Pollen use by Megalopta sweat bees in relation to resource availability in a tropical forest

1. Spatial and temporal availability of pollen helps shape bee foraging behaviour and productivity, which has been studied in great detail at the landscape level, but never in a diverse tropical forest. 2. To study the effect of spatio-temporal variation in resource distribution on pollen use and productivity, we identified pollen from spatially explicit nest collections of two generalist sweat bees, Megalopta genalis Meade-Waldo and M. centralis Friese, from Barro Colorado Island, Panama, a 50-ha forest dynamics plot during the 2007 dry and early wet seasons. Pollen from nests collected in 1998–1999 without spatial information was also identified. 3. Bees used pollen of at least 64 species; many of these occurred in only one collection. The 2007 collections contained pollen of 35 different species, but were dominated by five species, especially Hura crepitans L. and Pseudobombax septenatum (Jacq.) Dugand. 4. Temporal availability, but not distance from nest, influenced flower use at a 50-ha scale. 5. Body size was not associated with minimum flight distance as inferred from pollen collections. 6. Nest productivity and pollen diversity decreased from the dry to wet seasons, mirroring community-level availability of floral resources. 7. Results suggest that on a scale of 50 ha, bees are choosing certain host plant species regardless of distance from the nest, but adjusting foraging behaviour opportunistically based on the temporal availability of host flowers.