On Thu, 22 Jul 1999 Chris Slade wrote:

> the typical varroa mite is the product of a brother/ sister mating.
> This must dramatically reduce their genetic variability compared to
> honeybees where the opposite is the case.

Just out of curiosity, on what basis do you conclude that Varroa mites
are largely inbred?  Assuming a sufficiently large Varroa population in a
hive, and assuming even a moderate degree of bee drifting and bee robbing
(thus transferring mites among hives), I would think the Varroa mites
would have enough genetic diversity to prevent serious inbreeding.

> From this it follows that once resistant traits appear among honeybees,
> as they surely will in the fullness of time, the difference in mating
> habits will have a multiplier effect enabling bees to out-evolve varroa
> mites.

I wouldn't be so sure of this.  Remember, as honeybees evolve, so will the
mites.  And the mites will evolve faster than the honeybees, because their
life cycle is at least an order of magnitude shorter--if not two orders of
magnitude shorter--than honeybee queens.

Without human intervention, clearly, the mites would have decimated the
honeybee population.  Of course, by doing so, the mites would have
decimated their *own* population as well, because they would have
eliminated most of the population of their only host species.  In this
type of host/parasite relationship (e.g., one where the parasite generally
kills its only host, but only after completing several reproductive
cycles), the following equilibirum would probably be achieved:

    1.  The honeybee population density drops low enough that the average
        distance between hives is greater than the field force foraging
        range.

    2.  As a result of the mites not being able to travel to new hives,
        the mite population takes a drastic nose-dive as they "burn out"
        most of the existing infected hives.

    3.  As a result of the negligible mite population, the honeybee
        population begins to increase.  Eventually, the average honeybee
        population reaches the point where the average distance between
        hives is short enough to allow cross-infection.

    4.  The mite population increases rapidly as the mites spread
        throughout the honeybee population.  The honeybee population is
        decimated as hives die off at a rapid pace.

    5.  Go to step #1 and repeat.

Obviously, this type of natural equilibrium is unacceptable to beekeepers.
But the type of equilibrium beekeepers want--a high honeybee population,
and only a background level of mites--is going to be very difficult to
consistently achieve with any of the current (or upcoming) forms of mite
control.  We don't want the Varroa mite to keep the honeybee population in
check (which is essentially what the mites are doing).

What we *really* need to discover is, essentially, a Varroa mite parasite
that isn't harmful to anything else.  This would do unto the Varroa
population what Varroa does unto the honeybee population: when the Varroa
population builds up enough to start traveling, the mite parasite would
spread rapidly and decimate the mite population.  A parasite that kept the
Varroa mite population in check would prevent the Varroa mite from keeping
the honeybee population in check.

This technique has been used successfully for gypsy moth control.
Specifically, the bacteria Bacilus thuringensis var. kurstaki infects the
larvae of moths and butterflies and prevents them from developing.  Many
counties in western Pennsylvania had a huge gypsy moth problem a few years
back.  (There were so many moths that when you went outside, you could
actually hear them eating the trees.)  Sprayings of Bacilus thuringensis
were carried out.  Because the gypsy moth population was so great, the
Bacilus thuringensis infection spread like wildfire, and it simply
*destroyed* the gypsy moth population.  Although I'm sure the gypsy moths
are still around, their population levels are low enough that I haven't
seen any since then.

Unfortunately, I don't perceive any great effort being spent in
investigating biological forms of mite control.  Biological controls are
generally long-term, high-research-effort solutions.  Coming up with new
chemicals is much easier (and more profitable, if you're the company
selling the chemicals).

James