Specialization is common in most lineages of insect herbivores, one of most diverse groups of organisms on earth. To understand why and how specialization is maintained over evolutionary time, we hypothesized that phylogenetic distance among host plants, along with plant defenses and other plant ecological attributes play together influencing herbivore performance, thus constraining host range expansion.
In this study, we take a novel phylogenetic and functional trait approach to assess the determinants of host range in the cerambycid beetle Tetraopes tetraophthalmus, a monophagous herbivore of common milkweed Asclepias syriaca L. (Apocynaceae). We quantitatively assessed the relationship between phylogenetic distance from the host plant species and performance of the root feeding Tetraopes larvae on 18 Asclepias species grown in a common garden setting. Half of the host plant species were chosen being recorded experiencing herbivory from other Tetraopes species, whereas the other half of the plant species never encounter this type of herbivore. Subsequently, we used phylogenetically controlled analysis to investigate the relative importance of plant traits relating to defense, life history and habitat driving host specialization.
Results/Conclusions
Across 18 milkweed species, T. tetraophthalmus larval survivorship decreased with increasing phylogenetic distance from A. syriaca, suggesting that adaptation to plant traits drives specialization. Among several root traits measured, only cardenolides (a class of toxic defense chemicals) correlated with larval survival. Two additional host plant factors were predictors of larval survival: milkweed species having a known association with other Tetraopes beetles were better hosts than species lacking Tetraopes herbivores, and milkweeds that typically occupy extreme habitats (having very high or very low SLA values) were poor hosts compared to milkweeds from similar habitats as A. syriaca (intermediate SLA value). Pair-wise phylogenetically independent correlations among the three significant predictors of larval survival (cardenolides, SLA, and association with Tetraopes) show that species having higher SLA values have proportionally lower levels of cardenolides. Moreover, species having an association with Tetraopes have lower levels of cardenolide.
