I've been following the new method of OA and glycerin pretty closely. It's looking pretty promising. The biggest advantage is that only one treatment is needed. Randy Oliver has a good write up on it.
Yes, one varroa treatment each year would be a dream come true. But I'm not comfortable leaving our best mite treatment in the hive for weeks.
The questions become clear: 1) Will mites ever develop immunity to oxalic acid? 2) If yes, will extended exposure in the hive aid the mites in developing that immunity?
I suspect the answer is 'No' to both questions, but I'd like to hear some educated entomologists/biologists share their opinions on this.