What’s the attraction? Early season native bee visitation to a non-flowering shrub, Adenostoma fasciculatum, at Pinnacles national monument, California

The evolution of angiosperms’ often elaborate, costly floral displays has been driven, in large part, by the foraging behavior of their pollinators.  Bees respond to visual cues, using petal markings to navigate to nectar sources and visiting colored objects placed in their habitats.  Observations at California’s Pinnacles National Monument of native bee activity on pre-bloom Adenostoma fasciculatum, particularly on shrubs coated in a black sooty mold fueled by honeydew excretions of colonizing scale insects, hint at bees’ ability to find and utilize alternate resources not visually advertised.  This foraging flexibility may provide crucial access to “emergency” resources as changes in climate increasingly disturb the synchrony between blooms and their pollinators.  An experiment was conducted in early spring of 2012 to determine whether shrubs with sooty mold are more attractive to diverse species of bees and, if so, what attributes are responsible for the attraction.  Results confirm higher visitation rates to shrubs with sooty mold.  A 20% sugar solution applied to randomly selected A. fasciculatum shrubs drew more bees just an hour after application than plants sprayed with water.  A black paint solution, mimicking the dark visual cue of sooty mold, did not influence bee abundance, nor did shrub surface temperatures.  Extermination of scale insects decreased visitation, suggesting that the presence of live honeydew producers, rather than moldy residue alone, is important to bees.  These results illustrate the efficiency with which bees locate alternate, unadvertised sources of sugar and are interesting in light of the broad evolution of elaborate floral displays.