Introduction Nematodes are about 0.6mm long, live in the soil and have three life stages (egg, juvenile and adult). The genera Steinernema and Heterorhabditis are the most well-studied entomopathogenic nematodes and are lethal to many important soil-inhabiting insect pests. Almost eighty have been classified, of which twelve are presently used as biological control agents. These nematodes are the second most widely used microbial control agents after B. thuringiensis, with commercial sales at US$2-3 million each year (Lacey et al, 2001). Due to high production and formulation costs however their use is limited to high value crops. Advantages The nematodes can be mass produced, cultured in vitro, applied using the same equipment as conventional pesticides and are compatible with several synthetic pesticides. They do not infect plants, animals or humans therefore no safety equipment is needed. General Infection Process These nematodes are entomopathogenic due to their symbiotic relationship with bacteria in the genera Xenorhabdus, for steinernematids, and Photorhabdus, for heterorhabditids (Lacey et al, 2001). Steinernematids enter the insect via natural openings and mate whereas heterorhabditids entre via the cuticle and are hermaphrodites. The bacteria are extremely virulent and fast acting (unlike other microbial biopesticides), causing death by septicaemia within 24-48 hours after infection. The bacteria degrade the insect body, which the juvenile nematodes feed on before developing into adults and reproducing. Each species of nematode has a symbiosis with a specific species of bacteria. Transmission of infection is horizontal. Examples of Inundative Control - Phasmarhabditis hermaphrodita is used to kill slugs. - Steinernema kraussei is used to kill vine weevils (Otiorynchus sulcatus) (Figure 1). Infection Cycle (Lacey et al, 2001) Classical Control by Deladenus siricidicola The woodwasp (Sirex noctilio) is native to Eurasia and northern Africa. In Australia and New Zealand the woodwasp is an invasive species, freed from natural enemies and consequently causes considerable damage to economically important Monterey pine (Pinus radiata). The female woodwasp kills the tree by injecting toxic mucus and its symbiotic fungus Amylostereum areolatum when ovipositing (egg laying) inside the tree (Hurley et al, 2007). The fungus is food for the developing larvae.  S. noctilio mainly infects stressed trees thus several silvicultural practices are implimented to maintain the health of the forests. Parasitic wasps are also used to control this pest but inoculative introduction of the nematode Deladenus siricidicola is seen as the primary biological control agent, having greatest success in Australia (Hurley et al, 2007). D. siricidicola was estimated by Bedding (1993) to save the Australian pine lumber industry $4 billion (Lacey et al, 2001). The nematode is particularly effective as it actively seeks out its host using chemoreceptors (similar to parasitoids), detects larvae efficiently and is highly persistent. Figure 1 Pathogenic nematode on the surface of a vine weevil.