>>Oxalic in late fall is not a safe for the coming winter but for the 
winter after that.

This is an excellent point.  OA is very effective - from 85% to 100% 
depending on the study - in eliminating varroa.  It's a pleasure to 
see colonies of vibrant honey bees the following spring.  Barring 
mite re-infestation and perhaps susceptible genetics, these colonies 
will continue strong until the next fall.

That next fall I like to identify the best hives, in terms of honey 
production and mite tolerance, for queen rearing the following 
spring.  Hopefully, one day I'll be able to either skip the OA 
treatment or do it every other year.

As far as the effectiveness, I believe, if one uses precisely the 
prescribed method (ie. the 3.5% concentration and 4-5 ml of solution 
per a full Langstroth bee space) in the broodless period, one 
consistently approaches a 100% mite kill w/o serious harm to the bees.

Waldemar 

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