Effects of CO2 and temperature on Eucalyptus insect herbivores from individuals to communities

Abstract

Atmospheric CO2 concentrations and temperatures are predicted to increase dramatically during this century. Changes in these environmental factors may impact insect herbivore physiology, abundance and community structure. In general, elevated CO2 (CE) reduces foliar nitrogen concentrations while increasing the carbon to nitrogen ratio. These changes in foliar chemistry often result in slower development and increased mortality of insect herbivores. Elevated temperatures (TE) can directly accelerate the development of insects by increasing their metabolic rates. TE may also indirectly impact insect herbivores through plant-mediated effects. CE and TE may have opposing or interactive effects on insect herbivores, so it is important to understand how concurrent changes to these two climate change factors impact herbivorous insects. Further to this, the impact of CE on insect herbivores and insect-mediated processes (such as herbivory and nutrient transfer) is rarely quantified in mature forests. Field sites present the opportunity to understand how insects in complex environments respond to CE under conditions that are difficult to simulate in greenhouse environments. The main and interactive effects of CE and TE were examined for an insect herbivore feeding on two different Eucalyptus species (chapter 2). Paropsis atomaria (Coleoptera: Chrysomelidae) fed on the flush leaves of either Eucalyptus tereticornis or Eucalyptus robusta in a greenhouse. CE reduced the nutritional quality of both Eucalyptus species, while TE increased foliar concentration of nitrogen in E. robusta only. Larval developmental time and leaf consumption increased while female pupal weight decreased at CE via plant-mediated effects. Larval survival increased at CE on E. robusta but decreased on E. tereticornis. TE only accelerated larval developmental time. No interactive effects between CE and TE were observed in this study indicating CE is a stronger driver of changes in insect growth and survival via plant-mediated effects than TE under the experimental conditions in this study. As an extension to examining the effects of CE and TE on the growth and development of an insect herbivore, the immune response of P. atomaria was also assessed when it was feeding on E. tereticornis under CE and TE conditions (chapter 3). The cellular (melanisation) and humoral (phenoloxidase or PO activity) components of the insect's immune response to the implantation of a nylon filament was assessed and linked to changes in leaf chemistry. Haemolymph protein content and PO activity decreased at CE, however, the melanisation response increased at CE. TE had no effect on any immune parameters. Complex interactions of immune responses such as these occurring at CE may alter the outcomes of parasitoid or pathogen attack. Based on results obtained from these greenhouse studies (chapter 2 and 3), two field experiments were undertaken within a mature Eucalyptus woodland undergoing CO2 fumigation to investigate the effect of CE on herbivory and insect-mediated nutrient transfer. The impact of CE on insect-mediated nutrient cycling over two years at the Eucalyptus free-air enrichment (EucFACE) site is reported in chapter 4. CE did not impact the quantity or chemical composition of frass deposited at the site nor did it affect foliar nitrogen. Frass deposition showed a positive-lagged correlation with precipitation and average maximum temperatures likely linked to leaf phenology. CE may have a limited effect on insect-mediated nutrient cycling of mature forests in the short-term as the response of mature trees to CE may be lagged. The effect of CE on herbivory at the EucFACE site, and the role of leaf phenology on herbivory are reported in chapter 5. Young expanding leaves sustained significantly greater damage compared to fully-expanded or mature leaves. Thus, the availability of young expanding leaves drove monthly variations in leaf consumption. CE had no effect on leaf consumption or leaf age preference by herbivorous insects. Leaf phenology may be a significant factor in determining insect herbivory in sclerophyllous forests. Alterations in leaf phenology as a result of climate change may negatively impact insect herbivores particularly if insect phenology is synchronised with leaf phenology. The results of this Ph.D. research contribute to the understanding of (a) the main and interactive effects of CE and TE on the growth, development and immunity of insect herbivores; (b) the role of host-plant species in altering the response of insect herbivores to CE and TE; (c) the impact of CE on insect-mediated forest nutrient cycling and the interaction with rainfall and temperature; (d) the influence of leaf phenology and CE on leaf consumption. This work provides important information for the predictions of insect responses to CE and TE and this information is essential for the modelling of ecosystem responses. Results obtained from greenhouse studies in this thesis indicate insect herbivores may find refuge from the negative effects of CE in some growth, development and immunity traits particularly if they inhabit mixed-species forests. Furthermore, strong effects of CE on individuals of an important insect herbivore species of the EucFACE site in greenhouse experiments were not confirmed at herbivore community scales in the field due to complex interactions which may be unique to mature nutrient-limited forests.

Date of Award	2016
Original language	English