Mainly from Ferris et al (1992) Beyond Pesticides - Biological Approaches to Management in California, based on substantial contributions from B. A. Jaffee.
The history of biological control of nematodes can be divided into two phases.
Prior to 1975, most research was descriptive. Antagonists of nematodes, especially nematode-trapping fungi, were considered interesting wonders of nature with little relevance to agriculture. Nematicides were inexpensive and effective, and little research effort was devoted to studying the effect of antagonists on nematode populations in soil.
The second phase began in 1977 with the loss of DBCP (which was followed by the restriction of several other important nematicides). The research in the second phase, which continues into the present, is typified by attempts to replace nematicides with antagonists. Thus far, few of these efforts have resulted ineffective biological control and the research has done little to increase our understanding of how biological control may or may not be achieved.
Our greatest need is for sound, in-depth biological information on how organisms, populations, and communities operate in the soil. The next phase will be typified by basic investigations of organismal, population, and community ecology.
Because nematodes often occur in high numbers in soil, it is not surprising that a wide variety of soil organisms exploit nematodes as food, i.e., as sources of carbon, nitrogen, and energy. Those organisms that seek out and consume nematodes are called predators.
Predators of plant-parasitic nematodes include mites, collembola, flatworms, protozoa, and other predacious nematodes.
Many exploiters of nematodes
have prolonged and specialized interactions with nematodes; these
organisms are called parasites. Parasites of plant-parasitic nematodes
include fungi, bacteria, and mycoplasma-like organisms.
Other organisms may have a detrimental effect on nematodes without utilizing them as a substrate, by competing for food, space, and necessary resources. Competitors of plant-parasitic nematodes include other nematodes, bacteria, and fungi. However, interaction between plant-parasitic nematodes and other competitors may cause increased damage to a particular food source.
Some organisms may antagonize
nematodes by producing nematicidal or nemastatic compounds such as
ammonia, certain fatty acids, and avermectins. This mode of action is
referred to as antibiosis, and involves bacteria and fungi.
Of these four mechanisms of antagonism (predation, parasitism, competition, and antibiosis), parasitism has received the most research effort. Predators are difficult to handle and manipulate, while competition and antibiosis are less direct and more difficult to quantify than parasitism and predation. With the exception of one bacterium, Pasteuria penetrans, most effort has focused on fungal parasites of nematode eggs and females.
Introduction of organisms antagonistic to pests into an environment (one form of classical biological control) has been successful in entomology, but not in plant pathology or nematology. This approach involves the one-time or repetitive release of an antagonist in an area where it is not naturally present.
Introduction has been successful against exotic insect pests and weeds that have escaped their natural enemies when introduced to a new area. Most nematode pests are not exotic (or not recently exotic), and it is considered difficult to introduce an organism into soil where all ecological niches are expected to be filled by the wide variety of organisms already present (the soil foodweb). However, further research effort is warranted in this area.
A number of researchers have applied large quantities of nematode antagonists to soil with the hope of inducing biological control. Although occasional success has been reported, this approach has generally failed. The reason for failure is unknown because the researchers typically did not determine if the applied organisms became established.
An exception has been the application of rhizosphere bacteria through drip irrigation. Drip irrigation would seem to be a viable method of applying and maintaining large numbers of nematode antagonists (presumably antibiotic producers) in the rhizosphere, although research has shown that the distance microorganisms are delivered from the emitter depends on the size of the organisms; the delivery distance is usually less than 12 inches.
The greatest success in utilizing biological control of nematodes has involved the conservation and enhancement of antagonists naturally present in soil.
In England, the cereal cyst nematode, Heterodera avenae, is controlled successfully by growing monocultures of small grains which support high biomass of certain nematophagous fungi.
Natural suppression of plant-parasitic nematodes has also been documented in peach orchards in the United States and vineyards in Australia. Methods for enhancing natural biological control include chitin amendments (to stimulate chitinolytic organisms that can degrade the eggshells of nematodes), collagen amendments (to stimulate collagenolytic organisms that can degrade the cuticle of vermiform nematodes), and other organic amendments (to increase the C:N ratio and, thus, stimulate the activity of nematode-trapping fungi).
Biopesticides are materials that are the natural products of organisms that are shown to be toxic to nematodes and other pests. The frequent implication is that such materials are environmentally safe because they are produced naturally rather than synthetically. This assumption is unwarranted, however, as some naturally occurring toxins are known to be extremely carcinogenic.
There are some intriguing possibilities for nematode management in this area. Becker et al. (1988) have shown that plant roots can be protected from fungal and, possibly, nematode attack by the presence of certain bacteria in the rhizosphere. The most probable mechanism is antibiosis, since the bacteria produce compounds that are lethal to the plant pests. Considerable work has been conducted on attempting to isolate the effective rhizobacteria, culture them, and inoculate seedling plants, or to introduce the bacteria at appropriate intervals in irrigation water (Behme et al., 1988).
