Evolution and phylogeny of behavioural manipulation of insect hosts by parasites

The literature contains many examples of changes induced by parasites in the behaviour and/or other phenotypic traits of insects. From an evolutionary perspective, the nature of these changes is usually difficult to assess. Parasite-induced changes in host behaviour can be adaptations of either host or parasite, or they can be mere pathological consequences of infection. Of the many criteria and experimental tests necessary to distinguish between adaptations and non-selected consequences, two are particularly important: the demonstration of fitness benefits for either host or parasite associated with the behavioural change, and the elucidation of the proximate mechanism responsible for the behavioural change. Another approach can serve to identify adaptive changes in behavior: mapping specific behavioural alterations on a phylogeny of either hosts or parasites. The usefulness of this approach is illustrated with two examples, acanthocephalan-cockroach associations and insect-fungus associations. The adaptive nature of parasite-induced behavioural changes will always be difficult to evaluate because they are the product of two distinct but interacting genotypes. However, experimental and phylogenetic approaches can provide valuable insights into the evolutionary history of insect-parasite interactions.