Aaron Morris' "Varroa Answer" speculates on the way a mite-less colony
    gets infested.  Here in British Columbia, we have witnessed a progressive
    spread of Varroa from the most southern areas close to the US border
    northward.  Due to BC's topography marked by mountain ranges running
    north-south, Varroa has been following the valley floors where bees are
    kept, and where bees are being transported.  We have seen the typical
    'leaps & bounds' phenomenon of finding incidental Varroa infestations in
    apiaries that by themselves are pretty isolated (from other bee
    populations) but are always close to the highway.  Clear proof of
    infested bees escaping from trucks moving colonies from southern BC to
    the far north.
 
    The other form of spread is more localized but rapid nonetheless, which
    can only be explained through drifting and drone visitations.  I
    commented sometime ago on the distance of flight of bees, and referred to
    research done by Tibor Szabo in the early eighties.  Drones with free
    access to other colonies, coupled with their 'high motivation' (ie. sex
    drive) and physical ability to fly fast and far, are overwhelming likely
    playing a critical role as vectors of Varroa.
 
    Your question about how Varroa distinguishes between a drone cell/larva
    and a worker cell/larva is intriquing.  I have often wandered about that
    also, but perhaps some chemical or pheremonal cues may play an important
    role.
 
 
    Paul van Westendorp
    Provincial Apiculturist
    BC Ministry of Agriculture
        Fisheries & Food
    17720 - 57th Avenue
    Surrey, B.C.
    V3S 4P9  CANADA