Steinernematidae & Heterorhabditidae
By David I. Shapiro-Ilan, USDA-ARS, SEFTNRL, Byron, GA & Randy Gaugler, Department of Entomology, Rutgers University, New Brunswick New Jersey
Nematodes are simple roundworms. Colorless, unsegmented, and lacking appendages, nematodes may be free-living, predaceous, or parasitic. Many of the parasitic species cause important diseases of plants, animals, and humans. Other species are beneficial in attacking insect pests, mostly sterilizing or otherwise debilitating their hosts. A very few cause insect death but these species tend to be difficult (e.g., tetradomatids) or expensive (e.g. mermithids) to mass produce, have narrow host specificity against pests of minor economic importance, possess modest virulence (e.g., sphaeruliids) or are otherwise poorly suited to exploit for pest control purposes. The only insect-parasitic nematodes possessing an optimal balance of biological control attributes are entomopathogenic or insecticidal nematodes in the genera Steinernema and Heterorhabditis. These multi-cellular metazoans occupy a biocontrol middle ground between microbial pathogens and predators/parasitoids, and are invariably lumped with pathogens, presumably because of their symbiotic relationship with bacteria.
Entomopathogenic nematodes are extraordinarily lethal to many important insect pests, yet are safe for plants and animals. This high degree of safety means that unlike chemicals, or even Bacillus thuringiensis, nematode applications do not require masks or other safety equipment; and re-entry time, residues, groundwater contamination, chemical trespass, and pollinators are not issues. Most biologicals require days or weeks to kill, yet nematodes, working with their symbiotic bacteria, can kill insects within 24-48 hours. Dozens of different insect pests are susceptible to infection, yet no adverse effects have been shown against beneficial insects or other nontargets in field studies (Georgis et al., 1991; Akhurst and Smith, 2002). Nematodes are amenable to mass production and do not require specialized application equipment as they are compatible with standard agrochemical equipment, including various sprayers (e.g., backpack, pressurized, mist, electrostatic, fan, and aerial) and irrigation systems.
Hundreds of researchers representing more than forty countries are working to develop nematodes as biological insecticides. Commercially available since 1993, this species’ excellent ability to persist and provide long-term control contribute to overall efficacy.
Life Cycle of Insect-Killing nematodes
Steinernematids and heterorhabditids have similar life histories. The non-feeding, developmentally arrested infective juvenile seeks out insect hosts and initiates infections. When a host has been located, the nematodes penetrate into the insect body cavity, usually via 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 gut, which multiplies rapidly and causes rapid insect death. The nematodes feed upon the bacteria and liquefying host, and mature into adults. Steinernematid infective juveniles may become males or females, where as 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 days, and hundreds of thousands of new infective juveniles emerge in search of fresh hosts. 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. The bacterium further 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. Conversely, the bacterium lacks invasive powers and is dependent upon the nematode to locate and penetrate suitable hosts.
Production and Storage Technology
Entomopathogenic nematodes are mass-produced for use as biocontrols using in vivo or in vitro methods (Shapiro-Ilan and Gaugler 2002). In vivo production (culture in live insect hosts) requires a low level of technology, has low startup costs, and resulting nematode quality is generally high, yet cost efficiency is low. The approach can be considered ideal for small markets. In vivo production may be improved through innovations in mechanization and streamlining. A novel alternative approach to in vivo methodology is production and application of nematodes in infected host cadavers; the cadavers (with nematodes developing inside) are distributed directly to the target site and pest suppression is subsequently achieved by the infective juveniles that emerge. In vitro solid culture, i.e., growing the nematodes on crumbled polyurethane foam, offers an intermediate level of technology and costs. In vitro liquid culture is the most cost- efficient production method but requires the largest startup capital. Liquid culture may be improved through progress in media development, nematode recovery, and bioreactor design. A variety of formulations have been developed to facilitate nematode storage and application including activated charcoal, alginate and polyacrylamide gels, baits, clay, paste, peat, polyurethane sponge, vermiculite, and water-dispersible granules. Depending on the formulation and nematode species, successful storage under refrigeration ranges from one to seven months. Optimum storage temperature for formulated nematodes varies according to species; generally, steinernematids tend to store best at 4-8 °C whereas heterorhabditids persist better at 10-15 °C.\
Nematodes species used are abbreviated as follows:
- Hb = Heterorhabditis bacteriophora
- Hd = H. downesi
- Hi = H. indica
- Hm = H. marelata
- Hmeg = H. megidi
- Hz = H. zealandica
- Sc = Steinernema carpocapsae
- Sf = S. feltiae
- Sg = S. glaseri
- Sk = S. kushidai
- Sr = S. riobrave
- Sscap = S. Scapterisci
- Ss = S. scarabaei.
