Nematodes are simple, colorless, unsegmented, roundworms, lacking appendages. Nematodes may be free-living, predaceous, or parasitic, and many of the parasitic species cause important diseases of plants, animals, and humans. The only insect parasitic nematodes possessing an optimal balance of biological control attributes are entomopathogenic (also referred to as "beneficial" or "insecticidal") nematodes in the genera Steinernema and Heterorhabditis.

Insect parasitic nematodes are extraordinarily lethal to many important soil insect pests, yet are safe for plants and animals. Most biologicals require days or weeks to kill, yet nematodes, working with their symbiotic bacteria, kill insects in 24-48 hr. Dozens of different insect pests are susceptible to infection, yet no adverse effects have been shown against non-targets in field studies.

Biology
Steinernema and Heterorhabditis nematodes have similar life histories. The non-feeding infective juvenile seeks out insect hosts, especially in the soil environment. When a host has been located, the nematodes penetrate into the insect body, usually through natural body openings (mouth, anus, spiracles) or areas of thin cuticle. Once in the body cavity, a symbiotic bacterium (Xenorhabdus for steinernematids, Photorhabdus for heterorhabditids) is released from the nematode, which multiplies rapidly and causes rapid insect death. The nematodes feed upon the bacteria and liquefying insect, and mature into adults. Thus, entomopathogenic nematodes are a nematode-bacterium complex. The nematode may appear as little more than a biological syringe for its bacterial partner, yet the relationship between these organisms is one of classic mutualism. Nematode growth and reproduction depend upon conditions established in the host cadaver by the bacterium. In turn, the bacterium contributes anti-immune proteins to assist the nematode in overcoming host defenses, and anti-microbials that suppress colonization of the cadaver by competing secondary invaders. Steinernematid infective juveniles may become males or females, whereas heterorhabditids develop into self-fertilizing hermaphrodites although subsequent generations within a host produce males and females as well. The life cycle is completed in a few weeks, and hundreds of thousands of new infective juveniles emerge in search of fresh insect hosts.

Entomopathogenic nematodes are remarkably versatile in being useful against many soil insect pests in diverse cropping systems, yet are clearly underutilized. Like other biological control agents, nematodes are constrained by being living organisms that require specific conditions to be effective. Unlike pesticides, desiccation or ultraviolet light rapidly inactivates insecticidal nematodes. Similarly, nematodes are effective within a narrower temperature range than chemicals, and are more impacted by suboptimal soil type, thatch depth, and irrigation frequency.

Nematode Appearance
Nematodes are formulated and applied as infective juveniles, the only free-living and therefore environmentally tolerant stage. Infective juveniles range from 0.4 to 1.1 mm in length and can be observed with a hand lens or microscope after separation from formulation materials. Disturbed nematodes move actively, however sedentary ambusher species (e.g. Steinernema carpocapsae, S. scapterisci) in water soon revert to a characteristic "J"-shaped resting position. Low temperature or oxygen levels will inhibit movement of even highly active cruiser species (e.g., S. glaseri, Heterorhabditis bacteriophora). In short, lack of movement is not always a sign of mortality; nematodes may have to be stimulated (e.g., probes, acetic acid, gentle heat) to move before assessing viability. Good quality nematodes tend to possess high lipid levels that provide a dense appearance, whereas nearly transparent nematodes are often active but possess low powers of infection.

Insects killed by most steinernematid nematodes become brownish-yellow, whereas insects killed by heterorhabditids become red and the tissue assumes a gummy consistency. A dim luminescence given off by insects freshly killed by heterorhabditids is a foolproof diagnostic for this genus (the symbiotic bacteria provide the luminescence). Black rotting indicate that the host was not killed by entomopathogenic species. Nematodes found within such cadavers tend to be free-living soil saprophages.    

