Bumblebee death associated with Tilia x europaea L

Abstract

For more than six decades, bumblebee death, which mainly occurs in August, has been assumed to be associated with intake of presumed toxic nectar from linden trees (Tilia spp.), a hypothesis which has been sustained by observations of a significant number of dead bumblebees under these trees during their flowering season. Several theories exist in current literature to account for these observations. The nectar has been assumed to contain compound(s) toxic to the bumblebees including the monosaccharide mannose, which cannot be metabolized by bumblebees. The presence of toxic compounds such as the alkaloid nicotine or pesticides of anthropogenic origin has also been indicated. However, none of the above suggested compounds have hitherto been properly characterized from the nectar. In the current paper we report on characterization of the composition of nectar of linden trees, under which a significant number of dead bumblebees were observed. The structure determinations were performed by selective 1D TOCSY NMR and extensive applications of 2D NMR spectroscopy. The nectar of the linden flowers was readily available in significant amounts during the entire period when dead bumblebees were observed under these trees. The nectar only contained non-toxic sugars such as α-glucopyranose, β-glucopyranose, sucrose, fructose and minor amounts of xylose. The nectar did not contain mannose, nor alkaloids or pesticides. Initial toxicity studies using brine shrimp lethality assay showed that the nectar did not exhibit any toxic effects even at concentrations higher than 1 mg/ml dry weight, providing disproving evidence against the assumption of the nectar's toxic character.