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Conventional "wisdom" used to be that colonies in the north are broodless in December, making them prime candidates for varroa treatments most effective during broodless conditions, which is all of them, I know ;) Traditionally, OAV is applied around the winter solstice in December. Lately, I see people stating northern bees are broodless in October-November, which would suggest changing the timing of OAV by at least a month. Any input on when winter broodlessness is for northern US and lower Canada?
 

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I cannot give input into when they are totally broodless, because I never open them up in the dead of winter. Is it actually knowable, for all locations and situations?

I just aim for a OAV treatment sometime between Thanksgiving and Christmas, and call it good.
 

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I’m about 50 miles South of you (Zone 8, North Seattle area). When I had a deadout in mid January 2017, there already were capped brood. During winter 2017/2018, I had a plastic board inserted from the bottom entrance and checked hive debris every day. A small number of fresh-looking dead pupae could be found until 12/17/17. Weather permitting, my bees would collect pollen from early January until early December.

So, my bees seem to have a rather short period without capped brood, around mid-late December. Therefore, I always do 2nd OAV one week after the 1st (usually around X-mas), to make sure mite drop becomes near-zero. If not, I would continue weekly OAV until it happens (usually I can find warm enough days to do this).
 

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I am also in USDA zone 7a, although considerably more south. My queens typically stop laying toward the end of October and are back into production in January. Broodless might be a six week period between mid Nov and New Year's. Timing treatments of OAV for around Thanksgiving and Christmas works well for almost the entire country. Some bees never go entirely broodless, unless the queen died, so waiting for that mystical, magical moment may be in vain.
 

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Randy Oliver's graph of a data set developed in Canada (from a presentation available on Randy's Scientific Beekeeping website) indicates that the late-fall broodless period is actually more November than December or January.
Text Diagram Plot Line Slope


This November timing is consistent with data from the UK, which is described in the first ~12 minutes of a presentation at the 2015 National Honey Show by Ben Harden, "Bees in Winter". Ben also points out -- consistent with Randy's chart -- that some hives are never totally, truly broodless.

Personally, I live in Massachusetts and target late-November for my OAV teatments.

John
 

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What might be a correct time period for Italian habits bees will probably be way off for Carnis. Colony stores and population will skew things greatly too. It is perhaps a bit like the question, how long is a piece of string.
 
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