1. Department of Plant and Environmental Sciences, University of Copenhagen, Thorvaldsensvej 40, 1871 Fredriksberg C., Denmark
2. Laboratory of Virology, Wageningen University, Droevendaalsesteeg 1, 6700 AA Wageningen, the Netherlands
3. INRA, UMR 1319 Micalis-AgroParisTech, AgroParisTech UMR Micalis, 78352 Jouy en Josas, France
4. Honeybee and Silkworm Research Unit, Consiglio per la Ricerca in Agricoltura e l’Analisi dell’Economia Agraria, Padua Seat, Via Eulero 6a, 35143 Padova, Italy;