Effectiveness of managed populations of wild and honey bees as supplemental pollinators of sour cherry (Prunus cerasus L.) under different climatic conditions

Managed populations of Apis mellifera, Bombus terrestris and Osmia have been investigated rufa as sour cherry pollinators in two flowering seasons with different weather patterns. Flight activity of the three bee species during the pollination-receptive period of the cultivar 'Stevnsbaer' was recorded. Also, using insect-proof cages constructed around 'Stevnsbaer' trees, fruit set and yield were recorded and compared with self-pollination in the absence of insects and open pollination. When floral development and seasonal conditions are suitable, there is a potential role for introduced Osmia rufa and B. terrestris to add to the pollination activity of conventionally employed A. mellifera. However, in a season with a less favourable weather history, and despite comparable activity of the three species during the pollen-receptive period, introduced bees had only a limited effect on fruit set and yield and could not raise yield above that of freely pollinated branches. The contribution of appropriately dense populations of introduced bees in the orchard, given favourable weather conditions, is clear, but maintenance of populations of introduced, wild bees, compared with honey bees may well be impractical due to cost. Known ecological risks associated with species introduction also need to be considered. Consequently, if wild bee populations are to be used to secure the potential benefits of increased fruit set and yield, then positive habitat management will be necessary to sustain the required populations.