go on indefinitely, the nematodes feed on the wasp’s symbiotic fungi in
the traceid system of trees. The wood wasp also feeds on the fungi. When
juvenile nematodes encounter the microenvironment surrounding a sirex
larva, they mature into a different form of adult. After mating, males die
and females penetrate into the sirex larva’s haemocoel. Female nematode
reproductive systems do not develop until the host larva begins to pupate;
then juvenile nematodes are produced that migrate to the host’s repro-
ductive system and enter the host eggs. The now sterile female wasp then
deposits the nematodes back into trees to resume the free-living life cycle.
Juvenile nematodes will invade the testes of male wasps, but transmission
does not occur and males are considered to be dead-end hosts.
Obligate parasites typically occupy the insect’s body cavity, but some
species occur in the digestive tract, reproductive organs or cuticle. Some
of the best-studied examples of obligate parasites of the insect body cavity
occur in the family Mermithidae (Poinar, 1975). Nematodes in this family
infect a wide range of invertebrate species in both terrestrial and aquatic
ecosystems. Typically, eggs are deposited into the external environment,
where second-stage juvenile nematodes hatch and seek out and penetrate
into the host. The parasitic stages generally remain in the insect
haemocoel and absorb nutrients through their cuticle. One individual per
host is typical, and the nematode can become quite large and is usually
coiled up within the haemocoel in species-specific locations. The process
of exiting the host is typically lethal to the host, due to the large exit
hole produced. The postparasitic juveniles enter the soil or the aquatic
sediment and mature to adults, mate and lay eggs. Another important
group of obligate parasites is the entomopathogenic nematodes, which
will be discussed in detail below.Entomopathogenic Nematode Biology
Entomopathogenic nematodes (Heterorhabditidae and Steinernematidae)
have mutualistic associations with bacteria (Photorhabdus andXeno-
rhabdusspp.). These nematodes are lethal endoparasites, capable of
infecting a broad range of insect species (Poinar, 1975; Kaya and Gaugler,
1993). These two families are discussed together because of their similar
life cycles and bacterial associations, but these similarities are probably
the result of convergent evolution (Poinar, 1993; Forst and Nealson,
1996; Blaxteret al., 1998). The nematode–bacteria complex represents
a mutualistic relationship, because the bacterium creates a favourable
environment for nematode growth and development within the insect
host and the bacteria are vectored between insects by the nematode. Each
nematode species is associated with only one bacteria species, although
some bacteria species are associated with more than oneSteinernema
species (Akhurst, 1993).
Although these nematodes occur in soil and epigeal habitats through-
out most regions of the world, their population ecology and hostEntomopathogenic Nematode Host-search Strategies 17