CALIFORNIA buckeye is the culprit.  The big "horse chestnut" trees that
most people think of are different, and apparently not a problem.
 
California buckeye is a major pain if you're in the parts of California
that have it.  The nectar deforms or kills brood (symptoms are a little bit
like the deformity from heavy varroa infestation) and a tell-tale sign is
piles of dead bees in front of the hive that look bleached out or faded,
many with deformities.  I lost a couple of colonies to CB for the first
time this year.  I've seen increasing signs of it for the two preceding
years but thought it was mites.
 
I've heard, but don't know if it's true, that the pollen is also toxic and
that it can cause a secondary winter kill.  The idea is that the colony
stores CB pollen in season, then covers it over with other pollen later.
Then in winter or early spring, they uncover and use the CB pollen and
another generation of brood is affected.
 
I have noticed that only certain colonies are affected by CB.  I suppose it
has to do with different forage patterns for different hives.  Only 3 out
of 20 of my colonies at the affected yard seem to have been noticeably
affected by the buckeye problem.  Two of them died.
 
The only solution I know of is to move the bees during buckeye bloom.
 
> Seems like I remember somebody saying something about buckeye trees not
> being good for bees.  What's the story here?  I have a buckeye tree right
> in the middle of my bee yard.  Should I be worried about this?
> Please advise.