The disappearance is most often not a mystery, especially with what you described. You noted that the hive that was the weakest all season is now the strongest, while the others are gone. It sounds to me there is a healthy, thriving Varroa mite population (perhaps in addition to tracheal mites). Here in the sw BC we have been hearing beekeepers telling the same story. When there is relentless pressure of parasitism on the beebrood during the summer season, the rate of bee population replacement slows down, and the adults seem to be induced to seek other colonies. This exodus seems exponential over time and within a few the bee population collapses. The brood left behind has deteriorated rapidly and with some spotty appearance, one may be tempted to think of a bacterial disease. But, try to remove some of cappings and you may have the mites 'jumping' at you. A dissecting microscope is most helpful but a good magnifying glass will do the same. It appears that during the population collapse, adult bees use cues in selecting certain colonies, while others do not nearly receive equivalent numbers of bees. What these cues are, is not known. The sudden increase in population size most often causes the beekeeper to become complacent by thinking his/her colony is doing wonderful. Of course, a substantial number of the adults are infested with mites and the bee brood is being increasingly parasitized. Ultimately, these populated colonies will collapse also. We advise beekeepers not to use simple visual inspection of the bees as the only method to determine mite population. Frequent monitoring is key to effective control. These controls ought to be applied only after the level of mite infestation has been determined. We recommend an Apistan strip(s) for 24 hours and a sticky board as the most accurate detection method. I suggest you do the same and I suspect you may be startled at the number of mites present in you apparently healthy colony. Also, when you do find mites, you may wish to leave the strips in for the recommended 40+ days to bring the population down. In your part of Washington, where winter conditions are so mild that bees retain brood well into November and start up as early as mid/late January, the mites will proliferate along with the brood. So, make sure to bring the population down before winter. Good luck, Paul van Westendorp [log in to unmask] Provincial Apiculturist British Columbia