An approach of considerable commercial interest
involves growing effective bacteria in fermenters, using patented
processes and media, and marketing the antibiotic product as a
biopesticide. Abbott Laboratories is currently working with a toxin
produced by a soil fungus that may have potential for biological control (DiTera)
of plant-parasitic nematodes.
Researchers at the Mycogen Corporation isolated and patented a strain of Bacillus thuringiensis (Bt) that produces a toxin effective against certain plant-parasitic nematodes. Bt crystals, are protein crystals formed during sporulation in some Bt strains. Bt produces proteins that aggregate to form a crystal. In insects, the crystal proteins bind specifically to certain receptors in the intestine. Humans and other vertebrates do not have these receptors in their bodies and so are unaffected by the toxin.
The Bt gene responsible for toxin production effective against certain insect pests has already been transgenically inserted into the genome of some plants. Introduction of the nematode-effective Bt gene into a plant genome would ostensibly render that cultivar resistant to nematode parasitism through antibiosis, while retaining its desirable agronomic or horticultural characteristics. This approach is especially promising in woody perennial plant species that can be grown on rootstocks. Development of Bt-transgenic rootstocks would potentially confer nematode resistance with no effect on the characteristics of the scion.
Bt toxins relatively easy to make and are safe to humans and vertebrate animals. Wei et al (2003) found that the Bt crystal protein Cry5B destroyed the intestine of C. elegans. They also found that Bt crystal proteins, Cry6A and Cry14A, reduced C. elegans fecundity. They determined that six bacterivore nematode species, C. elegans, Pristionchus pacificus, Panagrellus redivivus, Acrobeloides sp., Distolabrellus beechi and the free-living stage Nippostrongylus brasiliensis are susceptible in varying degrees to certain Bt crystal proteins, either by killing them, damaging their intestines or reducing fecundity.
The toxic crystal molecules coded by the Bt gene are large; they range in molecular mass from 40 to >70 kDa (Wei et al, 2003). Crystals would need to be ingested by the plant-feeding nematode and there has been some speculation that stylet-aperture exclusion would be a problem. However, Meloidogyne incognita, Globodera rostochiensis and Rotylenchulus reniformis are able to ingest green flourescent protein molecules of >28 kDa size from plant cells (Goverse et al, 1998; Urwin et al, 1997; Urwin et al, 2000). The size of the Bt crystal is 40-70 kDa and there is currently no evidence to suggest that crystals of that size would be excluded by the stylet aperture. The inability of Heterodera schachtii to digest proteins >40 kDa has been attributed to the a zone of exclusion established in the cytoplasm when these nematodes feed in plant cells (Bockenhoff and Grundler, 1994; Urwin, et al, 1997).
Scientists are aware, however, that resistance conferred by Bt toxins may be short-lived. If Bt toxin is not specific to nematodes, it will be important to ensure that it remains in the rootstock; if it is also expressed in the scion, it may select for resistance in lepidopterous pests and jeopardize the effectiveness of a currently useful management approach. There are several examples of insect pests that have developed resistance to this single-gene controlled product. In general, however, nematodes are less migratory than most insect species, and if a line of resistant individuals developed, it would require some time for the whole root system of a tree to be colonized, or for spread through an entire orchard to occur. Certainly, the strategy should allow for establishment of vigorous trees, under minimal nematode pressure, even if the nematode population subsequently developed in the mature orchard.
In the mid-1990s the Mycogen Corporation transferred rights to the nematode-effective Bt to Monsanto. Currently, Monsanto is not pursuing development.
Where biological control of nematode pests appears to be occurring, it is essential that the mechanism of antagonism be established for the system in question. This is relatively easy for parasites and predators, but quite difficult for antibiosis and competition. In addition, we must be able to quantify the antagonist and antagonist activity. Without this information, we cannot understand and remedy the inconsistency of results that is characteristic of biological control research.
Quantitative assays are lacking for most antagonists of
nematodes. Biological control occurs when a population or community of
antagonists suppresses a population of nematodes. Quantitative assays
and models must be developed in order to understand the interaction of
pest populations and biological control agents.
The soil environment is physically, chemically, and biologically very complex. It varies in its characteristics and complexity both spatially and temporally. Biologically, it is most complex, and the pace of interactions greatest, around plant roots. Antagonists of nematodes occur in many groups of soil organisms, and depending on the composition of these organisms in the soil or rhizosphere, the dynamics of the nematode population may be substantially buffered. The basic context of interactions among organisms is that each group is a food source for others; each group of organisms in the soil constitutes a
source of required carbon and energy for those organisms feeding on it, the soil foodweb.
As their proximal sources, these food chains are dependant on plants and their
fixing of carbon dioxide and water into carbohydrate through the
process of photosynthesis. Primary consumers, including plant-parasitic
nematodes, are among the initial links in the chains. Other initial
links are direct release of plant materials and products into the soil
that fuel pathways of decomposition; the fuel includes plant litter,
rhizodeposition and root sloughings, and root exudates.
