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Did a balanced bee-mite relationship develop?
Everyone knows that honey bees are sweetness and light - producers of honey and beeswax - but we beekeepers know that these insects offer another gift: the quiet communion of the beeyard. "And I shall have some peace there," William Butler Yeats wrote, "for peace comes dropping slow." This is the peace of bees humming all around, the peace of harmonious union with nature. Lately, though, the beeyard has become less a place of serenity and contentment, more one of anxiety and concern, for our bees live in a world awash in pesticides, overrun by invasive pests, and afflicted by exotic diseases. Perhaps foremost among these threats is the well-named mite, Varroa destructor, which will kill almost any honey bee colony it infests unless a beekeeper provides protection.
Five years ago, I began wondering if honey bees and Varroa mites in North America might evolve a stable host-parasite relationship, one that would enable them to coexist without human intervention. I suspected that there may be little opportunity for the evolution of a balanced bee-mite relationship in places where most colonies of bees are managed by beekeepers. This is partly because we control mite populations to avoid loss of colonies, thereby weakening the selection for Varroatolerant bees, and partly because we manage our bees in ways that promote the "horizontal" transmission of the mites between colonies. By horizontal transmission, I mean the infectious transfer of mites among unrelated colonies. This occurs, for example, when Varroa-bearing workers drift into an uninfested colony or when Varroafree workers rob a colony with mites and bring them home. Many of our beekeeping practices - such as crowding colonies into apiaries, housing bees in look-alike hives, and transferring frames of brood between hives - promote the horizontal transmission of mites between colonies. This is unfortunate, because biologists studying the evolution of parasite virulence have learned that horizontal transmission of a parasite fosters the evolution of virulence in this parasite, by favoring those strains of the parasite that reproduce vigorously, and thus harmfully, in one host before moving on to infest another.
We might expect, however, the evolution of a balanced bee-mite relationship in a place with little or no beekeeping, hence where the population...