Currently, the most destructive force to beekeepers is Varroa Mite. As beekeepers, we have to have a good understanding of what it is, what the signs of attack are, and how to handle an infestation if it gets in our hives.
Here is a University of Florida link. They have a good basic introduction to Varroa: http://entnemdept.ufl.edu/creatures/misc/bees/varroa_mite.htm
As I breed my own bees, I try to raise mite resistant stock. Typically, if you have a hive with bad hygienic behavior, you can requeen that hive with good hygienic stock, and the problem will fix itself. That being said, there are times when some form of treatment is necessary.
There are many treatment options out there. For people who want to stay natural, there is powdered sugar and a screened bottom board. There is oxalic acid vaporization, formic acid, and for the essential oil folks: Thymol.
If you don’t mind using a synthetic treatment, there is Apivar, Hopguard II, Apistan, and several others.
I have used the powdered sugar method. It works on a hive by hive basis, however it is not very feasible for treating hundreds of hives.
I haven’t used the oxalic vaporization, but it looks cool, and has reasonable reviews.
If you go synthetic, I have used Apivar strips as a last resort a couple of times. They do work well in reducing mite levels, however, they have a lot of drawbacks.
The point is choose what works for you, but do something. Varroa Mites are Phoretic. This means they travel from place to place on their host. Bees drift from hive to hive, and they interact out in the world with other bees. You have an obligation to the beekeepers around you to keep your mite levels checked and keep your populations down.