Invasion of Varroa destructor mites into mite-free honey bee colonies under the controlled conditions of a military training area

The honey bee mite Varroa destructor can be spread between colonies by vertical transmission, particularly when heavily infested colonies are robbed by foraging bees from neighbouring hives. We quantified the invasion of V. destructor into mite free colonies on a military training area not accessible to other beekeepers. Ten "mite receiver colonies" continuously treated against V. destructor were placed at distances of one to 1.5 km from four heavily infested "mite donor colonies". Over a two month period from August to October, the population of bees, brood, and V. destructor in the donor colonies were estimated at three week intervals and the invasion of V. destructor into the receiver colonies was recorded every 7-12 days. During the experimental period, between 85 and 444 mites per colony were introduced into the receiver colonies. There were no significant differences in the invasion rates in relation to the distance between donor and receiver colonies. In total, 2,029 mites were found in the 10 receiver colonies, but these only correspond to 2.5% of the total mite population in the donor colonies at the start of the experiment. This means that the major part of the initial V. destructor population died together with the collapsed host colonies. Under natural conditions, a more benign behaviour should therefore be an adaptive strategy for V. destructor. From a practical perspective we could show that highly infested honey bee colonies present a substantial risk to already treated colonies up to distances of 1.5 km away.