Ok so the nematodes will kill the fleas. but then im going to have tons of nematodes, and who knows what their going to do to my plants
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Left: Juvenile nematodes emerging from a beet armyworm pupa. Note the large adult female (center left).
A.Hara
Center: An infective juvenile nematode.
P.Timper
Right: Steinernema carpocapsae infective juvenile nematodes are "ambushers" that may assume an upright or nictation position when seeking insect hosts.
J.Ogrodnick[/SIZE]
Nematodes
(Rhabditida: Steinernematidae & Heterorhabditidae)
By 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 soil 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, kill insects in 24-48 hr. Dozens of different insect pests are susceptible to infection, yet no adverse effects have been shown against nontargets in field studies (Georgis et al., 1991). Nematode production is easily accomplished for some species using standard fermentation in tanks up to 150,000 liters. Nematodes do not require specialized application equipment as they are compatible with standard agrochemical equipment including pressurized, mist, electrostatic, fan, and aerial sprayers. Application through irrigation systems has improved grower acceptance. Insecticidal nematodes are virtually without competition from other biological agents for control of soil-inhabiting and plant-boring insects.
Hundreds of researchers representing more than forty countries are working to develop nematodes as biological insecticides. Nematodes are sold in the U.S., Europe, Japan, and China for control of insect pests in high-value horticulture, agriculture, home and garden niche markets.
Life Cycle
- [SIZE=-1]Diagram courtesy of H. Kaya[/SIZE]
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 of 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.
Relative Effectiveness
Growers will not adopt biological agents that do not provide efficacy comparable with standard chemical insecticides. Technological advances in nematode production, formulation, quality control, application timing and delivery, and particularly in selecting optimal target habitats and target pests, have narrowed the efficacy gap between chemical and nematode agents. Nematodes have consequently demonstrated efficacy in an number of agricultural and horticultural market segments. Entomopathogenic nematodes are remarkably versatile in being useful against many soil and cryptic 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. Thus, desiccation or ultraviolet light rapidly inactivates insecticidal nematodes; chemical insecticides are less constrained. Similarly, nematodes are effective within a narrower temperature range than chemicals, and are more impacted by suboptimal soil type, thatch depth, and irrigation frequency (Georgis and Gaugler, 1991). Nematode-based insecticides may be inactivated if stored in hot vehicles, cannot be left in spray tanks for long periods, and are incompatible with several agricultural chemicals. Certain species cannot be applied with high-pressure application equipment; unused nematodes cannot be applied the following year; different species require different screen sizes. Chemicals also have problems (e.g., mammalian toxicity, resistance, groundwater pollution, etc.) but a large knowledge base has been developed to support their use. Accelerated implementation of nematodes into IPM systems will require users to be more knowledgeable about how to use them effectively.
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 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 cadavers with associated putrefaction indicate that the host was not killed by entomopathogenic species. Nematodes found within such cadavers tend to be free-living soil saprophages.
Habitat
- Steinernematid and heterorhabditid nematodes are exclusively soil organisms. They are ubiquitous, 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. In New Jersey, entomopathogenic nematodes were recovered from 22% of sites sampled (Gaugler, et al., 1992).
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.
