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]