>>But the usefullness of oxalic is limited in a place with no broodless period.>>

I am not sure about the toxic effects of OA, but I will comment on using OA when hives do not become broodless.  In my area of coastal California, hives do not become broodless and do not cluster for long periods during fall and winter.  

I did an experiment in 2006 where I caged all of my queens for long enough to create a one week long period where no capped brood existed in all hives during late November.  The hives were infested with mites and had not been treated all season.  I applied an OA dribble as recommended for broodless wintering hives.  I applied it during the evening after the majority of the bees were in the hives.  Plenty of dead mites dropped onto the boards beneath screened bottoms over the next few days.  

The queens were released from their cages and allowed to resume brood rearing.  The hives appeared healthy until the new brood cycle emerged.  The infestation of the hives then appeared similar or worse than it had been before treatment.  Hives collapsed apparently due to the mites as well as the lost brood rearing time during November.

In my opinion, not only is a nearly broodless condition required for an OA treatment to work, but there must also be a prolonged period of winter clustering.

--Jeremy Rose
San Luis Obispo, CA

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