Entomopathogenic nematode

Entomopathogenic nematodes are a group of nematodes (thread worms), causing death to insects. The term entomopathogenic has a Greek origin entomon, refers to insect, and pathogenic, which denotes causing disease. They are multi-cellular metazoans that occupy a bio control middle ground between microbial pathogens and predator/ parasitoids, and are habitually grouped with pathogens, most likely because of their symbiotic relationship with bacteria. 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. Entomopathogenic nematodes (EPNs) live parasitically inside the infected insect host, and so they are termed as endoparasitic. They infect many different types of insects living in the soil like the larval forms of moths, butterflies, flies and beetles as well as adult forms of beetles, grasshoppers and crickets. EPNs have been found in all over the world and a range of ecologically diverse habitats. They are highly diverse, complex and specialized. The most commonly studied entomopathogenic nematodes are those that can be used in the biological control of harmful insects, the members of Steinernematidae and Heterorhabditidae (Gaugler 2006). They are the only insect-parasitic nematodes possessing an optimal balance of biological control attributes. (Cranshaw & Zimmerman 2013).

Because of their economic importance, the life cycles of the genera belonging to families Heterorhabditidae and Steinernematidae are well studied. Although not closely related, phylogenetically, both share similar life histories (Poinar 1993). The cycle begins with an infective juvenile, whose only function is to seek out and infect new hosts. When a host has been located, the nematodes penetrate into the insect body cavity, usually via natural body openings (mouth, anus, spiracles) or areas of thin cuticle. (Shapiro-Ilan, David I., and Randy Gaugler. "Nematodes.") After entering an insect, infective juveniles release an associated mutualistic bacterium from their gut which multiplies rapidly. These bacteria of the genus Xenorhabdus or Photorhabdus, for steinerernematides and heterorhabditids, respectively—cause host mortality within 24-48 hours. The nematodes provide shelter to the bacteria, which, in return, kill the insect host and provide nutrients to the nematode. 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. When food resources in the host become scarce, the adults produce new infective juveniles adapted to withstand the outside environment. The life cycles of the EPNs are completed within a few days.(Shapiro-Ilan, David I., and Randy Gaugler. "Nematodes.") After about a week, hundreds of thousands of infective juveniles emerge and leave in search of new hosts, carrying with them an inoculation of mutualistic bacteria, received from the internal host environment (Boemare 2002, Gaugler 2006). Their growth and reproduction depends upon conditions established in the host cadaver by the bacterium. The nematodes bacterium contributes anti-immune proteins to assist in overcoming their host defenses.(Shapiro-Ilan, David I., and Randy Gaugler. "Nematodes.")

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