In any season, choosing the best Varroa destructor mite treatment requires some thought. In summer, however, it is complicated because most beekeepers have supers on their hives to collect honey for human consumption, which limits many of the options for food safety reasons.
Of the three products, formic acid (available commercially as Formic Pro) is the summer mainstay because it has two important features: It can be used when honey supers are in place and it kills varroa mites under brood cappings, meaning all of the varroa in the hive are vulnerable to it. It has one important downside to its use, however. It has an upper limit on the maximum outside air temperatures during the first few days of the 2-week long treatment period. Ideal daily air temperature maximums are between 50-85 degrees F. Temperatures above 92 F in the first three days may result in significant brood, bee, or even queen loss. In some areas of the U.S., summer daily highs remain well above 85 F for weeks on end, making Formic Pro inappropriate. But in many areas, it is possible to find a slightly cooler, 7- to 10- day period in summer in which to get the treatment underway. With all pesticides it’s important to carefully follow the use and application instructions on the label. With Formic Pro, details about leaving the entrance wide open and the screen bottom board closed are very important to the treatment success and safety. So please, take them to heart.
The second useful option for summer, HopGuard III, also allows the honey supers to be in place during the treatment period. However, it is not as effective at killing varroa mites protected under the brood cappings, like FormicPro. On the other hand, it doesn’t have the high temperature restraints of Formic Pro, making it a better choice in areas where daytime temperatures prevent use of formic acid. The duration of the treatment is 30 days, and it can be repeated up to three times per year. The active ingredient is made from the hop plant, the same plant that is used to make beer. (Beer, however, is not effective against mites and best reserved for drinking.)
The "one-half" option is oxalic acid. It has no upper air temperature limit, making it great for summer use. However, it is not labeled for use when supers are in place, nor does it kill varroa protected under the cappings.
From time to time, over the course of a summer, these two constraints are sometimes removed by unusual events. For example, during any period of broodlessness, where the supers are also not on the hive, oxalic acid offers the possibility of a single treatment event providing excellent varroa control. It can be applied in the form of a sugar-syrup mixture which is dribbled on to the surface of the frames (see instructions here), or heated in a specialized device (see here and here) with the resulting plume of tiny crystals retained inside the hive. Instructions for doing this can be read here. If oxalic acid is heated (a process often called vaporization), the operators must commit to wearing a respirator equipped with acid gas cartridges when applying the treatment. An excellent, and effective, summer use of oxalic acid vaporization, is after a new swarm has been hived and settled in, but before there is any capped brood. Any period of broodlessness, should be considered as a potential opportunity for oxalic acid. Even in a hive with brood, but without any supers on the colony, you may consider doing a short series of vaporizations as useful, though less effective, than when capped brood is entirely absent.