The composition of a plant’s neighborhood can influence its interactions with herbivores or pollinators, a phenomenon called associational effects (AE). AE are due to the density, frequency, or diversity of neighbors. However, host plant density can also affect these interactions. Although many studies have investigated AE in plant-insect interactions, most have used experimental designs that confound focal plant density with other factors. As a result, it has not been possible to disentangle AE (effects of neighbor density or frequency) from effects of intraspecific density. In this study, we asked how neighbor and host plant density and frequency affect pollinator visitation to self-incompatible perennial Solanum carolinense. We planted S. carolinense and Solidago altissima in a mowed and tilled field using a two-species Nelder fan design, which simultaneously manipulates the density and frequency of each plant. During peak flowering, we recorded pollinator visitation rates and duration at open S. carolinense flowers spanning the range of its density and frequency. We also recorded plant size, damage levels, and the number of open flowers within a 1 m2 area around the observed plant.
Over a three week period June-July 2016, we observed pollinators for a total of 66.25 hours (265 15-minute observation periods), including 782 visits to 265 plants. We used GLM to test for effects of S. altissima density and frequency, and S. carolinense density, on bumblebee visitation. Bumblebees had higher visitation rates to flowers in areas with more open S. carolinense flowers, suggesting they were attracted to high-density patches. These high density patches may allow for more efficient foraging. After accounting for floral density, bumblebees spent less time pollinating S. carolinense when it was surrounded by more S. altissima neighbors. The number of landings per plant declined as S. altissima neighbor frequency increased, but the duration of each visit was not affected by neighbor frequency. These results provide strong evidence that AE in pollination occur in this system. S. altissima flowers much later than S. carolinense. The AE we observed likely arise from S. altissima visually obscuring S. carolinense from bees rather than from any changes floral rewards (since visit duration was not affected by neighbor frequency).