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NematodesAlthough they are metazoan (nonsegmented) animals, many of which are commensuals and saprophytes, some entomogenous nematodes have characteristics that allow them to be considered with the pathogens. The most important insect pathogenic nematodes for biological control are very small (approximately 1 mm in length as adults) and use mutualistic bacteria to kill the host. Although nematode species in at least 20 families are primary or facultative parasites of insects, those in the Order Rhabditida have been most exploited as biological control agents. Species in the genera Steinernema and Heterorhabditis (Steinernematidae and Heterorhabditae, respectively), are particularly amenable to mass production and application in a variety of pest systems. Entomopathogenic nematodes enter the host via natural body openings or through the cuticle. Some species utilize an anterior stylet or a tooth to rasp the cuticle and gain entrance into the hemocoel. Others ingress by ovipositing on the host food source and the eggs hatch in the host midgut (Tanada and Kaya, 1993). Cycles of development and reproduction within the host vary according to species. Effects of nematode parasitism on the hosts can be sterility, reduced fecundity, reduced mobility and life span, behavioural and morphological changes, and death. Nematodes utilize several strategies to find the host, including active searching and ambushing. They respond to physical and chemical cues in the environment that signal the presence of the host, and genetic selection in the laboratory may enhance these responses (Gaugler et al., 1989). Nematode infections are usually not discernable from exterior examination, and although death or malformation of the host may result, dissections are required to confirm the identity of the agent (Tanada and Kaya, 1993). Commonly observed are the larger mermithid nematodes that ‘pop out’ of dissected hosts and are visible without the use of microscopes. |
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