Plant Protection: Nematodes: The parasites of insects

CROP PEST BIOCONTROL: Insect Parasitic Nematodes

Nematode Steinernema carpocapsae

Insects are killed by Nematodes

Introduction to Parasites

Etymology and technical words

-Etymology of the word “parasite”

First used in English 1539, the word parasite comes from the Medieval French parasite, from the Latin parasitus, the latinisation of the Greek (parasitos), "one who eats at the table of another"and that from (para), "beside, by (sitos), "wheat". Coined in English in 1611, the word parasitism comes from the Greek (para) +(sitismos) "feeding, fattening".

-Predators

In ecology, predation describes a biological interaction where a predator (an organism that is hunting) feeds on its prey (the organism that is attacked). Predators may or may not kill their prey prior to feeding on them, but the act of predation often results in the death of its prey and the eventual absorption of the prey's tissue through consumption.

In the nature, crop pest insects are killed by many predators such as animals, spider and other insects.

-Parasitism

Parasitism is a type of non mutual relationship between organisms of different species where one organism, the parasite, benefits at the expense of the other, the host. Traditionally parasitereferred to organisms with lifestages that needed more than one host (e.g. Taenia solium). These are now called macroparasites (typically protozoa and helminths).

Parasitism can take the form of isolated cheating or exploitation among more generalized mutualistic interactions.

Parasitism is differentiated from the parasitoid relationship, though not sharply, by the fact that parasitoids generally kill or sterilise their hosts.

-Parasites

The word parasite now also refers to microparasites, which are typically smaller, such as viruses, bacteria, protozoas, nemayodes… and can be directly transmitted between hosts of the same species.

Unlike predators, parasites are generally much smaller than their host. Parasites show a high degree of specialization, and reproduce at a faster rate than their hosts. Classic examples of parasitism include interactions between vertebrate hosts and diverse animals such as tapeworms, flukes, the Plasmodium species, and fleas.

Parasitoids are organisms whose larval development occurs inside or on the surface of another organism, resulting in the death of the host. This means that the interaction between the parasitoid and the host is fundamentally different from that of a true parasite and shares some of the characteristics of predation.

Parasitoidism occurs in much the same variety of organisms that parasitism does.

Types of parasites

Parasites are small organisms that complete most or all of their life cycle within a host, and many are capable of a high degree of within-host replication. Not all parasites kill their hosts, but parasites almost always have negative effects on host survival and reproduction.

Parasites are classified based on their interactions with their hosts and on their life cycles.

Parasites that live on the surface of the host are called ectoparasites (e.g. some mites). Those that live inside the host are called endoparasites (including all parasitic worms). Endoparasites can exist in one of two forms: intercellular parasites (inhabiting spaces in the host’s body) or intracellular parasites (inhabiting cells in the host’s body). Intracellular parasites, such as protozoa, bacteria or viruses, tend to rely on a third organism, which is generally known as the carrier or vector.

The vector does the job of transmitting them to the host. An example of this interaction is the transmission of malaria, caused by a protozoan of the genus Plasmodium, to humans by the bite of an anopheline mosquito. Those parasites living in an intermediate position, being half-ectoparasites and half-endoparasites, are sometimes called mesoparasite.

-Social parasites take advantage of interactions between members of social organisms such as ants or termites.

-An epiparasite is one that feeds on another parasite. This relationship is also sometimes referred to as hyperparasitism, exemplified by a protozoan (the hyperparasite) living in the digestive tract of a flea living on a dog.

The parasites of insects

Many parasites and disease-causing pathogens are known to attack insects, including viruses, bacteria, fungi, protozoans, nematodes, and mites.

The infective stages of most insect parasites must be consumed orally, although some can invade though pores or membranous joints in the insect cuticle. Many researchers are currently exploring the role of parasites and infectious diseases in regulating insect population size (E.G. Faeth and Simberloff 1981, Bowers et al. 1993, Jaenike 1998).

Insects can be parasited by many organisms such as:

-Insect parasites (parasitoids) of insects:

Such as: Anaphes species, Aphidius species, Aphytis spp, Bracon cushmani, Citrus mealybug parasite, Cottony cushion scale parasite, Cottony cushion scale parasite, Elm leaf beetle parasite, Encarsia formosa, Encarsia formosa, Hyposoter exiguae, Lysiphlebus testaceipes, Tachinid flies, Trichogramma spp., Trioxys pallidus…

-Nematode parasites:

Such as: Steinernema: S. carpocapsae, S. feltiae, S. riobravis,Heterorhabditis bacteriophora, H. megidis…

-Protozoan parasites:

Such as: many species of Sarcodina, Flagellata, Infosoria, Sporozoa (Coccidae , Neogregarinida , Cnidospora), Ophryocystis elektroscirrha and Nosema species…

-Fungal parasites of insects:

Such as: Metarhizium anisopliae, Beauveria bassiana, B.tenella,Hirsutella thompsonii, Cordyceps militaris , Nomuraea rileyi, Paecilomyces farinosus, Lecanicillium lecanii, Coelomomyces spp, Paearia rileyi , Entomophthora sp.

