3) and seed-parasitic beetles (Messina, Chapter 4), adult females search
for hosts in a way analogous to predator foraging. In addition, there are
some strategies that are unique to parasites. Some parasites avoid the need
to survive in the external environment by manipulating their host to
increase the probability that it is consumed by an upstream host (Combes
et al., Chapter 1; Poulin, Chapter 12), and some parasites use vectors to
facilitate transmission between hosts (Hamilton and Hurd, Chapter 13).
There may be significant ecological constraints on the infective stages
of parasites. Parasites are typically small relative to their hosts, they
have limited sensory and movement capabilities, and the environment in
which they search for a new host is large and complex. Small body size
alone has profound effects on how an organism perceives its environment
and how it can respond evolutionarily (Wehner, 1997). For example,
humidity is critical for parasitic nematodes, because their free-living
stages are so small that the thickness of water films on surfaces constrains
their behaviour (Campbell and Lewis, Chapter 2; Robinson, Chapter 5).
In addition, these infective stages have limited sensory systems to detect
the cues emanating from a host and limited behavioural repertoires with
which to respond. Nevertheless, even in the more primitive groups,
relatively simple sensory systems and limited behaviour repertoires are
combined in ways that produce elegant and sophisticated host-search
strategies (Combeset al., Chapter 1; Campbell and Lewis, Chapter 2).
In parasitoid wasps and seed-parasitic beetles, where the behavioural
sophistication and neurosensory systems of the parasite are similar to or
greater than those of their hosts, some of the most complex mechanisms of
host finding have evolved (Vetet al., Chapter 3; Messina, Chapter 4).
The conceptual model of host finding originally developed by Salt
(1935) and Laing (1937) proposes a hierarchical process of host-habitat
location, host location, host acceptance and host suitability. However, the
distinction between the steps in this hierarchical process is not always
clear and the sequence is not necessarily always the same. The process is
more dynamic in nature, and experimental studies show that the response
to stimuli depends more on the information content of the signals
than the sequence in which they are presented (Vetet al., Chapter 3).
The behavioural flexibility of infective stages that are actively transmitted
is more constrained than that of parasitoid insects. Entomopathogenic
nematode species that use different foraging strategies differ in the
rigidity of their responses to hierarchical sequences of cues (Campbell
and Lewis, Chapter 2). Parasitoid wasps are able to learn while foraging
and adjust their behaviour in ways that increase the probability of host
encounter (Vetet al., Chapter 3), but, even in these species, learning is
likely to be less important for parasite infective stages that search for a
single host.
Another conceptual model that has been useful for understanding
parasite host-search behaviour is the ambusher/cruiser continuum.
Foraging strategies can be divided into two broad categories, cruise
(widely foraging) and ambush (sit and wait), which represent end-pointsDiverse Perspectives on Parasite Behavioural Ecology 339