At rosybee we have never really had a big problem with varroa but we still diligently do a couple of treatments each year just to ensure they keep at a tolerable level, However, towards the end of last season I noticed that a couple of the hives had a large pile of dead bees outside the door which is often a sign of deformed wing virus. We did an autumn treatment with apivar but didn't failed to put the sheets under the open mesh floor so that we could note how many mites dropped.
Then this week we did a oxalic acid treatment and, because its still cold, the floors were in place so we could see the drop. Oh my!! After two days I estimate we had about 500 on each floor which is way above any safe limit so I am really glad we took action. I am now wondering if the apivar treatment didn't fully work for some reason as this is such a high number for this time of year.
The picture shows the volume we found and the mites in all their disgusting glory. I noticed there were a few baby mites in the mix which you normally would not see because they would be tucked up with the bee larvae but in winter they have to make do with adult bee carriers and so are equally impacted by the treatment.
Hopefully this has got a lot of them but we will have to keep an eye on the numbers and use further treatments if necessary. It just shows you have to be really vigilant.