Grazing on Australian Eucalypt Leaves by Insects

Abstract

Discrete samples of leaves from 28 species of eucalypts were collected from various habitats in southeastern Australia for measurement of herbivory in terms of (1) percentage of leaves damaged by folivorous insects and (2) the proportion of total leaf area consumed. There was high variation among samples in percentage of leaves damaged (range 27—100%). Proportion of leaf area consumed varied greatly from leaf to leaf and from sample to sample, even among closely located conspecifics. Mean values for different habitats, however, were remarkably similar, varying from 5.3% (urban cultivars) to 9.7% (roadsides) for a grand mean over all habitats of 7.6%. Distributions of values often were highly skewed and most median values were low. A review of the literature combined with the present data suggests that the baseline level of folivory by insects on healthy eucalypts is about 7.5%, with a secondary peak at about 15%. The latter is attributable, at least in part, to elevated folivory suffered by eucalypt seedlings, saplings and regrowth. The literature includes conclusions that chronic folivory of Australian trees is considerably higher than that on other continents. However, the values from Australian eucalypts overlap broadly with those elsewhere, and the differences, if real, are not so great as sometimes believed. An appeal is made for more process studies of the dynamics of folivory in which individual cohorts of leaves are followed separately and account taken of variation in leaf-longevity and the phenology of flushing.