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Three years in a row I have experienced what I think are serious horizontal spreading issues of Varroa or Varroa Bombs in my neighborhood. I have been trying to understand the source of the Varroa. Is it a home-grown or horizontal migration the issue. I think the answer is both but horizontal migration dominates in the Fall. Sequential Fall Varroa migrations result in re-infestation of brood - winter brood!

This year I was a little more methodical in my evaluation of the treatment steps I take. I have 9 colonies undergoing OAV treatments post "super" removal. I use a Dead Drop Count (DDC) via a sticky board as an evaluation method. One colony was intentionally put through a brood break, one was a July nuc which built-up to hive level, one was Spring "save" which grew and grew to be a sizable colony and six were strong spring survivors. All, except the nuc, were OAV treated, once last Christmas. Monitoring all summer was by sticky board observations + drone removal and inspection (nil observed Varroa) and a July inspection of 800 capped drones plus random samplings of worker brood and drones in other hives. 6 Varroa were found in 800 capped drones; none found on purple eyed pupa. It is not easy searching in the soup for Varroa. Six colonies have VSH type queens, one hive has a possible 4 year old Hawaiian queen, two others have mutt queens raised from swarm cells the prior year.

My hives are located on 5 platforms separated by approximately 100 feet or more with one or two hives to a platform. In one case two hives and a grown-up nuc share a platform (nuc hive to be moved soon).

These comments are not based on a large data pool of numbers but more on several years of Contrarian experiences using OAV. My observations so far are:

1. Winter OAV treatment has a dramatic impact on Varroa infestation and growth all Spring and Summer - low numbers until Fall.

2. My strong colonies find and rob other colonies but seldom within the apiary.

3. Less than a 100% efficacy, generally 85 to 95%, typical of all treatment methods, allows Varroa infestation of winter brood while treating. I need to try other treatment methods to truly compare results but that is a big, well- planned effort; possible APIVAR comaprison would be interesting. Simple logic /math supports this conclusion. No vendor claims 100% efficacy and OAV does not kill capped Varroa. Phoretic Varroa can have a 4.5 day turn-around in re-infestation of larva. Even less, a day perhaps, if a phoretic Varroa mite migrates to a new colony.

4. Brood break treatment method (sample of one) appears to verify low summer numbers but is affected by horizontal migration. This is suported by a sharp, 2-3 week Fall Varroa DDC rise and fall versus longer, 6-7 week elapsed time curves for other hives.

5. Varroa Resistance is possible. After 5 years, I have a colony with a NWC queen that has amazingly low Fall numbers. THe colony seems defend against immediate neighbors casuing horizontal spreading. They may also have a non-robbing trait. At the opposite end of the spectrum I have a mutt queened, strong hive which I predict will not survive the winter based on Dead Drop Mite Counts and infestation length of time .

6. Multiple OAV treatments appear to have no significant effect on colonies or queens ( observation, no data).

I have spent two full months, 8 treatments in total, this 2019 Fall period of repeated OAV treatments to kill off Varroa so that my Dead Drop Count falls below 50 and hopefully into single digits (5 colonies). One colony remains above 50 as of 11-18-2019, 1st treatment 9/18/2019. All colonies are still active and raising winter brood ( I think, by observation, as I do not disturb them now). I plan on OAV treating and counting once more before Thanksgiving to complete this testing sequence. I will also OAV treat twice, 14 days apart, around Christmas or New Years ( hopefully a 99.7% efficacy).

This effort begs the question - how do we stop Varroa at the entrance? Ankle biting? Elimination / reduction of weak, failing colonies? Modify robbing behavior? I think hygienic behavior is a latent, defensive response and not a preventative solution for Fall Varroa Bombs.
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