All honeybee colonies in the UK (excepting parts of Scotland and some islands) have the ectoparasitic mite Varroa destructor. Varroa acts as a vector for a range of viruses of honeybees which are transferred when the mite feeds on haemolymph (blood) from the developing pupa. The most important of these viruses is Deformed Wing Virus (DWV). We have demonstrated that mite infestation preferentially leads to amplification of a specific virulent form of DWV. In mite-exposed developing pupae this particular virus reaches levels almost 10,000 times higher than seen in the absence of the mite, despite the development of an immune response to the infecting virus. We want to understand why the virulent form of DWV observed in mite-infested colonies or mite-exposed pupae replicates to such elevated levels. Varroa-free honeybee colonies or Varroa-infested colonies will be used as a source of individual honeybee larvae, harvested and maintained in the laboratory under carefully controlled conditions. Individual larvae will be either fed or injected with a characterised virus population. The tissues and organs in which the virus replicates will be determined using exquisitely sensitive hybridisation techniques on either dissected pupae or sections. We will repeat these studies in larvae in which we have deliberately suppressed key components of the immune response by inhibiting expression of the genes Dicer and Argonaute. We will pre-expose larvae via feeding, to short RNA molecules designed to inhibit DWV replication. We will test whether DWV carries a gene that inhibits the effectiveness of RNA-based immune responses using well-established techniques. We will investigate the role of specific host genes implicated in components of the immune response or development to enhanced susceptibility to DWV-mediated disease. We will suppress individual genes of interest and then challenge larvae with known virus populations.