Let’s back up for a moment and first define Varroa sensitive hygiene or VSH. VSH is a trait found in some honey bees that causes them to remove Varroa-infected pupae from the brood nest. A number of breeders around the country have developed lines of honey bees with enhanced VSH. When these bees detect the presence of Varroa mites beneath the cell cap, they rip the cap off and haul the pupa out the front door. Videos of this process are amazing in a lurid sort of way.
For many years the VSH trait has been seen as the potential savior of honey bee populations. However, a number of beekeepers are now reporting colonies of VSH bees that have gone out of control, ripping out not only the infected pupae but all the pupae. While this certainly takes care of the mite population, it also takes care of the bee population. Oops. This trait is sometimes called hygienic behavioral disorder.
So what causes it? Researchers are not exactly sure. Perhaps all the brood in those hives were infected. Or perhaps the bees become so sensitive to Varroa that even the brood near to infected brood is hauled away.
It seems likely, though, that there is a distribution of Varroa sensitivity in the VSH lines. In other words, some bees will be extremely sensitive, some moderately sensitive, and some barely sensitive. So occasionally you will get a queen who has the extremely sensitive gene which she passes on to her daughters. These bees, in turn, rip out all the brood.
This trait should be somewhat self-limiting. If the queen can’t produce new queens because they all get ejected from the hive, the trait would eventually disappear.
Another complaint I’ve heard from some beekeepers is that VSH bees are poor honey producers. However, most beekeepers say there is no difference in honey production between VSH bees and other bees. It would be interesting to know whether those beekeepers reporting poor honey production have the extremely hygienic type. If so, it may be that low brood survival is causing low honey production in some VSH hives.