Its time to be thinking about varroa control. New beekeepers should especially take note and read this article. This article by Meghan Milbrath is a good refresher for everyone.
My take on this article and also based on my experience is that either formic aoid (MAQS) or thymol are the most effective mite controls, but require openning the hive. Other treatments, while easier to apply, are less effective and may require multiple applications. Using a combination of treatments may be the best strategy; such as MAQS right after honey harvest in mid August and then an oxalic vapor follow up treatment later in the fall.