10Insect Evolutionary

Parasites and pathogens pose a ubiquitous threat to all organisms. However, until relatively recently, the impact of parasites on the ecology and evolution of their hosts had been largely ignored by biologists. Now, of course, parasites are recognized as an important selective force on their hosts and a key factor influencing their population dynamics (Hudson et al., 2002; Grenfell and Dobson, 1995). The majority of studies examining the evolutionary ecology of host–parasite interactions have been conducted on vertebrate hosts, despite the fact that most animals are insects and other invertebrates. In recent years, though, this taxonomic bias has been challenged, as biologists have exploited the logistical advantages that insects and their parasites often provide. The aim of this chapter is to review recent advances in our understanding of insect host–parasite interactions. The theoretical framework underpinning this work has largely come from the field of ‘ecological immunology’ (Sheldon and Verhulst, 1996; Rolff and Siva-Jothy, 2003; Norris and Evans, 2000; SchmidHempel, 2003), a new and growing field concerned with understanding the evolutionary ecology of parasite resistance mechanisms. Ecological immunology is about the ecological and evolutionary causes and consequences of variation in immunity, defined in its widest sense (see below), to include any mechanism that improves the capacity of an organism to resist a parasite or pathogen. Essentially, ecological immunology is examining the proximate and ultimate causes of variation in disease resistance, and it takes an evolutionary ecology approach. In the past, this field has been dominated by biologists working on vertebrate systems, particularly birds (Norris and Evans, 2000). However, there is a growing belief that because the insect immune system is relatively simple in comparison with that of vertebrates, in that it lacks a conventional acquired immune system, ecological immunology studies are likely to be most successful when applied to insects.