Performance of adult psyllids in no-choice experiments on juvenile and adult leaves of Eucalyptus globulus

The presence of glaucous juvenile leaves and glossy adult leaves in the canopy of the heteroblastic plant Eucalyptus globulus Labill., is a source of within-tree variation that affects herbivory by three psyllid species (Ctenarytaina eucalypti Maskell, C. spatulata Taylor and Glycaspis brimblecombei Moore) (Homoptera, Psyllidae). In no-choice field experiments with adult female psyllids in clip-on cages, we compared (1) leaf-settling, honeydew production, and survival of the Ctenarytaina species on juvenile and adult leaves of E. globulus, (2) leaf-settling and survival of G. brimblecombei on juvenile and adult leaves, (3) leaf-settling and survival of the three species on waxy (untreated) juvenile leaves and de-waxed (rubbed) juvenile leaves, and (4) leaf-settling and survival of C. spatulata on juvenile leaves from which wax was repeatedly removed. Leaf-settling by C. eucalypti was significantly greater on juvenile than adult leaves, however, honeydew production and survival were equal on both leaf types. In contrast, leaf-settling, honeydew production, and survival by C. spatulata were significantly greater on adult than juvenile leaves. Similarly, leaf-settling and survival by G. brimblecombei were significantly greater on adult than juvenile leaves. Wax removal from juvenile leaves significantly increased leaf-settling and survival of C. spatulata and G. brimblecombei. Although wax removal decreased leaf-settling by C. eucalypti, it did not affect survival. Repeated removal of regenerating wax from juvenile leaves did not affect leaf-settling or survival of C. spatulata. These results suggest that the epicuticular wax of juvenile leaves plays a role in resistance to C. spatulata and G. brimblecombei, however other unknown factors are probably also involved.