Biological characteristics of key species
Steinernema carpocapsae: The most studied, available, and versatile of all entomopathogenic nematodes. Important attributes include ease of mass production and ability to formulate in a partially dried state that provides several months of room-temperature shelf-life. Particularly effective against lepidopterous larvae, including various webworms, cutworms, armyworms, girdlers, and wood-borers. This species is a classic sit-and-wait or "ambush" forager, standing on its tail in an upright position near the soil surface and attaching to passing hosts. Consequently, S. carpocapsae tends to be most effective when applied against highly mobile surface-adapted insects. Highly responsive to carbon dioxide once a host has been contacted, the spiracles are a key portal of host entry. It is most effective at temperatures ranging from 22 to 28°C.    

Steinernema feltiae: Attacks primarily immature flies, including mushroom flies, fungus gnats, and crane flies. This nematode is unique in maintaining infectivity at soil temperatures below 10°C. S. feltiae offers lower stability than other steinernematids.

Steinernema riobravis: This highly pathogenic species, isolated to date only from the Rio Grande Valley of Texas, possesses several novel features. Its effective host range runs across multiple insect orders. This versatility is likely due in part to its ability to exploit aspects of both ambusher and cruiser means of finding hosts. Trials have demonstrated its effectiveness against corn earworm and mole crickets. In Florida, tens of thousands of acres of citrus are treated annually for control of citrus root weevil with impressive results. This is a high temperature nematode, effective at killing insects at soil temperatures above 35°C. Only formulation improvements that impart increased stability are needed for this parasite to achieve its full potential.

It must also be noted that S. riobravis has been marketed for suppression of plant parasitic nematodes infesting turfgrass. There is substantial correlative data suggesting that some entomopathogenic nematodes may suppress plant species. Some skepticism may be healthy until this puzzling assertion can be fully confirmed by rigorously designed, multiple field experiments

Steinernema scapterisci: The only entomopathogenic nematode to be used in a classical biological control program, S. scapterisci was isolated from Uruguay and first released in Florida in 1985 to suppress an introduced pest, mole crickets. The nematode become established and presently contributes to control. Steinernema scapterisci is highly specific to adult mole crickets. Its ambusher approach to finding insects is ideally suited to the turfgrass tunneling habits of its host. Commercially available since 1993, this nematode is also sold as a biological

insecticide, where its excellent ability to persist and provide long-term control contributes to overall efficacy. Availability is severely restricted due to the small market niche this nematode occupies. This is aggravated by its specificity for a host that is very difficult to rear.

Heterorhabditis bacteriophora: Among the most important entomopathogenic nematodes, H. bacteriophora possesses considerable versatility, attacking lepidopterous and coleopterous insect larvae among other insects. This cruiser species appears most useful against root weevils, particularly black vine weevil where it has provided consistently excellent results in containerized soil. A warm temperature nematode, H. bacteriophora shows reduced control when soil drops below 20°C. Characteristic poor stability has limited the usefulness of this interesting nematode: shelf-life is problematic and most infective juveniles persist only a few days following field release.

Heterorhabditis megidis: First isolated in Ohio, this nematode is marketed in western Europe for control of black vine weevil and various other soil insects. Its large size, characteristic heterorhabditid instability, and dearth of field efficacy data limit its utility at present.    

Habitat
Steinernematid and heterorhabditid nematodes are exclusively soil organisms. They are found virtually everywhere, having been isolated from every inhabited continent from a wide range of ecologically diverse soil habitats including cultivated fields, forests, grasslands, deserts, and even ocean beaches.

Pests Attacked
Because the symbiotic bacterium kills insects so quickly, there is no intimate host-parasite relationship as is characteristic for other insect-parasitic nematodes. Consequently, entomopathogenic nematodes are lethal to an extraordinarily broad range of insect pests in the laboratory. Field host range is considerably more restricted, with some species being quite narrow in host specificity. When considered as a group of nearly 30 species, however, entomopathogenic nematodes are useful against a large number of insect pests, many of which are listed in the table below. As field research progresses and improved insect-nematode matches are made, this list is certain to expand. Regrettably, nematodes have yet to be found which are effective against several of the most important soil insects, including wireworms, grape phylloxera, fire ants, or corn rootworms.

H. bacteriophora, 
S. carpocapsae