Figure 11.  A. Adult female and immature wingless (apterous) bird cherry-oat aphid, Rhopalosiphum padi. This and other aphid species are vectors of the viruses that cause BYD (Courtesy G.W. Bishop and S.E. Halbert, from D’Arcy and Domier 2000). B. Diagram of an aphid vector feeding on a plant host showing the internal route of the viruses that cause barley yellow dwarf (Diagram courtesy L.L. Domier, from D’Arcy and Domier 2000).