Nematodes are usually considered pests because of the diseases they cause in humans and animals and the economic impact they have on many agricultural products. There are, however, a small but significant number of beneficial entomogenous nematodes, i.e., nematodes associated, often parasitically, with insects. Some of these nematodes are of considerable interest because of their potential as biological control agents of pest insects.

Poinar (1979) listed 19 families of nematodes containing members that are facultative or obligate parasites of insects. Nine families (Allantone-matidae, Diplogasteridae, Heterorhabditidae, Mermithidae, Neotylenchidae, Rhabditidae, Sphaerulariidae, Steinernematidae, and Tetradonematidae) include species that attack insects and kill, sterilize, or alter host development. This two-part work will concentrate on the more promising of these entomogenous nematodes. Part I covers the mermithids and Part II in a subsequent volume will cover the tylenchids, steinernematids, and heterorhabditids.

The mermithids are a large and important group of nematodes. They are obligate parasites of arthropods, principally insects, but have also been recorded from spiders, crustaceans, earthworms, leeches, and mollusks (Poinar, 1979). They are usually specific to a single species or to one or two families of insects and are almost always lethal to their hosts. One of the earliest records of mermithids is found in the writings of Aldrovandi in 1623; he observed dead grasshoppers associated with long worms and considered the worms responsible for the death of the insects. Lister in 1671 described worms (most likely mermithids) from beetles found in his garden (Poinar, 1975).

Mermithids have since been reported from a variety of arthropods, in a variety of environments, and often infecting large percentages of host populations. The study of these nematodes, however, has been slow to develop because most mermithid observations were made by entomologists with little interest or training in nematology. Thus, much of the early work was limited to parasite-host associations or observations on incidence of parasitism.

Most of our knowledge of this group has been developed in the last 20 years and has been stimulated by increased interest in biological control. Mermithids are particularly attractive because (1) they offer little or no environmental hazard; (2) they offer no threat from competitive displacement of other desirable organisms because of their life cycle; and (3) the potential exists for inundative release to give high initial host reduction, or inoculative releases to establish the nematode and give partial control for an indefinite period.