours the observed aphid preference was recorded.

All statistical analyzes were performed in R, using the "survival", "vegan" and "RcmdrMisc" package.