The western honey bee (Apis mellifera) is well studied as a pollinator in agricultural systems but its ecological role as a pollinator and potential competitor in natural systems is less well understood. Where studied in San Diego County, USA, non-native and predominantly feral honey bees are responsible for 75% of floral visits to native vegetation. This occurs despite the fact that San Diego County is home to ca. 700 species of native bees and a great diversity of other pollinating insects. We asked how floral visitation by honey bees and the native insect fauna is partitioned among species of native plants and how phenological patterns (i.e. species-specific changes in floral abundances through time) affect honey bee and native insect foraging patterns. At one intensively-studied site, the number of flowers on all flowering species was estimated and numbers of visits by honey bees and by native insects were recorded twice weekly from late February to June, 2016. Similar data were recorded at 12 less- frequently censused sites to examine the generality of observations. The number of floral visits was analyzed as a function of plant species, number of flowers and pollinator type (honey bee vs. native insects).
Results/Conclusions
After accounting for floral abundance, plant species had little or no effect on visitation rates by honey bees or by the native insect fauna as a whole. Honey bees showed a strong positive numerical response to increasing floral abundance while native insects showed little or no change in the number of visits as floral densities increased. These data show that honey bees achieve greatest numerical dominance on the most abundant floral resources, often accounting for > 90% of visits to plant taxa when they bloomed abundantly. Results for honey bees mirror foraging studies on single plant species that show increased honey bee use as patch size increases and likely result from the honey bee’s ability to recruit and direct workers to high quality resources. Lack of a numerical response by the native insect fauna to increased floral density may reflect the solitary nature of most native pollinating insects or result from competition from honey bees. Future experiments are planned to test the hypothesis that low visitation by native insects to highly abundant floral resources results from competition with non-native honey bees.