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The suite of behavioural traits associated with intermediate foraging

is more variable. Many intermediate foragers are attracted to host volatiles

and also switch to localized search in response to contact with host

cuticle (J.F. Campbellet al., unpublished data). However, most species

also exhibit standing and jumping behaviour. One intermediate forager,

Steinernema riobrave, was found to have short-duration standing bouts,

and standing giving-up time was not influenced by the presence of host

cues (J.F. Campbellet al., unpublished data). In this case, termination

of standing bouts appears to be a random process, which generates an

exponential distribution of leaving times. Intermediate foragers also tend

not to be triggered to jump by the sudden introduction of host cues and

not to jump towards the source of cues (Campbell and Kaya, 2000). All of

these characters suggest that intermediate foragers use host cues in a man-

ner consistent with cruise foragers, although perhaps not as effectively,

but because they stand and jump may encounter moving hosts as

well. Standing and jumping behaviours are unlikely to be adaptations

specifically for ambush foraging and use of these behaviours to attach to

passing insects may be unintentional but, given the opportunistic nature

of entomopathogenic nematodes, still an effective infection strategy.

Stable standing, chemical cues triggering changes in standing-bout

duration and controlling of the timing and direction of jumps are likely

to be adaptations to ambush foraging and these tend only to be found

in extreme ambush foragers.

Host Acceptance

Host acceptance for a parasite infective stage culminates in entering

the host haemocoel and the transition from the free-living infective

stage to the parasitic stage. Host acceptance is typically not reversible, so

selection of an appropriate host is a critical decision for an infective stage.

Encountered hosts will vary in quality, due to differences in suitability for

growth and development. Clearly nematodes should infect hosts that

maximize nematode fitness, but how nematode infective stages assess

host quality is not well understood. There have been a number of host-

acceptance models developed for parasitoids (Godfray, 1994), which,

while not directly applicable to parasite infective stages, do highlight

some of the important factors in the host-acceptance process. These

factors include host profitability (i.e. potential fitness benefits), risk of

mortality and infective juvenile age. Infection can be a risky undertaking,

but a large part of the risk is associated with the host immune response,

so this risk could be reduced by preferentially infecting hosts that are

already infected.

Multiple cues are involved in the process of host acceptance, perhaps

beginning with detection of host volatile cues, but, since most species

will initiate development if injected into the haemocoel, these cues are

not obligatory. For example, differences in giving-up time (i.e. return to

Entomopathogenic Nematode Host-search Strategies 27