When to treat for mites - It is all about the timing

 I like to treat my bees for mites usually around mid August. My mite treatment of choice is Formic Acid. There are two types of Formic Acid, Mite Away Quick Strips (MAQS) or Formic Pro. They are both formic acid but their application is slightly different. Formic Pro and MAQS are both made by the same manufacturer.  MAQS has a shelf life of about 6 months, while Formic Pro has a shelf life of two years. Expired MAQS should never be used because the delivery method has been compromised. Expired MAQS when applied will give 100% formic acid, the intensity of the vapors may injure the colony.
 Formic Acid should not be applied if the daytime temperatures are over 85 degrees for the first three days of treatment. This is a fumigant, the slide of a screen bottom board should be in while treating.
 Formic Acid is considered an organic mite treatment and can be applied with the honey supers on.
 But, sometimes the weather is too hot to put on the Formic Acid treatment. I have the mite treatment ready to use by end of July. So I watch the weather in August, if there is a window of three days of the 70's in early August, I will treat. By waiting until mid August, then a beekeeper thinks about treating, the weather can get too hot and stay there for several weeks. Sometimes not cooling off until the first week of Sept. Beekeepers need to get Formic on their hives sometime during the month of August. So watching the weather and long term forecasts is what I do. 
Here is a couple of manufacturers video's of using Formic Acid. Always follow the manufacturers label instructions when applying any miticide:

Formic Pro:




MAQS: