Thanks for the reply, dudelt. That is what I did last year, but since then I've read that the acid crystals condense on most surfaces in the hive. When the bees clean the hive they will get it on them and it will kill the mite then. I was just wondering if anyone knew the 'best' or 'most effective' temp to use at the winter broodless vap.
Possibly more important is that the colony is broodless ... I usually wait for the first long (3+ weeks) cold spell of the winter (night frosts) and then treat as soon as the weather warms. By 'warms' I mean just reaching double figures centigrade ... enough for cleansing flights but not foraging. The cluster breaks and the OA permeates better.
Last year this was early December, the year before it was late November. At both times colonies I checked were broodless.
Typically colonies in the UK are treated between Christmas and the New Year but experience shows that many have brood by then.
Since all manufacturers of OAV equipment recommend 37 degrees plus, I think the testing shows that OA sublimation and its movement in the hive is optimal at 37 and above. In my area, I do my "one shot" around xmas. They are broodless and you can usually get lucky with a day around 40 degrees. I give them a couple puffs of smoke to loosen them up if necessary but usually find that they are not tightly clustered at 40 with sun on the hives. J
Has anyone seen the effect of OAV on a cluster of bees, Oav not penetrating the cluster below a certain temperature, is this not another urban legend? Nothing to do with sublimation I am sure as the sublimation takes place in the vaporizer and most of the OA crystallizes within a few inches of leaving the nozzle of the vaporizer. Does anyone know the temperature that bees would form a cluster and not move. Just because something is written on a label or on manufacturers instructions does not make it so, one should question conventional wisdom and observe more to reach your own opinion.
I do OAV from the bottom entrance (I seal the opening with a rag), and usually see OA fume coming out of the small top entrance. But once I ignored the >37F instruction and did it at ~35F. Sublimation took place fine, but lots of fume came out of the bottom, nothing from the top. Because the bees were clustered in the top box, I was afraid OA did not reach the cluster, so I repeated treatment one week later, at ~44F (no fume from the bottom some from the top). The 2 nd treatment killed 2-3 times more mites compared with the first.
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