Faithful Reader Anita asked me to explain varroa.
Sigh. That's a huge topic, and one that is fraught with opinions and lots of things I don't completely understand.
The short answer is that there are two creatures living in my hives: honey bees (Apis mellifera) and varroa mites (Varroa destructor). Varroa is a mite that came from Asia. There, the mite causes little damage as the Asian bees are more tolerant. Since the 1980s, however, varroa has spread around the world. Our Western honey bees are much more susceptible to the varroa, which is credited as being one of the many things contributing to colony collapse disorder.
What happens is that the mites lay their eggs in the bees' egg cells. They are particularly fond of drone larva as those cells are bigger and the gestation period is longer and more to their liking.
Mite infestation weakens and maims the bees, causing them to be vulnerable to other parasites and diseases. Google for images of varroa and varroa damage. It's grim.
I do have varroa in my hives. Everyone does. When I did a basic count recently I was pleased to not see much. From time to time I see varroa on a bee. That makes me sad. One of my more experienced beekeeper mentors told me that this year has been good for bees in that the varroa doesn't seem to be as evident as in past years. Last year was apparently really bad. I don't know how much weather contributes to varroa levels in bee hives.
There are wildly disparate schools of thought on treating for mites from doing nothing (the idea being to encourage the bees to, via natural selection, breed themselves stronger as the mites only become resistant to treatment) to a variety of commercially available treatments. Somewhere in the middle are methods that include encouraging and then culling drone brood or splitting hives (artificial swarms) to disrupt the mite life cycle or sprinkling powdered sugar on the frames (the bees clean themselves, removing the mites, the mites have difficulty maintaining purchase and fall off).
I know beekeepers on both ends of the spectrum. I ask lots of questions, read as much as I can, but mostly I worry.
I worry a lot about my bees anyhow. Are they okay? Am I doing what is best for them? Am I too intrusive? Am I not reacting quickly enough? It's very stressful, and I lie awake at night often, fretting about my responsibility to all these creatures living under my watch.
This is a very short and vague answer, but I hope it helps.