Seed predators can increase nectar volumes in an alpine daisy: but do the insects benefit?

Pollinators and pre-dispersal seed predators can interact via the plants they share. We examined how pre-dispersal seed predators modify nectar quality and quantity and thereby influence pollinator behaviour. Working in a Tibetan alpine meadow, we hypothesized that increasing levels of pre-dispersal seed predation by larvae of tephritid flies would reduce nectar quantity and quality in Saussurea nigrescens (Asteraceae), and that this would make the flowers less attractive to honeybees, the most frequent floral visitors. Our field experiments showed that floret nectar volumes responded differently to high and low densities of fly larvae, with significant increases when there was one larva present, but decreases when two or more larvae were present in a capitulum. Experimental manipulations of fly larvae yielded the same result. The increases in nectar volume generated by a single larva are likely to be beneficial for these insects in locations where pollinators preferentially visit those S. nigrescens that produce more nectar. At our study sites, honeybees were the main pollinators and visitation rates were unaffected by the changes in nectar volumes, but they are introduced to the area, and native pollinators may be more selective.