Honeydew-mediated indirect interactions: magnet and spillover effects of an invasive willow-aphid complex.

Indirect interactions are key drivers of community structure, yet their role in invasion ecology remains understudied. In riparian ecosystems of northern Patagonia, invasive willows (Salix fragilis) frequently host dense colonies of the invasive giant willow aphid (Tuberolachnus salignus) that produces copious amounts of honeydew. This novel, sugar-rich resource may alter pollinators' foraging activity or spatial distribution, with cascading consequences for co-occurring native plants. We quantified the abundance and taxonomic richness of insect pollinators feeding on honeydew at invasive willows. We then conducted a randomized block experiment comparing pollinator visits to the native herb Solidago chilensis placed near (~ 25 m) and far (~ 150 m) from the complex. Honeydew from the willow-aphid complex attracted a diverse assemblage of pollinators (16 taxa across three orders), including both native and exotic species. Pollinator abundance and richness on So. chilensis were significantly higher near the complex, evidencing a positive associational effect consistent with a magnet-resource mechanism and spillover effect. Our results reveal an underexplored pathway through which biological invasions reshape ecological networks. By showing that an interaction between two invasive species can indirectly facilitate native plants via honeydew-mediated pollinator attraction, we highlight the need to consider both positive and negative indirect effects when evaluating the ecological consequences of species invasions.