There was once a thought that the linden tree had a sap that was toxic to bees. A paper that I found claims that the more likely cause of bee death is actually starvation. The flowers of fhe linden tree have little or no nectar late in their flowering stage, but the presence of small amounts of caffeine may mislead bees into believing they are finding a food source. This paper recommends planting alternate late summer flowering plants nearby.

royalsocietypublishing.org/…/Do-linden-trees-kill…