Fall MITE BOMB - ID and Treatment

Varroa Mites can explode in Fall from robbing of collapsed hives. This happened to one of mine.
Notes: All hives were treat in August with 6 rounds of Oxalic Acid Vapor at 4 day intervals. Mite drops were monitored with oil tray bottom boards. After the last treatment very few mites were left.
In October, ONE (only one) hive showed a tremendous amount of mites. This is likely due to that hive robbing out a weak and dying feral colony or kept colony from somewhere in the area. I am not aware of any other beekeepers close by, so my bet is a feral colony.
These fall mite bombs are tricky....if you have solid bottom boards you'd never find this unless you did a mite wash on each hive before winter. Screen bottoms or oil tray bottoms make micro-checking much easier, and that ability to easily monitor for mites probably saved this hive.