nematode hosts have patchy distributions at the soil surface. Therefore,
some locations where nematodes stand will have a higher probability of
host encounter than other areas. When a forager arrives at a patch, it has to
decide how long to remain in that patch. If no host arrives within a certain
period of time, there are two possible explanations: it is a poor patch or it
is a good patch but by chance no hosts have yet arrived. Many ambush
foragers appear to use relatively simple decision rules about when to leave
a patch in the absence of host encounter (e.g. Janetos and Cole, 1981;
Janetos, 1982; Kareivaet al., 1989). A number of rules of thumb for leaving
a patch have been proposed, including: a fixed number rule, a fixed time
rule, a giving-up time rule (leave after a certain amount of time without
success) and an encounter rate rule (leave when intake drops below a
certain rate) (Stephens and Krebs, 1986). The relative performance of a
particular rule is strongly influenced by the distribution of prey within
and between patches (Iwasaet al., 1981; McNair, 1982; Green, 1984). The
only rules that are appropriate for parasites are the giving-up time rule or
the fixed time rule, which become functionally the same because parasites
only use a single prey item. Infective juveniles, unlike predators or female
parasitoids, do not have information on patch quality obtained directly
by experience, but may use chemical cues to evaluate patch quality.
Chemical cues are used by some parasitoids to influence patch leaving
(e.g. Waage, 1979; Hemerik et al., 1993). Information gained within
a patch can influence both the tendency to leave the current patch and
the tendency to leave subsequently encountered patches (Shettleworth,
1984).
ForS. carpocapsae,the leaving tendency when standing tended to
decrease as the time standing increased (J.F. Campbellet al., unpublished
data).S. carpocapsaeinfective juveniles that recently initiated standing
bouts were more likely to terminate standing (0.081 terminations min−^1
for bouts less than 10 min) than individuals that have been standing for
longer periods of time (0.041 terminations min−^1 for bouts longer than
10 min). The survival function that described the patch leaving was
consistent with a Weibull function – rate changes over time – rather than
the exponential distribution associated with leaving at a constant rate.
What generates this pattern and its adaptive value is unclear, but the
termination of standing bouts was not consistent with a fixed time
rule for patch exploitation. When host-associated cues were present,
S. carpocapsae was less likely to terminate a standing bout; the
nematode’s average patch-leaving rate was 0.0064 terminations min−^1.
The tendency to use a fixed rate of leaving rule and to stay in patches
longer than optimal has been reported for several other ambusher species
(Janetos, 1982; Kareivaet al.,1989). Kareivaet al. (1989) predicted that for
an ambush-foraging spider the prey arrival rates may be so variable that no
one patch exploitation strategy is markedly superior. Although we do not
know the distribution of arrival rates of insects at nematode ambush
locations, it is reasonable to speculate that arthropod arrival rates at the
soil surface are also highly variable, both temporally and spatially.26 J.F. Campbell and E.E. Lewis