|Pest Common name||Pest Scientific name||Key Crop(s) targeted||Efficacious Nematodes *|
|Artichoke plume moth||Platyptilia carduidactyla||Artichoke|
|Banana moth||Opogona sachari||Ornamentals||,|
|Banana root borer||Cosmopolites sordidus||Banana||,,|
|Billbug||Sphenophorus spp. (Coleoptera: Curculionidae)||Turf||,|
|Black cutworm||Agrotis ipsilon||Turf, vegetables|
|Black vine weevil||Otiorhynchus sulcatus||Berries, ornamentals||,,,,,|
|Borers||Synanthedon spp. and other sesiids||Fruit trees & ornamentals||,,|
|Cat flea||Ctenocephalides felis||Home yard, turf|
|Citrus root weevil||Pachnaeus spp. (Coleoptera: Curculionidae||Citrus, ornamentals||,|
|Codling moth||Cydia pomonella||Pome fruit||,|
|Corn earworm||Helicoverpa zea||Vegetables||,,|
|Corn rootworm||Diabrotica spp.||Vegetables||,|
|Cranberry girdler||Chrysoteuchia topiaria||Cranberries|
|Crane fly||Diptera: Tipulidae||Turf|
|Diaprepes root weevil||Diaprepes abbreviatus||Citrus, ornamentals||,|
|Fungus gnats||Diptera: Sciaridae||Mushrooms, greenhouse||,|
|Grape root borer||Vitacea polistiformis||Grapes||,|
|Iris borer||Macronoctua onusta||Iris||,|
|Large pine weevil||Hylobius albietis||Forest plantings||,|
|Leafminers||Liriomyza spp. (Diptera: Agromyzidae)||Vegetables, ornamentals||,|
|Mole crickets||Scapteriscus spp.||Turf||,,|
|Navel orangeworm||Amyelois transitella||Nut and fruit trees|
|Plum curculio||Conotrachelus nenuphar||Fruit trees|
|Coleoptera: Scarabaeidae||Turf, ornamentals||,,,,|
|Shore flies||Scatella spp.||Ornamentals||,|
|Strawberry root weevil||Otiorhynchus ovatus||Berries|
|Small hive beetle||Aethina tumida||Bee hives||,|
|Sweetpotato weevil||Cylas formicarius||Sweet potato||,,|
Characteristics of Some Commercialized Species
This species is the most studied of all entomopathogenic nematodes. Important attributes include ease of mass production and ability to formulate in a partially desiccated state that provides several months of room-temperature shelf-life. S. carpocapsae is particularly effective against lepidopterous larvae, including various webworms, cutworms, armyworms, girdlers, some weevils, 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 is especially effective when applied against highly mobile surface-adapted insects (though some below-ground insects are also controlled by this nematode). S. carpocapsae is also highly responsive to carbon dioxide once a host has been contacted, thus the spiracles are a key portal of host entry. It is most effective at temperatures ranging from 22 to 28°C.
S. feltiae is especially effective against immature dipterous insects, including mushroom flies, fungus gnats, and tipulids as well some lepidopterous larvae. This nematode is unique in maintaining infectivity at soil temperatures as low as 10°C. S. feltiae has an intermediate foraging strategy between the ambush and cruiser type.
One of the largest entomopathogenic nematode species at twice the length but eight times the volume of S. carpocapsae infective juveniles, S. glaseri is especially effective against coleopterous larvae, particularly scarabs. This species is a cruise forager, neither nictating nor attaching well to passing hosts, but highly mobile and responsive to long-range host volatiles. Thus, this nematode is best adapted to parasitize hosts possessing low mobility and residing within the soil profile. Field trials, particularly in Japan, have shown that S. glaseri can provide control of several scarab species. Large size, however, reduces yield, making this species significantly more expensive to produce than other species. A tendency to occasionally "lose" its bacterial symbiote is bothersome. Moreover, the highly active and robust infective juveniles are difficult to contain within formulations that rely on partial nematode dehydration. In short, additional technological advances are needed before this nematode is likely to see substantial use.
Only isolated so far from Japan and only known to parasitize scarab larvae, S. kushidai has been commercialized and marketed primarily in Asia.
This novel and highly pathogenic species was originally isolated from the Rio Grande Valley of Texas, but has since been also been isolated in other areas, e.g., in the southwestern USA. 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, mole crickets, and plum curculio. Steinernema riobrave has also been highly effective in suppressing citrus root weevils (e.g., Diaprepes abbreviates and Pachnaeus species). This nematode is active across a range of temperatures; it is effective at killing insects at soil temperatures above 35°C, and can also infect at 15 °C. Persistence is excellent even under semi-arid conditions, a feature no doubt enhanced by the uniquely high lipid levels found in infective juveniles. Its small size provides high yields whether using in vivo (up to 375,000 infective juveniles per wax moth larvae) or in vitro methods.
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 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.
Among the most economically important entomopathogenic nematodes, H. bacteriophora possesses considerable versatility, attacking lepidopterous and coleopterous insect larvae, among other insects. This cruiser species appears quite 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 efficacy when soil drops below 20°C.
First discovered in India, this nematode is now known to be ubiquitous. Heterorhabditis indica is considered to be a heat tolerant nematode (infecting insects at 30 °C or higher). The nematode produces high yields in vivo and in vitro, but shelf life is generally shorter than most other nematode species.
First isolated in Ohio, this nematode is commercially available and marketed especially in western Europe for control of black vine weevil and various other soil insects. Heterorhabditis megidis is considered to be a cold tolerant nematode because it can effectively infect insects at temperatures below 15 °C.
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