the behavioural responses of a diverse range of cruise-type foragers (Bell,
1990).
Ambush foragers, in contrast, have a very different response to
host cues. First, although on smooth substrates ambush foragers like
S. carpocapsaeexhibit ranging search, they are not attracted to host
volatile cues like CO 2 (Lewiset al., 1993). This response does not appear
to result from a constraint in their sensory systems, but to the absence of
selection pressure that would lead to the character, because artificial
selection can produce lines that are attracted to CO 2 (Gaugleret al., 1989).
Also, ambush foragers do not switch to area-concentrated search in
response to contact with host cues such as cuticle and faeces, asS. glaseri
does (Lewiset al., 1992). Although initially it was assumed that ambush
foragers were just not as responsive to chemical cues as cruise foragers
(Lewiset al., 1992), it has since become apparent that they are responsive
to chemical cues but their response is fundamentally different.
Lewiset al. (1995a) first demonstrated that ambush foragers respond
to volatile cues, but that their response was context-specific. They found
that, althoughS. carpocapsaeinfective juveniles when crawling on the
substrate were not attracted to host volatiles, they did respond to volatile
cues after contact with host cuticle. Lewiset al. (1995a) hypothesized that
this change in response occurred because, after a nematode contacted a
host while standing, it would be triggered to seek routes of entry into the
host (e.g. triggered to orientate towards the spiracles). They proposed that
ambush foragers are presented with cues in a more sequential fashion
than cruise foragers and therefore only responded to cues if they were
presented in the appropriate sequence.
It was subsequently determined that ambush foragers do respond to
volatile cues, even prior to host contact. Their behavioural response to
host cues needed to be investigated from within the context of their
foraging strategy. Cruise foragers scan while moving through the environ-
ment or during short pauses, but ambush foragers scan the environment
during long standing pauses. If host attachment is not a passive process,
then we expect to find that standing infective juveniles change behaviour
in response to host-associated cues. This has been demonstrated in two
ways. First, ambush-foraging infective juveniles presented with host-
associated cues (air movement and the presence of volatile cues) when
standing wave back and forth and jump towards the source of the cue
(Campbell and Kaya, 1999a,b, 2000). This appears to result from two
types of cues: volatile cues and air movement. This behavioural response
increases the host-attack area surrounding the standing infective juvenile
and can increase the probability of attaching to a host. This result
indicates that standing infective juveniles are scanning the environment
and respond in a way that can increase the probability of host encounter.
However, as discussed later, not all nematode species that stand respond
to host cues.
Host volatile cues can also cause ambush foragers to increase their
giving-up time during standing. It is likely that entomopathogenicEntomopathogenic Nematode Host-search Strategies 25