Entomopathogenic nematode
Although many other parasitic thread worms cause diseases in living organisms (sterilizing or otherwise debilitating their host), entomopathogenic nematodes are specific in only infecting insects.
[1] Together, the nematodes and bacteria feed on the liquefying host, and reproduce for several generations inside the cadaver maturing through the growth stages of J2-J4 into adults.
Steinernematids infective juveniles may become males or females, whereas heterorhabditids develop into self-fertilizing hermaphrodites with later generations producing two sexes.
The nematodes bacterium contributes anti-immune proteins to assist in overcoming their host defenses (Shapiro-Ilan, David I., and Randy Gaugler.
The foraging strategies of entomopathogenic nematodes vary between species, influencing their soil depth distributions and host preferences.
In order to ambush prey, some Steinernema species nictate, or raise their bodies off the soil surface so they are better poised to attach to passing insects, which are much larger in size (Campbell and Gaugler 1993).
Many Steinernema are able to jump by forming a loop with their bodies that creates stored energy which, when released, propels them through the air (Campbell and Kaya 2000).
Intraspecific competition takes place among nematodes of the same species when the number of infective juveniles penetrating a host exceeds the amount of resources available.
In both cases, the individual nematodes compete with each other indirectly by consuming the same resource, which reduces their fitness and may result in the local extinction of one species inside the host (Koppenhofer and Kaya 1996).
Millar and Barbercheck (2001) showed that the introduced nematode Steinernema riobrave survived and persisted in the environment for up to a year after its release.
S. riobrave significantly depressed detection of the endemic nematode H. bacteriophora, but never completely displaced it, even after two years of continued introductions.
EPNs may persist as metapopulations, in which local population fragments are highly vulnerable to extinction, and fluctuate asynchronously (Lewis et al. 1998).
Recent studies suggest that EPNs may also use non-host animals, such as isopods and earthworms for transport (Eng et al.2005, Shapiro et al. 1993) or can be scavengers (San-Blas and Gowen, 2008).
Entomopathogenic nematodes have the potential to shape the populations of plants and host insects, as well as the species composition of the surrounding animal soil community.
Agriculture exploits this finding, and the inundative release of EPNs can effectively control populations of soil insect pests in citrus, cranberries, turfgrass, and tree fruit (Lewis et al. 1998).
If entomopathogenic nematodes suppress the population of insect root herbivores, they indirectly benefit plants by freeing them from grazing pressure.
This is an example of a trophic cascade in which consumers at the top of the food web (nematodes) exert an influence on the abundance of resources (plants) at the bottom.
Many familiar animals like earthworms and insect grubs live in the soil, but smaller invertebrates such as mites, collembolans, and nematodes are also common.
Since EPNs are applied in agricultural systems at a rate of 1,000,000 inhabitants per acre (2,500,000/ha), the potential for unintended consequences on the soil ecosystem appears large.
EPNs have not had an adverse effect on mite and collembolan populations (Georgis et al. 1991), yet there is strong evidence that they affect the species diversity of other nematodes.
One study reported that Steinernema felidae and Heterorhabditis megidis, when applied in a range of agricultural and natural habitats, had little impact on non-pest arthropods.
Nematode-host interactions are poorly understood, and more than half of the natural hosts for recognized Steinernema and Heterorhabditis species remain unknown (Akhurst and Smith 2002).
For example, parasitic nematodes were found to be completely ineffective against blackflies and mosquitoes due to their inability to swim (Lewis et al.1998).
Efforts to control foliage-feeding pests with EPNs were equally unsuccessful, because nematodes are highly sensitive to UV light and desiccation (Lewis et al.1998).
Each nematode species has a unique array of characteristics, including different environmental tolerances, dispersal tendencies, and foraging behaviors (Lewis et al. 1998).
