> historically beekeepers assumed that at least a third of their colonies
would not over winter, because they were subpar.
Prior to varroa, that was my experience as an almond pollinator--I'd allow
for combining in fall, and expect to take only 2/3rds of the summer high
count to almonds, but this certainly wasn't because I'd experience 33%
"natural" winter losses. I've been wondering about the validity of the
oft-reported 5-10% range as being the norm to which today's 20-30% loss
rates are being compared, especially with commercial beekeepers (as Pete
noted) trying to overwinter any colony with a pulse, hoping that it will
survive to fill their almond contracts.
It is of interest that Jones categorized the 18% losses during an
"exceedingly severe" winter as the heaviest in a long period of years. His
observations help to confirm a 10-15% range as being "normal."
It's been widely reported that the winter loss rate did its jump from
"normal" in 2006-7 (although the jump actually occurred in Calif beginning
in 2004-5).
So my question is, what is the baseline to which we should compare today's
20-30% *reported* winter loss rate (note that this is a reported rate, not
an actual measured rate). Is it really that much higher than that of the
time period between when the mites arrived and 2004-5?
As best I can remember, winter loss rates went up and down wildly from the
time the two mites arrived in the U.S. (1984 for tracheal, 1987 for
varroa). We got hit hard in Calif several years apart, with tracheal
greatly leading varroa, and then again each time miticides failed, or the
varroa-associated viruses evolved. Winter losses were often in the 50%
range, and sometimes much higher (I experienced 70% from tracheal one year,
and ~97% from varroa).
Is the 30% loss rate the new normal? But as I posted recently, in a
California field trial that I ran last winter starting with 150
intentionally weak colonies (but with relatively low mite levels), winter
loss was only 5% (in a Calif winter). I don't keep detailed records for
the rest of my operation, so can't say exactly, but that rate seems to be
my norm. So I'm curious as to whether that is the norm for other
beekeepers who keep mites under control.
--
Randy Oliver
Grass Valley, CA
www.ScientificBeekeeping.com
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