You only occasionally actually see them in a hive — the pinhead-sized crablike Varroa destructor mites that have in the past seventy years become the nemesis of beekeepers in every continent save Australia. Varroa is indisputably the number one problem affecting bee health today.
The main problem is called the “varroa-virus complex.” Honey bees are host to about 20 viruses, most of which were formerly considered relatively benign. Varroa changed all that — not only does the mite vector viruses from one bee to another (similar to how mosquitoes vector malaria), but it also injects an immune suppressor into the bee, which then activates “latent” viruses into bee-killing machines (similar as to how HIV induces infections to explode in AIDS patients).
The second problem started as beekeepers worldwide desperately sought out miticides that could be safely applied to bee hives in order to kill this “bug on a bug.” And they worked — at first. But in a few short years varroa developed resistance to the first two miticides, resulting in a wave of colony losses of 50-90 percent beginning around 2004. An unforeseen consequence of chemical control of varroa is that residues of those miticides now constitute the most prevalent pesticides currently found in bee hives worldwide. These residues have changed the baseline toxin load with which bees have long had to deal — the natural plant toxins frequently found in nectar and pollen, industrial pollutants, as well as agricultural pesticides.
Varroa isn’t the only recently-introduced parasite to be affecting bee health. At the same time that varroa really started to be an issue, the bee populations of both North America and Europe were invaded by the intestinal pathogen Nosema ceranae. United States Department of Agriculture (USDA) data indicates that its rapid increase in prevalence in the early 2000’s closely tracked the increase in colony mortality. I’ve spent the past few years intensely following nosema in my own bee operation, and can’t help but feel that Nosema is also a contributor to our recent problems.
I don’t typically see nosema as being the main problem — that distinction still belongs to varroa. The most recent USDA survey of hives across the country found that the average varroa infestation rate in the United States in autumn is above the danger level for virus epidemics. In any year that I hear reports of beekeepers having a hard time with controlling varroa, winter losses go up.