Resolving the apparent inconsistencies in reports of colony mortality
from tracheal mites, etc. pointed out by Andy and Allen, is not possible
in a quick message, but a lot more good information is available.
The "truth" of the situation varies between different areas and likely
over time as well.
I've monitored a lot of colonies in western Canada for more than 10
years, from before the time the mites were present here (yes, they were
NOT present before 1986, and are still absent from some areas) and have
studied the effect of several miticides, and worked for several years on
developing more resistant bee stock. I'll try to put down some
statements that are consistent with what I've observed:
In the bee colonies here, tracheal mites regularly increased to levels
that would be considered very high and unusual compared to the
populations described by Bailey in Britain. This could result from
differences in the bees and/or the mites.
I was unable to see any colony symptoms of even very high (more than 80
% of bees have tracheal mites) infestations, during the summer foraging
season.
The effect on winter mortality of a certain level of infestation in
colonies here has been quite similar to the effect reported from Europe.
A rough description of this effect is: the added probability of colony
winter mortality is about equal to the tracheal mite infestation
percentage in the fall. ie. if a beekeeper had 10 % winter mortality
without mites, then had colonies with 40 % of the bees having tracheal
mites, the mortality would be (10 + 40 =) 50 %. (This "rule"
overestimates the effect at the low end and perhaps underestimates it at
the high end: we see little effect when infestations are below 20 %, but
above 80 % almost no colony would survive). It may not be the same in
Florida.
Our area varies from a temperate climate with wet winter and a bit of
snow (like Britain) to a continental climate with longer winter (mostly
frozen from November 1 into April, but a much better honey flow). The
longer winter results in poorer winter success anyway, but also
accentuates the winter effect of a given level of mites and makes
earlier intervention necessary. However, bees in the continental area
have typically had less frequent high tracheal mite populations.
(My feeling about this is that the usual more intense honey flows (50 to
100 kg crop) in the continental area wear out mite-infested foraging
bees before the bees have lived long enough to produce a good crop of
tracheal mites. Combined with the rapid growth rate of bee colonies in
this area, the tracheal mite population often stays lower than in the
south coastal area.)
Treatment for tracheal mites? I use 20 % (of bees examined, having
tracheal mites) in fall as a threshold: the further above this, the
greater the risk of winter loss, so the greater the potential benefit of
treatment. The further below 20 %, the less benefit per cost of
treatment. In the north, effective fall treatment of high infestations
is difficult because brood rearing decreases greatly in September, and
high infested colonies have increased mortality even if the mites are
killed by late fall treatment. In the south, September treatment is
effective (the mites get killed, then the bee population is replaced).
Since Varroa became a major influence, less attention has been paid to
tracheal mites, and things may have changed.
Resistant bees? From the first year that tracheal mites were found in
large apiaries, there were colonies that remained low-infested in the
same yards where others became highly infested. I worked on a project to
select from commercial North American stocks (with Page and Gary's
newly-emerged bee assay) over 5 to 10 generations, stocks that picked up
the least tracheal mites. It wasn't a "black and white" effect (we used
isolated yard natural mating rather than AI), but by 1994, the 3 lines
in this group compared to 3 Buckfast lines were at least as resistant as
the European. It's very likely that natural selection was operating all
over North America to result in the same effect, and perhaps also to
reduce the survival of very high reproducing mite varieties (this isn't
necessarily a contradiction to survival of the fittest, it's a part of
most host-parasite relationships).
There is clear good evidence of increased winter mortality of colonies
at higher tracheal mite infestations, and some environments seem to
favour higher mite levels. We don't know whether the frequency of such
higher infestations is decreasing, but it seems likely.
Kerry Clark, Apiculture Specialist
B.C. Ministry of Agriculture, Fisheries and Food
1201 103 Ave
Dawson Creek B.C.
V1G 4J2 CANADA Tel (250) 784-2231 fax (250) 784-2299
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