Honey Bees Reduce Pollen Viability While Foraging

Pollen acts as both a gametophyte for plant reproduction and a vital nutrient source for bees. Adult honey bees (Apis mellifera) mix pollen with nectar, enzymes, and microbes to create 'bee bread', diverting pollen from plant reproduction and re-appropriating it as larval food. However, the point at which corbicular pollen becomes nonviable is largely unknown. This question is important not only because it explicitly addresses pollen viability while bees pollinate, but also because it informs the food vs. fertilization tradeoff at the center of bee-angiosperm mutualisms. Here, we investigated changes in pollen viability during foraging bouts of honey bees. We observed pollen germination across two plant species: Allium tuberosum and Solidago rigida. Bee-collected pollen was contrasted against fresh pollen directly from floral anthers, de-ionized water-soaked pollen, and sucrose solution-washed pollen (a nectar substitute). The bee-collected pollen exhibited significant reductions in germination for both A. tuberosum and S. rigida pollen, compared to controls and the sucrose solution. Pollen viability, therefore, was greatly reduced while the bees in our study were foraging, suggesting that honey bees render pollen nonviable as they pollinate. These findings reveal why corbicular pollen contributes little to plant fertilization, highlighting the importance of non-corbicular 'body pollen'.