Ambush vs. cruise foraging
Differences in entomopathogenic nematode foraging strategies can be
most easily identified by comparing the ability of infective juveniles to
locate mobile versus sedentary insects (Campbell and Gaugler, 1997).
More infective juveniles of cruise-foraging species are able to find
sedentary hosts compared with mobile hosts and the opposite is true for
ambush foragers (Fig. 2.1). Intermediate foragers are similar in their ability
to find both types of hosts. By comparing attachment to mobile and con-
strained individuals of the same insect species, confounding factors asso-
ciated with host-specific differences in nematode behaviour are reduced.
Variation in the time allocated to crawling and standing among species of
Steinernemais consistent with variation along a continuum between
ambushing and cruising (Fig. 2.1), but all testedHeterorhabditisspp.
appear to be cruise foragers. Cruise foragers do not exhibit standing or
jumping behaviour, ambush foragers have stable standing periods and
exhibit high rates of jumping and intermediate foragers have lower rates of
standing and jumping (Campbell and Kaya, 2002; J.F. Campbellet al.,
unpublished data). It is also apparent that some species along the
continuum of the host-finding mode vary in their response to host cues,
which will be discussed in more detail later.
Although some foragers change foraging strategy in response to
experience or changes in internal state, this does not appear to be true for
entomopathogenic nematode infective stages. Time after emergence does
influence foraging behaviour, but there is no evidence that infective
juveniles change foraging strategies as they age. The general pattern is for
infective juveniles to become less effective at their original foraging mode
(i.e. become less mobile and less able to stand) and less able to infect a
host over time and this decline in infectivity has been correlated with
a decline in lipid levels (Lewiset al., 1995b, 1997).
Cruising is a more energetically expensive foraging strategy than
ambushing. Because infective juveniles are non-feeding and have a fixed
amount of stored nutrients, primarily in the form of lipids, we predicted
that cruise foragers will be larger (i.e. store more lipids) than ambush
foragers to compensate for their more energetically expensive search
strategy. Campbell and Kaya (2002) have noted that theSteinernema
spp. cruise foragers tend to have longer infective juveniles than ambush
foragers. Selvanet al. (1993a) reported that largerSteinernemainfective
juveniles had more stored lipids: the estimated energy content of a
single infective juvenile was 0.123 J forS. glaseri (a cruise forager),
0.065 J forS. feltiae(an intermediate forager) and 0.030 and 0.029 J for
S. carpocapsaeandS. scapterisci, respectively (ambush foragers). Lipid
levels have been shown to influence parasite infectivity and movement
(Croll, 1972; Lee and Atkinson, 1977). Lewiset al. (1995b) demonstrated
how lipid levels declined with infective juvenile age and how this was
correlated with changes in nematode behaviour and infectivity. In some
cases, lipid level was a better predictor of infectivity than infective20 J.F. Campbell and E.E. Lewis