Despite decades of reporting on bumblebee mortality associated with Tilia, new work suggests no actual link:

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. 

In sum, the now available facts about the chemical composition of the nectar presented in this paper together with the fact that no poisonous effects of the nectar were observed ... is that the bumblebees die only as a result of natural causes as a normal part of their life cycle.

Fossen, T., Holmelid, B., & Øvstedal, D. O. (2019). Bumblebee death associated with Tilia x europaea L. Biochemical Systematics and Ecology, 82, 16-23.

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