Killing varoa mites - formic acid treatment

During the last month I noticed that I got higher and higher varoa mite count in the bottom plate of the hive. Just before the last harvest I counted more than 100. After the last harvest I fed the bees with inverted sugar so they had something to feed on and then it was time to treat the bees with formic acid. This is a kind of harsh treatment and lots of bees will be killed but the hive will be almost cleared for mites afterwards.

The procedure is simple. For a 12x10 box with 10 plates you need 3 ml of 60% formic acid per plate i.e. 30 ml which is sprayed out on the bottom plate with a syringe.

This is repeated 4 days in a row. The bottom plate is sprayed with formic acid preferably at night where the temperature is lower and the bees are all in the hive. In this way all bees are exposed without the concentration in the air is excessively high.  There are several other procedures but this one is simple, easy, and not other equipment is needed. 

The result is obvious already after the first day. 

... Every small black dot is a mite

After the 4th day I estimate I had killed about 1000 mites (every dark brown dot). 

I changed the cloth in the bottom plate and a week after I counted about another 1000 mites......

..... And about 500 the week after (no pic).

Hereafter I have only counted 1-3 mites. But I was surprised that I had this many mites in the hive. Fortunately the number has only been high for a few weeks so the family had not been damaged by the invasion.