REINFESTATION OF AN ACARICIDE-TREATED APIARY BY VARROA-JACOBSONI OUD

The sources of reinfestation of a treated apiary by Varroa jacobsoni Oud. were studied in Friuli (North-Eastern Italy), in an area with a high density of colonies. Ten colonies, initially Varroa-free, were treated with Apistan or Bayvarol strips and mites killed by these treatments were counted twice a week for 1 year. Five hives were provided with ''drone excluders'', to avoid the entrance of drones from other apiaries. Nectar and honeydew flow were monitored. The reinfestation rate was low during spring, varied between 1.6-13.7 mites/day/colony during June, July and first week of August, and rose impressively during September and October (up to a mean of 75.6 mites/day/colony); it was relatively high when nectar flow was scarce. The presence of drone excluders did not help to limit the number of mites imported: drones did not seem to be the main cause of reinfestation. The coincidence between the increase in the reinfestation rate and the scarcity of nectar flow and the massive importation of mites, observed especially in September and October, suggest that reinfestation was mainly caused by robbing of infested colonies (mostly feral swarms) by the bees of treated colonies.