>15% normal losses number
Charlie Those of us who are in cold climates, who have been keeping large numbers of colonies over winter, and who preceded the introduction of varroa mites, and when most colonies stayed put, rather than go to sunshine states in the winter, had a more definitive ability to count colony annual losses. We didn't have the option of mid-winter splits or any replenishment of bees and queens until spring, when the hives were opened and inspected, and the replacement ordered from the queen and package producers.
Both the literature and common practical, experience lead to these overwintering survival rankings: 1) 5% loss, about as good as it ever got, it was a really good winter (and colder winter conditions tended to display higher survival rates than warm winters, when the bees were more active and consumed more honey stores), 2) 10%, which was more or less a realistic goal, 3) 15%, a bit of a tough winter, but things happen. Keep winter losses to 15% or less over several years, and one felt that they were good beekeepers.
Mites hit, and losses were initially spotty and catastrophic - tracheal in it's first appearance resulted in losses as bad as any severe CCD incident. Then things settled down, and wintering losses of around 20% became the new norm. Add varroa, and the new norm was 30-35%.
10 to 15% is the new 5% - it can still be realized, but it is less common.
For current times, I look to Tom Seeley's feral colonies - over wintering colony losses in an ecotype group, mid-20s. And, if you properly examine BIP survey results, that's the norm for commercial beekeepers, around 23-25%, years after year.
I use Seeley because that's the equivalent of the bees of years ago, who didn't get hauled all over the country, who more or less stayed in fixed apiaries, and who were replenished, restocked, or split once a year (usually spring or early summer).
Trying to compare to modern migratory beekeeping with dual warm seasons - bees have become snowbirds, leads to different management practices and options.
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