Tilia trees: toxic or valuable resources for pollinators?

To investigate whether Tilia trees are poisonous or valuable resources for bee visitors, we determined the nectar and pollen quantities and composition of the four main Tilia species planted in Western Europe (T. cordata, T. platyphyllos, T. tomentosa, and T. × europaea). We developed a new method to detect nicotine in nectars. We observed insect visitor diversity and abundance. We also assessed bumblebee death when individuals were only fed with Tilia flowers. No traces of mannose or nicotine, incriminated in the Tilia toxicity, have been detected in the nectars of the studied species. Huge numbers of insect visitors, mainly bees and syrphids, visited the trees which offer large numbers of flowers, plenty of sugar rich nectar, and protein-rich pollen. Bumblebees only fed with Tilia flowers did not present any particular mortality. We discuss the different hypotheses of the supposed toxicity and propose future research to solve this debate.