1. Department of Plant and Environmental Sciences, Faculty of Science, University of Copenhagen, Thorvaldsensvej 40, 1871 Frederiksberg C, Denmark.
2. Laboratory of Entomology, Wageningen University, Droevendaalsesteeg 1, 6708 PB Wageningen, the Netherlands.
3. Laboratory of Virology, Wageningen University, Droevendaalsesteeg 1, 6708 PB Wageningen, the Netherlands.