The effects of heteroblasty in Eucalyptus globulus on herbivory by three species of psyllids

As plants mature from seedlings to reproductive adults they go through a process termed phase change or ontogenetic aging. Heteroblasty, which is an expression of ontogenetic aging, describes plants with vegetative juvenile shoots that are morphologically different from their adult vegetative shoots. Little is known about the effects of heteroblasty on plant-insect interactions. This dissertation describes field studies on the effects of heteroblasty in E. globulus ssp. globulus Labill. on herbivory by three species of psyllids (Ctenarytaina eucalypti Maskell, C. spatulata Taylor and Glycaspis brimblecombei Moore) in the East Bay region of San Francisco, California. Preliminary observations revealed that C. eucalypti colonized the juvenile shoots, and C. spatulata and G. brimblecombei colonized the adult shoots. Ovipositional choice experiments showed that psyllid distributions were due to ovipositional preferences. Despite differences in the color and shape of juvenile versus adult shoots, both Ctenarytaina species were more attracted to visual cues from juvenile shoots. It is unclear whether the psyllids use visual cues to distinguish between the juvenile and adult shoots. In no-choice experiments with adult psyllids, C. eucalypti survived equally well on juvenile and adult shoots, however, C. spatulata and G. brimblecombei survived longer on adult shoots. Based on honeydew production and stylet probing behavior, C. eucalypti fed equally on both leaf types, but C. spatulata and G. brimblecombei fed only on adult leaves. Removing the epicuticular wax from the juvenile leaves increased the stylet probing behavior, and survival of C. spatulata and G. brimblecombei on juvenile leaves. In adhesion experiments, C. eucalypti adhered better than C. spatulata and G. brimblecombei to the waxy surface of juvenile leaves. Observations under light microscopy suggested that the tarsi of C. eucalypti are better adapted than those of the other two species for adhering to slippery waxy surfaces. Per unit of body weight, C. eucalypti had four times more total pulvillar area than C. spatulata, and eight times more pulvillar area than G. brimblecombei. These studies show that heteroblasty affects insect herbivory, and in the case of E. globulus, these effects were related to the presence of epicuticular wax.