Hi Alistair,
Varroa mites invade colonies using bees as means of transport. Drones from
other bee colonies will surely enter your hive possibly taking mites with
them. Other ways of colony invasion are 1. drifting of mostly young
unexperienced workerbees, which can occur over pretty long distances, and 2.
your bees may have robbed Varroa-weakend (wild) honeybee colonies, from which
they take, together with the stolen goods, mites with them back home. (For
some experiments read Sakofski et al, Apidologie 1990 and since you have the
ABJ available also read E. Rademacher, ABJ 1991 How Varroa mites spread)
 
Johan Calis
Honeybee research section,
Wageningen Agricultural University,
The Netherlands
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