Update 12.11.2018

Totally forgot to show you what effect the formic acid has:

The dark spots are those mites, they die, fall off of the bees and down onto the marked squares. We count them and can estimate how affected the hive is/was. If you don't do this (of course a few bees die too), your hive will most likely die.

If you can't spot them (I won't blame you!) here's two of them circled:

It's this time of the year again. To not have lots of dead bee colonies next year, the beekeepers have to help the bees fighting the mighty varroa mite.

We do this by vaporizing formic acid in each hive. Most of the bees survive this (a few very young and very old ones can't handle the vapors and die), the food they have left gets slightly contaminated but isn't used for honey production anyway.

We add a tray with some kind of blotting paper, which wicks the formic acid and evaporates it over the next week.

The mites die and the bees brush them off of each other. To count the mites we add a so called diaper or nappy/napkin under the hive:

The mites (and other stuff) falls onto this sheet so we can estimate by counting the dead mites how much the hive is affected by them.

We'll know more in about one week from now.