> Their statement does not suggest to me 
> that the feed was fatal for many of the 
> bees.  Is there something that I'm missing?

Of course their statement does not say "the feed was fatal".   They claimed
it was "sub-lethal exposure in the title of the paper".  

But the concentration of poison itself said "fatal for many of those who
partook". 

I was not speaking of the conclusions drawn by Lu, et al.  I was speaking of
the conclusions that arise from the numbers as presented by Lu, et al.  In
the below, I take their statements backed by quantitative measurements as
"true", and I ignore their qualitative assessments and conclusions.  (The
Jack Webb method: "Just the facts, Ma'am") 

This is speculation of course - I am merely trying to piece together an
explanation for why the hives lived as long as they did under conditions of
such abuse.  

a) They mixed up around a 135ppb of poison in feed.
b) They fed only 1/2 gallon at a time, once per week
c) 135ppb is almost, but not quite at the LD50 for Imidacloprid
d) Not quite half the bees eating a reasonable amount of the poisoned nectar
would die quickly
e) It would be unreasonable to expect a mere half-gallon of syrup to feed to
every bee in the hive, or to be fed to every bee in the hive.

It follows that the "dose per bee" in most cases would be "zero" (didn't eat
any), and in some cases would be the fatal (ate enough to get the full
dose).

So, they simply did not notice the impact of the poisoned syrup on the
hives.  Weird, because Imidacloprid poisoning is hard to miss.
Tragic-looking quivering bees out front of the hive.  But there is no way
that feed with that level of pesticide could be fed without some level of
causalities directly resulting.

Given bees' preference for real nectar over any sort of feed, scented or
not, I could imagine a scenario where the feeder was utilized only on
non-flight days, and at night, as real nectar coming in the door would be
favored.  

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