Specialised chemistry affects insect abundance but not overall community similarity in three rare shrub willows: Salix myrtilloides, S. repens and S. rosmarinifolia

Abstract

Willows serve as a keystone host-plant genus for insect herbivores. The diversity of insect herbivore assemblages harboured by willows is typically affected by the diversity of specialised metabolites that willows produce. Here, we studied three small, shrubby willow species (Salix myrtilloides, S. repens and S. rosmarinifolia) that primarily occur at sites of high conservation value in the Czech Republic. We explored if associated insect communities reflect the specialised chemistry in these uncommon host plants. We measured the three willow species for overall metabolomic profiles and salicinoids using non-targeted metabolomics and sampled them for caterpillars, leaf-chewing beetles (adults and larvae), sawfly larvae, and sap-sucking Hemiptera. We detected 2,067 metabolites across the three willow species. Most of them were shared by S. repens and S. rosmarinifolia, while S. myrtilloides showed a distinct chemical profile. Salix repens and S. rosmarinifolia also had significantly higher concentration and richness of salicinoids than S. myrtilloides. The abundance of all insect species and generalists that also feed on host-plants outside Salicaceae was higher on S. myrtilloides than on S. rosmarinifolia or S. repens. The abundance of Salicaceae specialists did not differ among the three willow species. Insect community composition, in contrast, did not show pronounced differences among the three willows. Our results suggest that salicinoids may be responsible for the low abundance of generalist herbivores. Furthermore, our study indicates that herbivore community composition does not reflect the specialised chemistry in the three willows we studied. Therefore, we hypothesise that the presence of some of the insect species is primarily determined by other factors, such as the habitat type where the respective willow species occur. Although the studied willows possess some characteristic specialised chemistry, we conclude that their importance as hosts of specific and sometimes threatened insect fauna may be mediated by willow habitat preference.