Impact of control strategies for Varroa destructor on colony survival and health in northern and central regions of Italy

Abstract

SummaryA comparison among four different summer strategies for the control of Varroa destructor was carried out from summer 2010 until spring 2011, in collaboration with national beekeepers associations in Italy. A total of 170 colonies located in Lazio, Piedmont, Tuscany, Veneto were involved. The control strategies included chemical treatments and biotechnical methods (thymol, formic acid, brood removal and brood interruption). In each colony, bee samples were collected before and after treatment to assess the quantity of Varroa destructor and Nosema spp. and the presence of the main honey bee viruses. Colony strength was assessed several times during the trial. The most effective strategies for maintaining low V. destructor infestation levels and higher colony survival in spring in all regions were brood interruption (caging of the queen for the duration of a brood cycle), followed by brood removal. Brood interruption was the only treatment which yielded a decrease in varroa infestation rates (from 4.36 mites/100 bees (5.61 SD) before treatment to 3.24 mites/100 bees (4.0 SD) after treatment, while the highest average overwintering index (ratio of spring vs. autumn bees) and a lower virus prevalence was observed in the brood removal group. Analyses for virus presence showed that in spring 2011 the most prevalent virus was DWV. Quantification of Nosema spp. spores showed that only N. ceranae was present, with regional differences in infection levels.