>That said, as Brian points out, don't expect a strong reduction in mites
from vapor treatments while there is active broodrearing.

This is a subject that we have visited for the last few years. My current thoughts unsupported by scientific study;

As is clearly illustrated by playing with Rand's Varroa Model (available at his site and a great tool) any number of factors will have large impact on any treatment protocol. Perhaps the most challenging is trying to bring numbers down from a high load in fall. Mite drop is a poor indication of efficacy because you don't know the beginning population. Much better to do pre and post 300 bee samples. Then you get an accurate sense of the load. If the colony in question has a very high load to start with, repeated treatments that kill only a fraction of the mites will take a very long time to make headway. If brood is present you may not ever get ahead. Another factor is the efficacy rate. We are all lulled into a false sense of security by the claims of "up to 90 or 99 % mite kill". Well try working it out if you are really getting 50 or 60%. In the case of OAV it may well be closer to 25% per application. 

A lot of variables play into any treatment efficacy. Starting mite and bee population, colony configuration, weather, season, and in the fall, invasion from collapsing colonys nearby.  Tom Seely did a study illustrating the influx in his location in NY last year, it is dramatic and fast. I have observed that the influx is far from uniform in an apiary. I had a particularly difficult yard last fall. This site is  vary rural and far removed from any other managed colonys. However I have used it for decades and assume that my own escaped swarms are the source. As long as weather permitted flight the mite population persisted in spite of reaped OAV at 5 day intervals in a few colonys, while others in the yard tested  at less than 1%. This year I am doing a test of moving my annual Apivar  application from spring to fall on 25 colonys to see if a continuous application during the fall shutdown solves the problem. It is now too cold to do samples and I can't do anything about it anyway so we will see in the spring.

As far as repeated OAV applications I have never seen any ill effect after 3 years of use. This fall everyone got 6 treatments at 5 days. It worked well with most colonys testing below 1%. I believe that it is by far the least bad control in my northern inter-mountain location. A few that had visible PMS in late Aug. still had 3-5% after the 6 treatments and got the Apivar. 

Paul Hosticka
Dayton WA

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