Honey Bees Reduce Pollen Viability While Foraging

TL;DR

Honey bees reduce pollen viability while foraging, making it less likely to contribute to plant fertilization.

Contribution

This study is the first to show that corbicular pollen becomes nonviable during foraging, explaining its minimal role in pollination.

Findings

Bee-collected pollen had significantly lower germination than fresh floral pollen.

Pollen viability decreased even when mixed with a nectar substitute, suggesting other compounds reduce viability.

Corbicular pollen contributes little to plant fertilization, emphasizing the role of body pollen in pollination.

Abstract

Foraging honey bees consolidate pollen within ‘pollen baskets’ (corbiculae), dual structures on their hind legs that facilitate large pollen loads. The bees mix pollen with nectar and other compounds, creating a sticky mass that keeps the pollen affixed to their corbiculae. While foraging, pollen grains on the corbiculae may contact the floral stigma, but the viability of corbicular pollen is largely unknown. This question is important not only because honey bees are critical pollinators in agriculture globally, but also because the vast majority of collected pollen is consolidated on their corbiculae. Here, we examined corbicular pollen while honey bees were actively foraging. Such pollen exhibited significantly lower viability than pollen from the floral anther. Further, bee-collected pollen exhibited lower germination than pollen mixed with a nectar substitute, suggesting that compounds other than nectar sugars play a role in reducing viability. Thus, actively foraging honey bees rendered most of their pollen unavailable for plant fertilization, which explains why corbicular pollen contributes little to fertilization. This underscores the importance of non-corbicular pollen (‘body pollen’) in bee-mediated pollination.

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’.

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