Hi all
Please don't misunderstand me, I don't doubt that bees are getting mites from crashing hives in late summer. This is the standard explanation and has been corroborated by Randy's and other's observations. 

An alternate scenario, one which seemed plausible to me, was that bees from crashing colonies could disperse mites by abandoning their hives and joining healthy ones. This would have great benefit to the mites. However, it does not seem to be supported by research:

> The average percentage of bees drifting from untreated hives infested with varroa into neighbouring treated hives did not exceed 3% (Table 1). There were no significant differences (P > 0.1) between the percentage of drifting bees from the varroa-infested colonies and the treated colonies at any stage throughout the trial. 

> This suggests that the reinfestation of treated colonies does not predominantly result from heavy varroa infestations causing worker bees to drift to other colonies, more than they generally do in practically varroa-free colonies. This supports Neumann et al. (2000). The normal level of drift in apiaries will, however, spread varroa from untreated to treated colonies. This observation supports the practice of treating all colonies in an apiary simultaneously.

> The presence of the queen and a small number of workers in the dying colonies suggests that the reinfestation was not due to colonies absconding and flying into colonies in the apiary. The alternative cause of reinfestation of treated colonies may be that the bees from treated colonies rob dying colonies and transport varroa back to their colonies. 

Goodwin, R. M., et al. "Drift of Varroa destructor-infested worker honey bees to neighbouring colonies." Journal of apicultural research 45.3 (2006): 155-156.

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