-Bacterial parasits of insects:

Such as:

-Pseudomonas:Ps. aeruginosa, Ps. chlororaphis, Ps. reptilivora, Ps. septica, Ps. putida

-Proteus: Pr. vulguris, Pr. mirabilis, Pr. rettgeri.

-Clostridium: Cl. brevifaciens, Cl. malacosomae.

-Bacillus: B. popilliae, B. fribourgensis, B. lentimorbus, B. euloomarahae, B. cereus, B. Thuringiensis.

-And many others: Serratia marcescens,

-Viral parasites of insects:

Such as: NPV (nuclear polyhedrosis virus), GV (granulosis virus) and CPV (cytoplasmic polyhedrosis virus).

Use of Nematodes as Biological Insecticides

Nematodes are simple, colorless, unsegmented, round worms, 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.

The Common Current Use of Nematodes as Biological Insecticides

Berries	Root weevils	Heterorhabditis bacteriophora
Citrus	Root weevils	Steinernema riobravis
Cranberries	Root weevils	H. bacteriophora, S. carpocapsae
	Cranberry girdler	S. carpocapsae
Mushrooms	Sciarids	S. feltiae
Ornamentals	Root weevils	H. bacteriophora, H. megidis
	Wood borers	S. carpocapsae, H. bacteriophora
	Fungus gnats	S. feltiae
Turf	Scarabs	H. bacteriophora
	Mole crickets	S. riobravis, S. scapterisci
	Billbugs	H. bacteriophora, S. carpocapsae
	Armyworm, Cutworm, Webworm	S. carpocapsae

Conservation

Conservation strategies are poorly developed and largely limited to avoiding applications onto sites where the nematodes are ill-adapted; for example, where immediate mortality is likely (e.g., exposed foliage) or where they are completely ineffective (e.g., aquatic habitats). Minimizing deleterious effects of the aboveground environment with a post-application rinse that washes infective juveniles into the soil is also a useful approach to increasing persistence and efficacy.

Native populations of insect parasitic nematodes are highly prevalent, but other than scattered reports of epizootics their impact on hosts populations is not well documented. This is largely attributable to the cryptic nature of soil insects. Consequently, guidelines for conserving native entomopathogenic nematodes have not been advanced.

USE OF NEMATODES AS BIOLOGICAL INSECTICIDES

Pest Common name	Pest Scientific name	Key Crop(s) targeted	Efficacious Nematodes
Artichoke plume moth	Platyptilia carduidactyla	Artichoke	Sc
Armyworms	Lepidoptera: Noctuidae	Vegetables	Sc, Sf, Sr
Banana moth	Opogona sachari	Ornamentals	Hb, Sc
Banana root borer	Cosmopolites sordidus	Banana	Sc, Sf, Sg
Billbug	Sphenophorus spp. (Coleoptera: Curculionidae)	Turf	Hb,Sc
Black cutworm	Agrotis ipsilon	Turf, vegetables	Sc
Black vine weevil	Otiorhynchus sulcatus	Berries, ornamentals	Hb, Hd, Hm, Hmeg, Sc, Sg
Borers	Synanthedon spp. and other sesiids	Fruit trees & ornamentals	Hb, Sc, Sf
Cat flea	Ctenocephalides felis	Home yard, turf	Sc
Citrus root weevil	Pachnaeus spp. (Coleoptera: Curculionidae	Citrus, ornamentals	Sr, Hb
Codling moth	Cydia pomonella	Pome fruit	Sc, Sf
Corn earworm	Helicoverpa zea	Vegetables	Sc, Sf, Sr
Corn rootworm	Diabrotica spp.	Vegetables	Hb, Sc
Cranberry girdler	Chrysoteuchia topiaria	Cranberries	Sc
Crane fly	Diptera: Tipulidae	Turf	Sc
Diaprepes root weevil	Diaprepes abbreviatus	Citrus, ornamentals	Hb, Sr
Fungus gnats	Diptera: Sciaridae	Mushrooms, greenhouse	Sf, Hb
Grape root borer	Vitacea polistiformis	Grapes	Hz, Hb
Iris borer	Macronoctua onusta	Iris	Hb, Sc
Large pine weevil	Hylobius albietis	Forest plantings	Hd, Sc
Leafminers	Liriomyza spp. (Diptera: Agromyzidae)	Vegetables, ornamentals	Sc, Sf
Mole crickets	Scapteriscus spp.	Turf	Sc, Sr, Scap
Navel orangeworm	Amyelois transitella	Nut and fruit trees	Sc
Plum curculio	Conotrachelus nenuphar	Fruit trees	Sr
Scarab grubs**	Coleoptera: Scarabaeidae	Turf, ornamentals	Hb, Sc, Sg, Ss, Hz
Shore flies	Scatella spp.	Ornamentals	Sc, Sf
Strawberry root weevil	Otiorhynchus ovatus	Berries	Hm
Small hive beetle	Aethina tumida	Bee hives	Yes (Hi, Sr)
Sweetpotato weevil	Cylas formicarius	Sweet potato	Hb, Sc, Sf

