A few years back in 2006 as part of collaboration with Betterbee Inc. and Penn State University, the idea of a tower hive set up was piloted in New York as a method of Varroa mite control.
A tower hive configuration is simply two hives placed side by side. When the hives are ready to be supered, the super is placed over the top of the center of the adjoining hives. In order to keep the queens separate, a flat queen excluder is placed under the super, on top of each brood area ensuring the queens will never meet, but that the workers will get acquainted with one another. Two modified covers (aka fancy, schmancy plywood) are placed on either side of the super to cover half of each of the two brood boxes. In each hive, underneath the modified cover, there is a drone comb frame.
Once every month this frame is removed from the hive and placed in the freezer overnight. The idea is that the drone comb frame will act as a magnet for Varroa mites, as they prefer to lay their eggs in drone comb. The mites will concentrate on this frame and when it is removed and placed in the freezer the mites will die. The hope is to catch the mites right before they hatch alongside of the drones. The frame is then placed back in the hive and the workers clean out the remains of the dead mites and drone larvae. Now, we all know that honey supers in the summer can be heavy and awkward to lift and this is why the drone comb frame is placed directly under the modified cover for quick, easy removal.
Not only does the tower hive seek to be a method of Varroa control, but also has numerous other benefits. One such advantage is that if one queen dies over the course of the summer the other queen can sustain the population of both hives, laying eggs in one hive while the other side is filled with honey from workers. This will keep the hive from becoming infested with wax moth. Although the information is somewhat limited on the effect of honey production, it is quite possible that the tower hive configuration could increase the honey production of hives in the tower. This needs further research as the reasons for this are currently unclear.
One thing is for sure, Varroa mite levels were lower in colonies in a tower setup in New York. Note that Varroa were not completely eliminated, but prevalence was certainly lower. To say that putting hives in tower configurations will cause you to be Varroa free and eliminate the need for a possible chemical treatment (if that is your method of Varroa control) is not so. However using a tower configuration can stave off the need to treat for longer and lower the treatment amount.
Because this study was done in a small scale in the past, we are looking to carry it out in Maryland. All interested in participating should go to http://www.msbeea.org/ to find out more information. If the window for sealed drone brood has already past, as I know it has in USDA colonies, we would love to invite your participation for next season.