Chapter 1: General Introduction 1.1 Predicted impacts of climate change Climate change is a major current and future threat to agroecosystems. It is already modifying, and will continue to modify, species’ geographical ranges, structure of community and ecosystems; as well as increase the range of pest species and potentially will reduce natural enemy effectiveness (Andrew and Hill 2017). To properly assess the direct and indirect effects of climate change, critical observational and experimental data on both populations and species needs to be collected, and then be embedded into predictive models to assess responses of species to rapid changes into the future (McCarty 2001). For Australia, recent evidence of climate change is compelling: there have been no other warmer periods than post-1950 in the last 1000 years (Gergis and Members 2012) and the most recent decade (2000-09) was the hottest on record. In addition, 2015 was hottest year on record in Australia (Braganza and Church 2011; CSIRO-ABM. 2014; Karoly and Black, 2015). Prolonged heat exposure can have extremely detrimental impacts on animal populations. For example, during 2010 in Australia, a two-week heat wave led to the death of thousands of birds in the wild: this happened when environmental temperatures exceeded the maximum critical thermal limits of these birds (Miller and Stillman 2012; Bozinovic et al. 2013). In the wild, ectotherms are continuously exposed to several short-term variations in environmental conditions. In this context, the study of the impact of thermal conditions on the performance of individuals and their plastic responses to it, such as lethal and sub-lethal effects, are key to 1 understanding the responses of biota to the environment and to climate change (Pörtner et al. 2006; Somero 2011; Bozinovic et al. 2013). Based on climate change models, it is predicted that heatwaves (extended periods of abnormally hot weather) will be extended, more frequent and severe under continued climate change, there will be increases in mean temperatures over the next century (Parmesan 2006 ; Meehl and Tebaldi 2004; IPCC 2007; Gillespie et al. 2012; IPCC 2007; Ju et al. 2013; IPCC. 2014). With exposure to these extreme heatwave events, particularly when insects are under thermal stress at the warmest period of their life cycles (summer), ectotherms (including insects) may be highly susceptible to deleterious effects of extreme heat exposure (Musolin et al. 2010). The direct impacts of climate change including change in behaviour, physiology and ecology, and indirect effects of climate change including CO2, O3, temperature and participation will influence insect herbivore population dynamics, ecosystem function and community structure (Cornelissen 2011). Insects able to successfully adapt to host plant and environmental climatic factor, can complete their life cycle, too. By understanding mechanisms behind the strategies in extreme conditions we can develop hypothesis related to impacts of climate change on insect’s life-history and responses to climatic change in the future (Bale et al. 2002). 1.2 Global climate change and insects Ectotherms are very sensitive to climate change due to vital biological characters being strongly influenced by temperature including development time, number of offspring and their locomotion (Wilson 1992). Ectothermic insects cannot regulate their temperature above and 2 below ambient temperature relative to endotherms and may consequently be more adversely influenced by climate change (Walther et al. 2002). All insects are responsive to temperature changes, so under a warming climate we would expect this to impact the lifecycle of an insect and the ecosystems they inhabit. Climate change has substantive impacts on physiological characteristics and habitat use by of insects (Stange and Ayres 2010). Climate change may have a variety of impacts of biotic and abiotic factors of any ecosystem, as direct and indirect effects on several aspects of the life cycle of insects including the population dynamics, physiology, distribution and abundance. To better understand current and future impacts of climate change on insects we need to consider the changes in climate in relation to ability and plasticity performance traits of species concurrently (Stange and Ayres 2010). Climatic warming tends to influence (and frequently amplify) population dynamics of insects directly through effects on survival, generation time, fecundity and dispersal (Bale et al. 2002; Andrew 2013). Climatic warming also reduces or increase the risk of death in insect winter populations mortality due to extreme cold (Ayres and Lombardero 2000; Williams et al. 2014). 