I was helping manage the Portage Beekeeper's club hives this summer. Over winter, I treated with OAV, and the hive occupying one deep had a mite drop of 200, the one occupying 2 deeps had a mite drop of 300.
In May, when both were broodless and had recently swarmed, the mite count was 3/300. In July, when both had lots of brood, the mite count was 0/300. In mid August, the one hive had a mite count of 18/300. Because of high temps, summer honey supers, and new queens, we did not treat with MAQS in July.
In contrast, my hives (13 production, only treated fall/winter, with multiple OAVs) are still at 0-2/300, now. Any robbing genetics may have been, ah, largely weeded out, shall we say? during 2016, with my failed treatment free experiment. The survivors didn't rob, seems like. But that's another story.
Either the mite count for the Portage hives in July was incorrect, or mites came from somewhere other than in the hive. That's just too big of a jump. One way to tell: find brood comb, look for mite frass. TIny white dots on the ROOF of the cells. A high in-hive mite load through the summer should show, well, should show Parasitic Mite Syndrome by now. And should show lots of tiny white mite frass - at 6% infestation on adults, should see a couple mite frass in a 5x5 grid (25 cells). And older capped brood would have fewer mites than young capped brood - we would expect like a 50% increase with the mites reproducing, but not 2-3x as many in the young brood.
So.... at any rate.... I am concluding that the Portage hive went a-robbing, and found a weaker hive with a high mite load. This is where mite counts shine - the hives seemed fine in July and if we went by that alone, we would have been lulled into a false sense of security.
This is where current theories of mite peril fail - the threat from a hive bringing back massive numbers of mites due to robbing is real. But it's a subtle thing, to tell the difference btween a high mite count due to a high initial infestation, or a high mite count due to imported mites.
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