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Oviposition site selection by the eucalypt herbivore Chrysophtharta bimaculata (Olivier) (Coleoptera : Chrysomelidae) and the implications for larval establishment


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Howlett, Bradley G 2000 , 'Oviposition site selection by the eucalypt herbivore Chrysophtharta bimaculata (Olivier) (Coleoptera : Chrysomelidae) and the implications for larval establishment', PhD thesis, University of Tasmania.

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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.

Item Type: Thesis - PhD
Authors/Creators:Howlett, Bradley G
Keywords: Chrysomelidae, Beetles, Eucalyptus
Copyright Holders: The Author
Additional Information:

Thesis (PhD.)--University of Tasmania, 2000. Includes bibliographical references

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