Comparing the spring diet of buff-tailed bumble bees and honey bees (Hymenoptera: Apidae) in a forest landscape: a metabarcoding approach.

The declining diversity of pollinators is a major threat to ecosystem conservation, pollination services, and global food security. Honey bees (Apis mellifera L.) dominate managed pollination, but their dominance can affect other pollinators. Competition for resources can lead to decreased foraging success and survival rates for other bee species, including other eusocial bees such as bumble bees. This study explores the floral diet of honey bees and buff-tailed bumble bees (Bombus terrestris L.) using metabarcoding of honey (for honey bees) and of wax pots' contents (for bumble bees), based on 3 molecular markers (ITS2, trnL g-h, and trnL c-h) in Médoc, France. Molecular markers detected different species pools, indicating a high diversity of plants visited by both species. The 'marker' effect was more important than the 'pollinator' effect in distinguishing plant taxa. The Schoener index revealed a diet overlap in plant taxa used by honey bees and bumble bees, but the value was highly dependent on the molecular marker. Thus, metabarcoding on different biological matrices for both species proved to be efficient, despite some biases. Although the matrices enabled avoiding capturing individuals and disturbing colonies, and were easy to sample, comparisons of results between 2 different matrices should be made with caution due to different storage and consumption strategies used by both studied bees. Nonetheless, this study provides a fast and inexpensive approach to study eusocial bees floral resource sharing, and gives insights to improve metabarcoding effectiveness in order to better describe dietary niche of eusocial pollinators by noninvasive sampling.