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
   INTERNET [log in to unmask]