Local, landscape, and density drivers of phorid-bee parasitism in urban gardens in the California central coast
Changes in urban landscapes and habitat characteristics of urban green spaces may negatively affect pollinator diversity and may alter interactions between bees and their parasitoids. Declines in abundance and diversity of native pollinators, like bumble bees, increase dependence on imported honeybee hives to maintain agricultural production. Introduction of Apis mellifera does not directly affect Bombus spp. populations even though they share floral resources. However, A. mellifera may indirectly affect Bombus spp. via a shared parasitoid. Until recently, bumble bees and paper wasps were the only known hosts for Apocephalus borealis, but the phorid also parasitizes honeybees. Little is known about the ecology of this parasitoid, or the habitat or landscape features that may facilitate host detection or parasitism. We examined whether bee population density or local or landscape characteristics of urban gardens influence Apis and Bombus parasitism. We sampled bees using aerial nets, measured vegetation and ground characteristics of gardens, and used USGS National Land Cover Database to classify the landscape surrounding of gardens at 500m, 1km, and 2km scales. We kept bees in rearing containers and observed fly larval emergence. We then examined the correlations between host density, local and landscape variables, and parasitism rate using generalized linear models.
We collected 2113 bees, 1822 A. mellifera, and 291 B. vosnesenskii. Parasitism rates varied at the sites between 0% and 17%. Several local garden characteristics, but only one landscape factor influenced parasitism rates. Parasitism rates of both species increased with decreases in herbaceous cover, increases in the number of white flowers, and increases in the number of yellow flowers within gardens. B. vosnesenskii parasitism rates were positively correlated with increases in high-density urban development within 200m of gardens. Thus, local factors, associated with garden management, appear to be more important determinants of phorid-bee interactions. Local factors that influenced both genera were the same, suggesting that phorid abundance or host location is influenced by vegetation characteristics of a foraging site more so than by host population density. Given the host switch of A. borealis from B. vosnesenskii to A. mellifera is newly discovered, much research is needed to assess what facilitated the host switch, or influences parasitism rates. Given that both honeybees and bumble bees are important pollinators, understanding what drives their parasitism rates may affect ecosystem services in urban gardens.