1.2.1 Insects at high temperature Abiotic factors such as extreme high and low temperature are major threat to insects, and can change their distribution by influence on development time , number of offspring and life span (Hutchinson and Bale 1994; Colinet et al. 2015). 3 Thermal mean and variability at local scales interact in a non-additive way to determine physiological performance thermal and thermal tolerance . This occur via change in performance curve parameters which in consequences may produce changes in activity patterns, timing of breeding, and finally may alter synchronization between trophic levels and species community structure (Ashton et al. 2009; Bozinovic et al. 2013). A thermal ‘performance’ or ‘fitness’ curve serves as a convenient descriptor of how a change in body temperature (Tb) influences physiological sensitivity and fitness of ectotherms (Huey and Stevenson 1979; Angilletta 2009). There are two endpoint temperatures for ectotherms in their performance curve. Very low and high Tb reduces an ectotherm’s performance and can be lethal in the extreme: these endpoints Tb are called the ‘critical temperatures’ (Figure 1.1; CTmax, CTmin). Within those critical limits, performance reaches a maximum at an optimal temperature region (To), and then typically plummets at higher Tb. These endpoints experimentally measured by the organism performance (Huey and Stevenson 1979; Gilchrist 1995; Frazier et al. 2006; Martin and Huey 2008) and this character in thermal performance curve can be change by acclimation time, duration and frequency of temperature exposure. The type of insect reactions and responses to climate change depend on whether a species is a thermal generalist or specialist (Janzen 1967; Huey and Slatkin 1976; Huey and Kingsolver 1993; Gilchrist 1995; Deutsch et al. 2008; Amarasekare and Savage 2012; Huey et al. 2012). For example, a given increase in Tb from warming will usually have a larger impact on a thermal specialist than on a thermal generalist: specialist have narrow performance curve whereas generalists have expanded performance curve. 4 Fig 1.1 Thermal fitness (performance) curve for a hypothetical ectotherm, with key descriptive parameters CTmax, CTmin, tolerance range, performance breath and optimal temperature (To) identified (adapted from Huey et al. (2012)). Insects use different strategies to survive at unfavourable high temperatures which can be separated into chemical and behavioural responses and these strategies can induce increases in heat tolerance or to prevent exposure to unfavourable conditions). The strategies are outlined below. 1.2.2 Effective compounds in increase tolerance to high temperature Polyhydroxyl compounds play key roles in unfavourable conditions such as extreme, high and low temperatures, desiccation and toxication of which, trehalose and sucrose are major part of these molecules (Kempf and Bremer 1998; Jain and Roy 2009; Arguelles 2000). The organic compounds that dominate the plant phloem sap are sugars, commonly sucrose, and amino acids (Wilkinson et al. 2001). Sucrose is the prevailing natural compound in plant phloem sap and is an essential carbon source for phloem feeding insects such as aphids (Fisher 2000; Douglas 2003). Sucrose is the primary respiratory substrate for aphids and gives the carbon skeleton to lipid and protein-amino acid creation (Rhodes et al. 1996; Febvay et al. 1999; Salvucci and Crafts-Brandner 2000; Pescod et al. 2007). 5 Sucrose is a major and vital source of energy in respiration time in the pea aphids (Febvay et al. 1995; Rhodes et al. 1996). Sucrose can be hydrolysis by the sucrase enzyme to glucose and fructose which can be used in respiration or synthesis of osmolytes compounds such as mannitol in aphids and sorbitol in whiteflies in high temperature (Ehrhardt 1962; Febvay et al. 1995; Rhodes et al. 1996; Hendrix and Salvucci 1998; Ashford et al. 2000). Higher concentrations of sucrose in the diet delayed high-temperature mortality, possibly a reflection of the high sucrose requirement for sorbitol synthesis in whiteflies (Hendrix et al. 1998). Based on studies phloem tissue of plants are rich in carbohydrates which are necessary and important for aphids and whiteflies performance (Fisher and Gifford 1986; Winter et al. 1992). Artificial studies with radiolabel sucrose have shown that majority of carbohydrate in aphids and whiteflies are used to perform basic metabolic process (Mittler and Meikle 1991; Rhodes et al. 1996; Wilkinson and Ishikawa 1999; Salvucci and Crafts-Brandner 2000). Osmolytes compounds can stabilise the structure of proteins in high extreme temperature (Back et al. 1979). In addition, first report studies have shown that aphids and whiteflies are the examples of organisms that accumulates polyols compounds at high temperature conditions (Hendrix and Salvucci 1998 ; Henle et