females from the same cadaver and therefore be in a better position to
mate with females when they infect. Lewis and Gaugler (1994) found that
maleS. glaseriinfective juveniles were more sensitive to volatile cues
from uninfected hosts than female infective juveniles and that females
were more attracted to infected hosts than to non-infected hosts. How-
ever, when a population of male and femaleS. glaseriinfective juveniles
was exposed toGalleria mellonella, there was no trend for males to infect
first (Stuartet al., 1998).S. carpocapsae, an ambush forager, showed no
sex bias in early emergers, and inS. feltiae, an intermediate forager,
females tended to emerge first (E.E. Lewis, unpublished data).Hetero-
rhabditis megidisinfective juveniles that emerge early from a depleted
host were better host finders, more tolerant of warm temperatures and less
tolerant of desiccation than later-emerging infective juveniles (O’Leary
et al., 1998). Further research is needed to assess how differences
in emergence patterns are related to foraging strategy and infection
dynamics.
Environmental conditions influence the emergence pattern of
entomopathogenic nematodes. Koppenhöfer et al. (1997) found that
emergence was delayed under low soil-moisture conditions and that they
could survive longer in dehydrated cadavers than they could exposed to
dry conditions in the soil. It is not clear if infective juveniles are trapped
in the cadavers or if this represents an adaptation to persist in low-
moisture conditions. The pattern forS. carpocapsaenot to be recovered at
or near the soil surface during daylight hours also suggests that infective
juvenile emergence from cadavers may be timed to favourable environ-
mental conditions (Campbellet al., 1996).
Exudates from a depleted host influence nematode behaviour
(Shapiro and Glazer, 1996; Shapiro and Lewis, 1999).H. bacteriophora
andS. carpocapsaeinfective juveniles allowed to emerge into sand from
their host cadaver dispersed farther than infective juveniles first collected
in water and then applied to the sand (Shapiro and Glazer, 1996).
H. bacteriophorainfective juvenile infectivity was also reduced if they
were collected in water and applied artificially, but infectivity could
be restored by applying the nematodes with an infected host (Shapiro
and Lewis, 1999). Many questions remain about the adaptive value of
this response. As currently understood, it suggests thatH. bacteriophora
infective juveniles are stimulated to disperse and infect by materials
associated with the depleted host. It may be that cues are picked up from
the depleted host that may be useful in identifying encountered insects as
acceptable hosts. This phenomenon has been noted for some parasitoid
wasps that learn cues from their natal host (Turlings et al., 1992).
However, given the many changes in the host during the infection process
of entomopathogenic nematodes, it is not clear what relevance these cues
might have.
Entomopathogenic nematodes live in soil and epigeal environments,
where potential hosts are likely to be clumped or patchy in distribution.
Location in the soil profile may be one of the most significant host-habitatEntomopathogenic Nematode Host-search Strategies 23