Nectar robbers and simulated robbing differ in their effects on nectar microbial communities

Floral nectar contains microbes that can influence nectar chemistry and pollinator visitation, and these microbial communities can be affected by pollinators in turn. Some flowers are also visited by nectar robbers, which feed on nectar through holes cut in floral tissue. If nectar robbers alter nectar microbial communities, they might have unexpected impacts on pollinator visitation. We investigated whether robbing could affect nectar microbial communities directly, by introducing microbes, or indirectly, by triggering a plant response to floral damage. We applied four treatments to flowers of Tecoma x "Orange Jubilee" (Bignoniaceae) in an arboretum setting: flowers were (1) covered to exclude all visitors; (2) available to both pollinators and nectar robbers and robbed naturally by carpenter bees; (3) available to pollinators only but cut at the base to simulate nectar robbing damage; or (4) available to pollinators only. We found that nectar in flowers accessible to any visitors was more likely to contain culturable microbes than flowers from which visitors were excluded. Microbial community composition and beta diversity were similar across treatments. Among flowers containing culturable microbes, flowers available to pollinators and nectar robbers had higher microbial abundance than flowers with simulated robbing, but there were no differences between flowers available to pollinators and robbers and unwounded flowers from which robbers were excluded. Overall, our results suggest that floral damage can affect some features of nectar microbial communities, but specific effects of nectar robbing are limited compared with the influence of visitation in general. This study experimentally tested whether nectar robbers and pollinators differ in their effects on nectar microbial communities of Tecoma x "Orange Jubilee" and whether any differences are due specifically to floral damage caused by robbing. Simulated robbing damage (but not natural robbing) decreased microbial abundance, relative to flowers from which robbers were excluded. Nectar microbial communities were otherwise similar in robbed and unrobbed flowers, and we found much greater differences between flowers that were unvisited versus flowers that received any kind of visit (robber or pollinator).image