Effects of nest invaders on honey bee (Apis mellifera) pollination efficacy

The work of pollinators is crucial to the sustainability of plant communities in natural and agricultural ecosystems; however, pollinators are declining in much of the developed world due to a variety of parasites, diseases, and environmental stresses. These experiments are the first to examine directly the impact of honey bee, Apis mellifera, nest invaders on plant pollination and fitness. A cost to pollination could occur under two scenarios: (1) at the colony level where nest invaders compromise the health of the foraging cohort and reduce their efficacy as pollinators or (2) at the community level where invaders simply kill bee colonies and reduce the local pollinator population. Honey bee colonies were manipulated to achieve different levels of the parasitic mite Varroa destructor or nest-invading beetle Aethina tumida and tented under one of two model plants: canola (Brassica napus) or rabbiteye blueberry (Vaccinium ashei). On the basis of single-bee flower visits, fruit-set was reduced in blueberry with bees from varroa-parasitized colonies. However, on the basis of colonies, there were no differences in blueberry fruit-set, number of blueberry pollen tetrads deposited on the stigma, and pod-set in canola among colonies with different levels of nest invaders or no-invader controls. Thus, within the range of nest invader densities used in this study, individual inefficiencies were erased by compensatory multiple flower visits by this colonial pollinator. By failing to affirm the functionality of scenario (1) this study indirectly supports scenario (2): the major contribution of honey bee nest invaders toward a pollinator deficit is the simple eradication of colonies. (C) 2008 Elsevier B.V. All rights reserved.