Paropsis atomaria (Coleoptera; Chrysomelidae) is a eucalypt feeding leaf beetle endemic to southern and east coast Australia, and it is an emergent pest of the eucalypt hardwood industry. Paropsis atomaria was suspected to be a cryptic species complex based on apparent differences in life history characteristics between populations, its wide geographical distribution, and extensive host range within Eucalyptus. In this study genetic and ecological characters of P. atomaria were examined to determine the likelihood of a cryptic complex, and to identify the nature and causes of ecological variation within the taxon. Mitochondrial sequence variation of the gene COI was compared between populations from the east coast of Australia (South Australia to central Queensland) to assess genetic divergence between individuals from different localities and host plant of origin. Individuals from four collection localities used for the molecular analysis were then compared in a morphometric study to determine if observed genetic divergence was reflected by morphology, and common-garden trials using individuals from Lowmead (central Qld) and Canberra (ACT) were conducted to determine if morphological (body size) variation had a genetic component. Host plant utilisation (larval survival, development time, and pupal weight) by individuals from Lowmead and Canberra were then compared to determine whether differential host plant use had occurred between populations of P. atomaria; individuals from each population were reared on an allopatric and sympatric host eucalypt species (E. cloeziana and E. pilularis). Finally, developmental data from each population was compared and incorporated into a phenology modelling program (Dymex(tm)) using temperature as the principle factor explaining and predicting population phenology under field conditions. Molecular results demonstrated relatively low genetic divergence between populations of P. atomaria which is concomitant with the single species hypothesis, however, there is reduced gene flow between northern and southern populations, but no host plant related genetic structuring. Morphometric data revealed insufficient evidence to separate populations into different taxa; however a correlation between latitude and size of adults was discovered, with larger beetles found at lower latitudes (i.e., adhering to a converse Bergmann cline). Common garden experiments revealed body size to be driven by both genetic and environmental components. Host plant utilisation trials showed one host plant, E. cloeziana, to be superior for both northern and southern P. atomaria populations (increased larval survival and reduced larval development time). Eucalyptus pilularis had a negative effect on pupal weight for Lowmead (northern) individuals (to which it is allopatric), but not so for Canberra (southern) individuals. DYMEX(tm) modelling showed voltinism to be a highly plastic trait driven largely by temperature. Results from across all trials suggest that P. atomaria represents a single species with populations locally adapted to season length, with no evidence of differential host plant utilisation between populations. Further, voltinism is a seasonally plastic trait driven by temperature, but with secondary influential factors such as host plant quality. These data, taken combined, reveal phenotypic variability within P. atomaria as the product of multiple abiotic and biotic factors and representing a complex interplay between local adaptation, phenotypic plasticity, and seasonal plasticity. Implications for pest management include an understanding of population structure, nature of local adaptation and host use characteristics, and predictive models for development of seasonal control regimens.