There's always _Tilia tomentosa_ (from S.E. Europe) which is reported as
killing (bumble) bees.

Sounds very far-fetched. As far as I know, bumblebees do not travel in swarms
and do not pollinate trees. They are solitary, they live in the ground, & work
on herbaceous flowers. In most cases where large numbers of honeybees have
fallen to the ground, they were drunk on fermented nectar. I don't think it
kills them. If they died suddenly, it was from something else, not a tree.
Iris,
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
train."
Robert Lowell (1917-1977)