associations in the field are poorly understood. Nevertheless, natural
populations of these nematodes can play an important role in ecosystems
(Sexton and Williams, 1981; Akhurstet al., 1992; Campbellet al., 1995;
Stronget al., 1996). Entomopathogenic nematodes have been the focus
of considerable research because of their potential as biological control
agents against a wide range of insect species in a wide range of crops
(Gaugler and Kaya, 1990; Kaya and Gaugler, 1993).
There is only one free-living stage, the infective juvenile, which is
non-feeding, non-developing and non-reproductive; its sole function is to
bridge the gap between a depleted and a new host. Infective juveniles
carry their symbiotic bacteria in their digestive tracts. When a potential
host is located, infective juveniles enter the host through natural body
openings, such as spiracles, mouth or anus, but in some cases can
penetrate areas of thin cuticle. After penetration into the insect
haemocoel, the nematodes begin feeding and development. The symbiotic
bacteria are released into the haemocoel, where they multiply, and the
host is usually killed by septicaemia within 24–48 h. Nematodes feed,
develop, mate and lay eggs within the host. The size and species of the
host and the number of founding nematodes influence the number of
generations that can occur within a host and the number of new infec-
tive-stage juveniles produced. AdultSteinernemaspp. are amphimictic
and therefore require at least one individual of each sex to infect a
host for reproduction. In contrast,Heterorhabditisspp. infective juveniles
develop into hermaphroditic adults, so only a single individual is needed
to initiate an infection. In subsequent parasitic generations ofHetero-
rhabditis, both amphimictic and hermaphroditic individuals are
produced (Strauchet al., 1994; Koltaiet al., 1995). As time passes in the
infection, nematode density increases and nutrient levels decrease, which
lead to the formation of infective-stage juveniles instead of parasitic third-
stage juveniles. When the host becomes depleted, these infective juveniles
emerge and seek new insects to infect.Entomopathogenic Nematode Foraging Behavior
Behavioural repertoire
Nematodes’ small size, lack of appendages and limited sensory organs
constrain their host-seeking behaviour. Many nematode infective stages
are often looking for hosts that are considerably larger and more mobile
than they are. In addition, the surface tension of the water film in which
they move holds them firmly to the substrate. The surface-tension forces
of water films on nematodes have been estimated to be 10^4 –10^5 times the
force of gravity (Crofton, 1954).
Three behaviours – crawling, standing and jumping – play important
roles in determining where along the continuum between ambush and18 J.F. Campbell and E.E. Lewis