Randy >The authors' hypothesis that the bumblebees were simply dying of old age
may be correct. 


Others suggest that they may be dying of starvation due to an over-reliance on a waning nectar source that may also have an addictive quality. 

The Koch and Stevenson paper states that although mannose was proven to be absent in many Tilia nectars (Baal et al. and Krasenbrink et al.) the mannose poisoning myth persists. My guess is the mannose theory got its footing from the fact that it was von Frisch who discovered its toxicity, and the rest was folks running with an unproven assumption about Tilia nectar. Of course, natural poisoning cannot be ruled out because another unknown compound may be present at times.   



>For decades, linden trees (basswoods or lime trees), and particularly silver
linden (Tilia tomentosa), have been linked to mass bee deaths. This phenomenon
is often attributed to the purported occurrence of the carbohydrate
mannose, which is toxic to bees, in Tilia nectar. In this review, however, we
conclude that from existing literature there is no experimental evidence for toxicity
to bees in linden nectar. Bee deaths on Tilia probably result from
starvation, owing to insufficient nectar resources late in the tree’s flowering
period. We recommend ensuring sufficient alternative food sources in cities
during late summer to reduce bee deaths on silver linden. Silver linden metabolites
such as floral volatiles, pollen chemistry and nectar secondary
compounds remain underexplored, particularly their toxic or behavioural
effects on bees. Some evidence for the presence of caffeine in linden nectar
may mean that linden trees can chemically deceive foraging bees to make
sub-optimal foraging decisions, in some cases leading to their starvation.

Do linden trees kill bees? Reviewing the causes of bee deaths on silver linden (Tilia tomentosa)
Hauke Koch and Philip C. Stevenson

https://royalsocietypublishing.org/doi/pdf/10.1098/rsbl.2017.0484

Bill Hesbach
Cheshire CT

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