What Does the Specialist Say About This: Use varroa-resistant/tolerant strains of bees in your operation (these include Russian queens, Minnesota Hygienic Queens, and New World Carniolan queens).  Make sure colonies are not suffering from other maladies (foulbrood diseases, chalkbrood diseases, etc.).  If varroa populations reach damaging levels, the beekeeper can treat with thymol-based products registered for the control of varroa (ApilifeVAR or Apiguard) or formic acid products (MiteAway II). The problem with all three products is the outside temperature often exceeds suggested use levels on the product labels. So, using these products while it is too hot is detrimental to the bees. Also, it is important to keep a safe and consistant source of fresh water close for the bees during warmer weather when they are under stress. That said, the conventional varroacides (the fluvalinate and coumaphos-based varroacides) can be harmful to bees and varroa has documented resistance to both.  For more information read: http://edis.ifas.ufl.edu/in855 and watch: http://entnemdept.ifas.ufl.edu/honeybee/extension/index.shtml 

Jamie Ellis, Ph.D., Assistant Professor of Entomology, Honey Bee Research and Extension Laboratory, UF Department of Entomology and Nematology