The phoretic stage of the mite lifecycle is 5-7 days, so it's possible with a 7 day OA treatment interval that a mite can emerge and make it back into another capped larvae before the next OA treatment. I have been looking for (but haven't found) good data on how many days after treatment the OA is still killing mites. I've seen drop rates, but these don't distinguish between mites that took a few days to die after being exposed as opposed to a mite that mite have emerged a day or more after the treatment and was exposed to lingering OA. Oxalic acid is very hygroscopic, so humidity in the hive will quickly convert the tiny crystals to a reactive solution, and my bet is the OA dissipates very quickly as it "bleaches the woodwork".
I have treated at 3-4 times at 5 day and at 7 day intervals, and anecdotally the 5 day treatment was more effective, but I need to do more controlled testing to be sure. With brood in the hive, OA is definitely not as effective as it is when the hive is broodless. My last round of several treatments at 7 day intervals in July only reduced the mite counts by approximately 50%. (Measured using the powdered sugar shake method, shaking for a solid minute to ensure reasonable accuracy of the count) I'm leaning towards either doing more frequent OA treatments during summer, or switching to MAQS for summer and OA for winter when the mite counts indicate it's time to treat (>2 mites per hundred bees sampled).
For what it's worth, I have seen nearly zero negative effects from using OA (Heilyser JB200). The bees perk up nicely when the mite counts drop.
-Knute
I've been using OAV at 7-10 day intervals, 4x. I've been pleased with the results. Oldtimer suggests 4x at 5 day intervals. Can someone familiar with the mite life cycle comment on most effective treatment interval?