> 1: The oxygen level in that hive was already dangerously low then adding
> the fume board just pushed it beyond the limit killing the bees by
> affixation? (Not sure how long that would take)
>

It would take a while.  Bees, unless vigorously flexing their wing muscles,
breathe only once or twice a minute.  They are able to survive in the
winter cluster at a reduced oxygen concentration of around 16%.

The stimulus of introducing a repellent on a hot day would indeed likely
cause many bees exposed to the fumes to begin fanning, but the oxygen would
not suddenly disappear from their surrounding atmosphere.

Thus, I don't see evidence to support the asphyxiation hypothesis.


> 2: A chemical reaction happened with other vapors (acid, alcohol, water,
> bases) in the hive producing a strong enough to kill (unknown) pesticide.
>

Occam's Razor suggests that we need not look further than direct toxicity
of butyric acid fumes to explain the kill.  I don't see a reason to suspect
that a new toxic chemical was formed.

When I arrived at the scene shortly after my sons called me, we inspected
the bottom boards of a number of hives.  There were a few freshly-dead or
dying bees on several, but only a serious kill in one hive.  The
temperature was around 100F, the hive bodies dark wood, and the yard was in
direct noon sun.  The fume board covers were dark anodized aluminum--very
effective at absorbing solar energy, and conducting that heat to the liquid
repellent.

The most parsimonious explanation would be acute toxicity due to a sudden
off gassing of butyric acid fumes due to an unfortunate combination of
extreme environmental conditions and operator error.

-- 
Randy Oliver
Grass Valley, CA
www.ScientificBeekeeping.com

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