selection behaviours. Some species of entomopathogenic nematode tend
to be located at or near the soil surface and others prefer deeper locations
(Moyle and Kaya, 1981; Campbellet al., 1996). Ambush foraging requires
a substrate favourable for standing and is most effective near the soil
surface. Mobile insects are also most prevalent at the surface. In turf
grass, endemic populations ofS. carpocapsae, an ambush forager, were
greater at the soil surface and nematodes were most prevalent during
crepuscular and dark periods, when environmental conditions were
favourable for nematode persistence (Campbellet al., 1996). In contrast,
H. bacteriophorawere recovered throughout the upper soil profile. In
sand columns containing growing grass, most ambush and intermediate
foragers preferred the region above the soil surface and most cruise-
foraging species preferred the subsurface regions (J.F. Campbell, unpub-
lished data).Host Finding
Cues that indicate the proximity of a potential host are most likely
to occur in the host habitat. These cues may be contact or volatile
stimuli from a host or its immediate environment. How foragers respond
behaviourally to these stimuli varies depending on the type of search
strategy they use, the information contained in the stimuli and their inter-
nal state. Here we shall evaluate how infective juveniles interact with
stimuli associated with hosts within the context of their foraging strategy.
We focus onSteinernemaspecies, which show the greatest variation and
have been the most extensively studied.
Cruise-forager infective juveniles move using relatively linear move-
ment patterns that are typical of ranging search (Lewiset al., 1992).
Ranging search maximizes the search area. During ranging search, species
characterized as cruise foragers respond to volatile cues from hosts. For
example, Lewiset al. (1993) found thatS. glaseriresponded positively to
volatile cues from an insect host in a y-tube olfactometer and that this
response was eliminated if CO 2 was absorbed. Grewalet al. (1994) found a
similar level of response to volatile cues for other cruise-foraging species
inSteinernema(S. arenarium) and inHeterorhabditis(H. bacteriophora
and H. megidis). Recently, this strong response to volatile cues was
extended to manySteinernemaspp. that are effective at finding sedentary
hosts (E.E. Lewis, unpublished data). Cruise foragers also respond to host
contact cues. Lewiset al. (1992) found thatS. glaseriswitched to localized
search (e.g. speed decreased, distance travelled decreased, proportion of
time spent moving decreased) after contact with host cues (e.g. cuticle,
faeces). Localized search can maximize the chance that a searcher will
remain in a patch or re-establish contact with a host. Thus, the scenario
for cruise foragers is that they range through the environment, move
toward the source of volatile cues and switch to localized search after
they contact host-associated cues. These mechanisms are consistent with24 J.F. Campbell and E.E. Lewis