Oviposition site selection by the eucalypt herbivore Chrysophtharta bimaculata (Olivier) (Coleoptera: Chrysomelidae) and the implications for larval establishment
Creators: name: Howlett, BG
Publisher: name: University Of Tasmania
Year: 2023

description: Chrysophtharta bimaculata (Olivier) is a chrysomelid folivore of several Eucalyptus species, including E. (Monocalyptus) regnans, E. (M) delegatensis and E. (Symphyomyrtus) nitens. Both adults and larvae feed on the same tree species and eggs are laid directly onto host foliage. Adults feed and oviposit in aggregated swarms and, as a result, subsequent larval feeding may cause severe host tree defoliation and larval resource depletion. Further, previous research on a related chrysomelid has shown that factors associated with changing leaf age ( eg variation in toughness and nitrogen concentration) can seriously impact on larval survival. Because oviposition site selection is likely to be of fundamental importance to larval survival in C. bimaculata, the factors affecting oviposition site selection and the impact that the selected site had on subsequent larval establishment were chosen as the primary foci of this thesis. Research followed three main thrusts. In the first (Chapters 2-6), I documented exactly where C. bimaculata placed its eggs, both under natural and controlled conditions, from the individual leaf up to the level of tree species. Manipulative and correlative studies were used to determine what factors might affect site selection. C. bimaculata prefers to oviposit near the leaf tip and although there was no evidence that conspecific egg batches directly deter ovipositing beetles, leaves with egg batches on their tip are less preferred for oviposition. Other factors demonstrated to negatively influence oviposition site selection between leaves were increasing leaf toughness and conspecific beetle feeding damage. By altering leaf position it was demonstrated that leaf toughness, rather than leaf position, influenced C. bimaculata oviposition preference. In chapter 5, I document the effects of egg batch placement on larval establishment. Wild, aggregated populations regularly deposit approximately one-third of egg batches on mature leaves unsuitable for neonate establishment. However, neonates had the ability to migrate to suitable leaves and establish with no increase in mortality. This suggests a strong relationship between oviposition site selection and larval establishment within host trees. Finally, I examined host plant phenology under natural conditions (Chapter 7). Significant differences in leaf toughness development, size and chemistry between and within host species were found. Leaf chemistry may influence host plant selection between species. However, the rate of leaf toughness development in current season leaves is likely to determine host vulnerability to defoliation under high egg batch densities. This thesis indicates that C. bimaculata oviposition site selection is influenced by direct and indirect plant and conspecific factors. The interaction of these factors determine the egg distribution and often lead to high egg density within hosts. The strong relationship between C. bimaculata oviposition site selection and larval establishment increases the potential for high larval densities.