* At least one scientific study reported 75% suppression of these pests using the nematodes indicated in field or greenhouse experiments. Subsequent/other studies may reveal other nematodes that are virulent to these pests. Nematodes species used are abbreviated as follows: Hb=Heterorhabditis bacteriophora, Hd = H. downesi, Hi = H. indica, Hm= H. marelata, Hmeg = H. megidis, Hz = H. zealandica, Sc=Steinernema carpocapsae, Sf=S. feltiae, Sg=S. glaseri, Sk = S. kushidai, Sr=S. riobrave, Sscap=S. scapterisci, Ss = S. scarabaei.

** Efficacy of various pest species within this group varies among nematode species.

Source:(Lewis and Grewal, 2005).

Commercial Availability

Of the nearly eighty steinernematid and heterorhabditid nematodes identified to date, at least twelve species have been commercialized. A list of some nematode producers and suppliers is provided below:

SOME COMMERCIAL PRODUCERS/SUPPLIERS*

A-1 Unique Insect Control Telephone: 916-961-7945; FAX: 916-967-7082	Andermatt Biocontrol AG Switzerland. Hb, Hmeg, Sc, Sf.	ARBICO, Inc. Telephone: 520-825-9785, FAX: 520-825-2038 Hb, Sc, Sf.
Becker Underwood Telephone: 800-232-5907 Hb, Hmeg, Sc, Sf, Sk, Sr, Ss.	The Beneficial Insect Co. PO Box 471143 Telephone: 704-607-1631 Hb, Sc.	BioLogic Company Springtown Road, P.O. Box 177 Willow Hill, PA 17271 Hb, Sc, Sf.
CropKing Inc. Telephone: 800/321-5656, FAX: 330-302-4204 ; FAX: 330-722-2616	E ~nema Germany. Telephone:+49-4307-8295-0; FAX: +49-4307-8295-14 Hb, Sc, Sf	Gardens Alive! Lawrenceburg, IN 47025. Telephone: 513-354-1482
Gardener's Supply Company Telephone: 888-833-1412, 802-660-3500 FAX:800-551-6712 Hb, Sc (mixture)	Greenfire Inc. Telephone: 530-895-8301, 800-895-8307; FAX: 530-895-8317 Hb, Sc (mixture)	Green Spot, Ltd. Telephone: 603-942-8925; FAX 603-942-8932 Hb, Sc, Sf.
Harmony Farm Supply & Nursery Telephone:707-823-9125; FAX: 707-823-1734 Sc.	Hydro-Gardens, Inc. Telephone: 888-693-0578, FAX: 719-495-2266	IPM Laboratories, Inc. Locke, NY Telephone: 315-497-2063; FAX: 315-497-3129
Koppert (The Netherlands) Telephone:1-800- 928-8827 FAX: 734 641 3799 Hb, Hmeg, Sc, Sf.	M & R Durango, Inc. Telephone: 800-526-4075; FAX: 970-259-3857. Hb, Sc, Sf.	Natural Insect Control Canada, L0S 1S0. Telephone: 905-382-2904; FAX: 905-382-4418.
Natural Pest Controls 8864 Little Creek Drive Orangevale, CA 95662 Telephone: 916-726-0855	Nature's Control Telephone: 541-245-6033; FAX: 800-698-6250 Hb, Sc.	Peaceful Valley Farm Supply Telephone: 888-784-1722, 530-272-4769
Rincon-Vitova Insectaries Inc. Telephone: 805-643-5407, 800-248-2847; FAX: 805-643-6267 Hb, Hi, Hmar, Sc, Sf.	Southeastern Insectaries, Inc. Telephone: 478-988-9412, 877-967-6777; FAX: 478-988-9413. Hb, Hi, Sc.	Territorial Seed Company Telephone: 800-626-0866, 541-942-9547; FAX: 888-657-3131.
Worm's Way Inc. Telephone: 800-274-9676, 812-876-6450; FAX: 800-466-0795.	Yardlover Telephone: 866-215-2230. Hb, Sc, Sf.	Gulf Coast Biotics United States ph: 1-800-524-1958 fax: 940-458-5188 marta@gulfcoastbiotics.com