on a continuum of strategies (Pianka, 1966; Schoener, 1971; Huey and
Pianka, 1981). Ambush foraging, in particular, is likely to be an important
search strategy for many parasite species, due to their limited ability
to capture hosts via pursuit (Campbell and Lewis, Chapter 2). A system
of host-habitat location followed by ambush foraging with short-range
behavioural mechanisms to facilitate host attachment may be a common
strategy for active parasite infective stages (e.g. Combeset al., Chapter 1;
Campbell and Lewis, Chapter 2). However, parasites that search primarily
for sedentary hosts (e.g. plant parasites) will have to use a more active
search strategy, and most parasitoid wasps also appear to use active host
search. For parasites with multiple hosts, different foraging strategies may
be used during different phases of the life cycle. Combeset al. (Chapter 1)
describe how differences in host activity and environment at different
phases in a multihost life cycle can explain differences in the search
behaviours of trematode cercariae and miracidia.
Optimal-foraging models assume that natural selection favours
behavioural strategies that result in the highest reproductive success
(MacArthur and Pianka, 1966). By weighing the costs and benefits
of different strategies, predictions can be made about which strategy
or strategies will maximize a particular currency, usually a surrogate
of fitness (Stephens and Krebs, 1986; Krebs and Davies, 1997). These
theoretical models have great heuristic value and are excellent tools
for addressing several questions on parasite behaviour, including host
manipulation (Poulin, Chapter 12). However, there can be problems with
the unrealistic assumptions of some optimality models applied to some
parasite situations, e.g. in the application of optimal-foraging models to
parasite infective stages that seek a single host. These models often use
currencies (e.g. rate of gain) that are appropriate for parasites searching for
more than one host, such as female parasitoids and seed-parasitic beetles.
In addition, there has been a tendency to focus on active cruise foragers.
Optimal decisions for ambush foragers, when the assumption of complete
knowledge is relaxed, become complex and difficult to solve (e.g.
Nishimura, 1994; Beachly et al., 1995). Empirically, many ambush
foragers appear to use relatively simple decision rules or rules of thumb
(Janetos and Cole, 1981; Janetos, 1982; Kareivaet al., 1989), such as the
patch-leaving rules of ambush-foraging entomopathogenic nematode
infective stages (Campbell and Lewis, Chapter 2), and these may
represent the best solutions in environments where predictions about
host encounter are difficult to make.Host Acceptance and Infection
Habitat assessment is fundamental to many studies of behavioural ecology
(Krebs and Davies, 1997). Studies of cost/benefit relationships have
shown that most animals make decisions about which habitats they
will exploit, based upon some measure of habitat quality. For example,340 E.E. Lewiset al.