Life Cycle

Life cycle of entomopathogenic nematode includes the egg, four juvenile stages and adult. The third stage is a free-living infective juvenile (dauer stage). The infective juveniles of both steinernematids and heterorhabditids carry in its gut bacteria of the genus Xenorhabdus and Photorhabdus, respectively (Boemare et al. 1993). The infective juvenile enters the host through mouth, anus or spiracles or penetrate through the intersegmental membranes of the insect cuticle as in case of Heterorhabditis sp. (Bedding and Molyneux 1982; Peters and Ehlers 1994) and reaches the haemocoel. In the haemocoel, infective juvenile releases cells of bacterial symbiont from its intestine. The nutrient-rich haemolymph of insect helps in the rapid multiplication of bacteria and ultimately results in killing the host within 48 h (Woodring and Kaya 1988). The infective juvenile then becomes feeding juvenile or functional third-stage juvenile and feed on the multiplying bacteria and degrading host tissues. The nematodes moult to fourth stage and finally develop into adult .The life cycle of steinernematids from infection to emergence of infective juveniles ranges from 7 to 10 days and for heterorhabditids ranges from 12 to 15 days (Sundarababu and Sankaranarayanan 1998). The number of generations may be more than one within the host cadaver depending upon the available resources.

Infective juveniles of Steinernematids develop into amphimictic females and males and never develop into hermaphrodites, whereas Heterorhabditids always develop into hermaphrodites in the first generation. Subsequent generation of heterorhabditids produces males, females and hermaphrodites (Dix et al. 1992). First-generation adults of steinernematids are termed as giant adults due to their larger size. This condition is believed to be due to the abundant available nutrition. The progeny of next generation, in most cases, find gradually depleting food supply due to regular progeny development. A full third-generation progeny may be observed when the food supply is in plenty (Adams and Nguyen 2002). Juveniles developing with adequate food supply mature to adults, while those developing in crowded conditions with limited food resources results in infective juveniles. Under suitable condition infective juveniles exit the cadaver to seek new hosts.

The eggs are initially laid into the host medium but in older female or hermaphrodite, eggs hatch in the uterus, and the developing juveniles consume the parental tissues. This process is known as endotokia matricida (Johnigk and Ehlers 1999), i.e. intrauterine birth causing maternal death. The infective juveniles are provided with two layers of external membrane, the cuticle of the third and second stages, due to superimposed first and second moults. The sheath of Heterorhabditis spp. in particular helps in protection against desiccation, freezing and fungal pathogens (Timper and Kaya 1989; Campbell and Gaugler 1991a; Wharton and Surrey 1994). This tight-fitting sheath of heterorhabditids do not lose easily, whereas the loose-fitting sheath of steinernematids is soon lost, as the nematode moves through the soil (Campbell and Gaugler 1991b; Dempsey and Griffin 2003). The physiology of infective juveniles may also bestow resistance or hardiness. In addition, oral and anal openings of infective juveniles remain closed in soil, thus preventing entry by microbial antagonists and toxic chemicals.

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