Influence of insect herbivory on the decline of black box (Eucalyptus largiflorens)
Stone, C & Bacon, PE 1995, Australian Journal of Botany, vol. 43, pp. 555-564.
A study of the influence of insect herbivory on the dieback of black box (Eucalyptus largiflorens) was undertaken in southern central New South Wales in 1993. Insecticide was applied to half of the tree samples to investigate the relationship between insect herbivory and different tree characteristics. Insecticide treatment, together with reduced rainfall, reduced insect herbivory damage greatly. The least trunk diameter increment was measured in trees that suffered the highest level of insect herbivory. There was surprisingly little evidence to show a consistent relationship between herbivory and crown condition. Moreover, two distinct foliage morphologies, both broad- and narrow-lanceolate, were observed in adjacent trees, as opposed to having only narrow-lanceolate foliage which is typically described for the species. Trees with broader foliage were found to be more susceptible to insect herbivory.
Premature decline of trees (dieback) can be caused by many factors, one of which is insect herbivory. Herbivory by leaf-mining, leaf-galling, and sap-sucking insects can cause leaf damage and removal of leaf tissue that is important for photosynthesis. The level of insect herbivory in trees can be determined by foliage quality, foliage defenses, rainfall and insect population pressure. In this study for example, the application of insecticide and the coincidental rainfall reduction led to a 18% herbivory reduction in treated trees, and 9% reduction in untreated trees. Insect herbivory can alter many aspects of the trees’ physical characteristics. Trunk diameter increment was significantly negatively correlated with insect herbivory, indicating that herbivory may have a significant effect on black box growth at the study sites. Interestingly, crown condition did not appear to be affected by herbivory which is contrary to what the authors assumed initially. It was then suggested that although insect damage contributes to dieback of black box, there may be other more influential factors involved, such as groundwater salinity and soil moisture. Black box is typically described having narrow-lanceolate juvenile and adult foliage. In this study, however, the authors observed both narrow-lanceolate and broad-lanceolate leaf shapes in trees that were adjacent to each other, and those with broader leaves seemed to suffer more from insect herbivory in both insecticide-treated and untreated trees. The authors believed these differences were driven by genetics rather than environmental factors. The former difference could be due to hybridisation or intra-specific competition, while the latter difference could be attributed to differences in the physical and chemical traits of the foliage. As the study was not initially aimed at addressing this specific question, it is worthy of further research to examine these interesting results regarding leaf morphology.
Lastly, this study highlighted the benefits of selecting trees with higher herbivory tolerance (narrow-leaved) for planting in this region to reduce black box dieback.
Summarised by Cherie Chan
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