The mite treatment calander goes like this:
Treat for Varroa in August or early Sept. Use Miteaway Quick Strips, ApiGuard or ApiVar strips. These mite treatments have good results. Then in late October treat with Oxalic Acid. The late treatment will kill any mites that have reinfested the hive during the robbing period of September and early October. When bees try to rob honey during those times, mites can hop off the robber bees and then reinfest your treated colonies and you may get a high level of mites again.
If you wait to treat past the August to early September treatment window the bees will be getting damaged by Varroa. The longer you wait to treat, the more damage Varroa does to the bees. Waiting to treat in October, your bees may be so far gone they will die over the course of the winter.
How to apply Oxalic Acid. This is a great article from Randy Oliver at his website: Oxalic Acid Dribble Method
Randy's bees are in California so after reading his article you need to form your own conclusions of how effective it would be in northern climates.
The dribble method may not be an option if it is cold in late October. You may not to want to get your bees wet.
The other method is the Vaporizer Method.
Nature's Nectar LLC does now carry the Varrox Vaporizers. The vaporizers are widely used in Europe. The following video is put out by the Varrox manufacturer. Double click on the video for full screen.