Causes and consequences of variation in immunity within insect populations

Parasitism is widespread, yet not all hosts appear to defend themselves against infection. The goal of my thesis was to highlight some consequences and in particular the causes of variation in insect immune expression. Costs of resistance against parasitism, particularly under environmentally relevant conditions will not only have consequences for host population dynamics but also may be related to whether or not resistance is expressed. My results suggest that likelihood of exposure to parasitism, the immune trait induced in response to parasitism, and in particular the likelihood of experiencing low temperatures best explains when the costs of resistance are realised under natural conditions. Researchers also have identified the relevance of temperature for the development of the parasite (costs of parasitism) and for host immune expression. Although I tested for other determinants of variation in resistance, my findings suggest that temperature likely accounted for the observed seasonal increases in resistance against parasitism for a damselfly-parasitic water mite association. Although not often examined, characteristics of the parasite also can influence resistance against parasitism. However, I found that timing influenced the ability of a water mite to attain parasitism on a damselfly host but not host resistance. Overall my studies on damselflies will contribute to a better understanding of how host sex, external factors such as temperature and biology of ectoparasitic mite larvae influence variation in insect immunity within host populations.; Host density is thought to be important immune expression in insects, whereby greater allocation to immunity compensates for the density-dependent increases in host susceptibility to parasites. Many insects under high-densities also exhibit dark coloration (greater cuticular melanism) which for these insects is associated with greater immunity. However, my results indicated that immunity is not associated with greater cuticular melanism in New Zealand mountain stone weta, a species where a genetic colour polymorphism occurs with no correlation to density. Thus, earlier findings of greater immunity associated with greater cuticular melanism in phase polyphenic insects cannot always be extended to insects with forms of discrete melanin variation.; The novelty, salient findings and future directions as a result of this dissertation are also discussed.