Nematodes of the families Heterorhabditidae and Steinernematidae are obligate parasites of a wide range of insects (Poinar 1979). The third stage juvenile of these nematodes is a non-feeding infective form which carries symbiotic insect-pathogenic bacteria (Xenorhabdus spp.) in its intestine. Under favourable conditions the juveniles can survive for months in the soil. They are attracted to and enter insects. After invading the haemocoel they release the symbiont. The bacteria multiply, kill the host by septicaemia, and provide suitable conditions for nematode growth and reproduction. After one to two weeks, the newly formed infective juveniles leave the cadaver and seek new hosts.