A diverse supply of pollen is an important factor for honey bee health, but information
about the pollen diversity available to colonies at the landscape scale is largely missing. In this
COLOSS study, beekeeper citizen scientists sampled and analyzed the diversity of pollen collected by
honey bee colonies. As a simple measure of diversity, beekeepers determined the number of colors
found in pollen samples that were collected in a coordinated and standardized way. Altogether,
750 beekeepers from 28 different regions from 24 countries participated in the two-year study and
collected and analyzed almost 18,000 pollen samples. Pollen samples contained approximately six
different colors in total throughout the sampling period, of which four colors were abundant. We ran
generalized linear mixed models to test for possible effects of diverse factors such as collection, i.e.,
whether a minimum amount of pollen was collected or not, and habitat type on the number of colors
found in pollen samples. To identify habitat effects on pollen diversity, beekeepers’ descriptions
of the surrounding landscape and CORINE land cover classes were investigated in two different
models, which both showed that both the total number and the rare number of colors in pollen
samples were positively affected by ‘urban’ habitats or ‘artificial surfaces’, respectively. This citizen
science study underlines the importance of the habitat for pollen diversity for bees and suggests
higher diversity in urban areas.