Effect of epicuticular wax on adhesion of psyllids to glaucous juvenile and glossy adult leaves of Eucalyptus globulus Labillardiere

In the heteroblastic plant Eucalyptus globulus, the glaucous juvenile leaves are coated in epicuticular wax that is relatively absent on the glossy adult leaves. The adhesion of three psyllid species (Ctenarytaina eucalypti Maskell, C. spatulata Taylor and Glycaspis brimblecombei Moore) to glossy adult leaves, adult leaves to which the juvenile leaf wax was added, glaucous ('waxy') juvenile leaves, and 'de-waxed' juvenile leaves was compared. Wax reduced adhesion of all three species but C. spatulata and G. brimblecombei were most affected. Unlike the other two species, C. eucalypti could consistently climb the adaxial and abaxial surfaces of vertically inclined 'waxy' juvenile leaves. Ctenarytaina spatulata and G. brimblecombei were able to climb the 'waxy' juvenile leaves only by clinging to protruding veins of abaxial leaf surfaces. Psyllid adhesion to adult leaves was reduced when juvenile leaf wax was transferred to glossy adult leaves. These results are consistent with previous findings that juvenile leaves are resistant to C. spatulata and G. brimblecombei but are susceptible to C. eucalypti. Observations under light microscopy suggested that the tarsi of C. eucalypti are better adapted for adhering to the epicuticular wax-coated surfaces than those of the other two species. Ctenarytaina eucalypti used all legs to adhere to flat surfaces while the other species used only the front two pairs of legs. All three psyllid species had adhesive pulvilli on their basal tarsomeres; but C. eucalypti had eight and 11 times more total pulvillar area per unit of body weight than did C. spatulata, and four and five times more total pulvillar area than did G. brimblecombei, for males and females, respectively. Based on the 'foot prints' of the psyllids on soot-coated glass, C. eucalypti caused less exfoliation than the other two species when walking.