juvenile age alone (Patelet al., 1997).S. carpocapsae, an ambush forager,
had a slower metabolic rate thanS. glaseriandHeterorhabditis bacterio-
phora, the two cruise-foraging species tested (Lewis et al., 1995b).
H. bacteriophorahad the highest metabolic rate and the shortest lifespan.
S. glaserihad an intermediate metabolic rate, but lived the longest due to
its larger size and corresponding lipid reserves.Heterorhabditisspp. tend
to be cruise foragers (Campbell and Kaya, 2002), but have low energy
contents (Selvanet al., 1993b), are similar in size to ambush-foraging
Steinernemaand have a short lifespan in the laboratory (Lewiset al.,
1995b) and persistence in the field (Bauer and Kaya, 2001). Clearly,
it is difficult to draw any firm conclusions from these comparisons,
but the results withinSteinernemaare consistent with the predictions.
The comparison betweenHeterorhabditisspp. andSteinernemaspp. is
difficult, since these genera are distantly related.Host-habitat Selection
The point when an infective juvenile first emerges from a depleted host is
that at which we regard the process of host infection as starting. Infective
juveniles emerge from a depleted host over a period of time that ranges
from several days to several weeks, depending upon host quality,
nematode species, number of founders that initiated the infection and
environmental conditions. While the interaction between infective
juveniles and a depleted host has not received as much attention as the
interaction with a potential host, recent studies suggest that the depleted
host influences nematode ecology and behaviour. There are two phases
where there are potentially significant interactions between infective
juveniles and depleted hosts. Firstly, during the process of emergence and
dispersal, there are endogenous and exogenous factors that determine the
timing of emergence. Secondly, the depleted host and/or its exudates can
continue to influence infective juvenile behaviour during the process of
host finding.
Variation in the sex ratio of nematodes that emerge at different times
from a host cadaver has been reported. Infective juveniles ofS. glaserithat
disperse early from a depleted host cadaver tend to be larger and to have a
more male-biased sex ratio (Lewis and Gaugler, 1994; Nguyen and Smart,
1995; Stuartet al., 1996). Stuartet al. (1996) found that there was a genetic
basis toS. glaseriemergence time and that there was selection for a highly
skewed early emergence in a population recently isolated from the field.
Increased size is likely to be correlated with improved fitness due to
increased lipid storage and longevity. However, it is not clear if larger size
is correlated with being male or if infective juveniles that are formed first,
and presumably developed under the best environmental conditions,
disperse first. Lewis and Gaugler (1994) proposed that this phenomenon
is analogous to adult males emerging before females (i.e. protandry). By
dispersing first, males are more likely to infect uninfected hosts before22 J.F. Campbell and E.E. Lewis