Several types of fungi are important antagonists of nematodes. Among them are the trapping fungi that capture nematodes in various forms of traps. In many of these fungi, trap formation is stimulated when nematodes are present, and research indicates that a variety of proteins and amino acids originating from the nematodes may be involved.
The fungi, Arthrobotrys dactyloides and A. brochopaga, form ring traps that constrict around the body of a nematode passing through them. Other fungi produce sticky nets (A. oligospora), sticky knobs (Dactylellina haptotyla and Nematoctonus spp.), or sticky spores (Drechmeria coniospora and Hirsutella rhossiliensis). In each case, the fungal trap initially binds to the nematode, a penetration peg grows into the nematode body, and fungal hyphae ramify through the nematode. After the resources of the nematode have been exploited, hyphae emerge from the nematode carcass into the soil, and new traps are formed.
The spores of some fungi are ingested by nematodes, but this would not occur with plant-feeding nematodes. Other fungi (e.g., Catenaria spp.) have motile zoospores that swim through the soil solution and encyst on natural body openings of the nematode; zoosporangia then fill the body cavity, and new zoospores are released into the soil solution. Questions arise as to whether such water-loving fungi might be delivered to nematode-infested fields through irrigation.
A final, important group of fungal parasites of nematodes are the egg parasites, including Pochonia chlamydosporia, Paecilomyces lilacinus, and Dactylella oviparasitica. These fungi may be of particular significance with those nematodes that aggregate their eggs and fungal food source in masses.
The soil also contains bacteria that are direct parasites of nematodes,
including Pasteuria penetrans, which has adhesive spores. Considerable
research has been conducted with this organism, but consistent
application tactics have not yet emerged.
The predacious nematodes are formidable natural enemies of plant-parasitic and other nematodes. Seinura, Labronema, and Eudorylaimus pierce their prey with a hollow stylet and remove the body contents, while Mononchus and Odontopharynx incise the nematode body with a large tooth, then ingest the body. Predacious nematodes tend to be susceptible to soil disturbance, and probably act as important population buffers in relatively undisturbed ecosystems, including perennial crops.
Other soil-dwelling organisms that obtain their source of carbon and energy from nematodes include certain amoebae (phylum Sarcomastigophora), tardigrades (phylum Tardigrada), flatworms (phylum Platyhelminthes), mites (phylum Arthropoda, class Arachnida), and colembolla (phylum Arthropoda, class Insecta). For example, the mesostigmatid mite Gamasellodes vermivorax has nematodes as a preferred food source and will also feed on soft-bodied mites and colembollans. In culture it devours nematodes voraciously and is considered an important nematode predator in semi-arid grassland soils of the Great Plains (Walter, 1987).
Clearly, there are myriad interactions and potential interactions among soil organisms that require detailed study. These complex interactions, if better identified and understood, might be manipulated to reduce the population of plant-parasitic nematodes.
Advances in the
techniques of biotechnology introduce additional possibilities of
biological engineering of nematode antagonists. The strategy would
involve selection or induction of variants of potential biological
control agents with characteristics that enhance their effectiveness; it
would also require development of cultural practices that promote the
growth of beneficial biota in the soil. Increased effectiveness might
involve the introduction of an organism that is a more successful
competitor, a more aggressive predator, a more virulent parasite, or
have enhanced survival characteristics. The approach may involve
mutagenesis and screening, or the direct insertion of genes for the
desired characteristic into the genome of the organism. There is an
important arena of basic and applied research opportunities in the
genetic tailoring organisms for effectiveness in specific environmental
and cultural situations.
Rhizosphere bacteria that are natural colonists and inhabitants of the root surface and adjacent soil, appear to have many important, although little understood, properties. These bacteria may enhance plant growth or might suppress nematode populations by exerting an antibiotic effect (Suslow et al., 1980). Additional research is needed to determine which organisms are most effective, and also to suggest methods for enhancing their activity in the soil. Such enhancement might include introducing the organisms into the soil at strategic intervals or introducing a food substrate to promote their activity.
The tactics of Introduction, Inundation, and Augmentation as biological controls of soil-inhabiting nematodes raise a series of implementation challenges.
How will organisms most effectively be introduced into soil?
In other cases, the organisms will already be present in the soil and will be augmented by alteration of the environment or introduction of a food substrate.
opportunity, which has not received adequate field testing, might
involve the introduction of a preferred crop or a soil amendment with
the antagonistic organism. Pochonia chlamydosporia, for example, is damaging to cyst nematodes and also pathogenic to some daisy-type plants. In all situations, the optimal approach will need to be case-specific and will depend on the characteristics of the organism, the biological and physical nature of the soil, and the nature of the crop. Certainly, the organisms must retain their effectiveness through the introduction process. Other important research and implementation questions include quality control in the production of the organisms, and their shipping and shelf-life characteristics.
Given an adequate foundation of basic information on how the system in question functions, researchers may proceed to the third phase of biological control work, in which the understanding achieved through basic research is integrated and tested in field implementation strategies. This may involve formulation and application of specific antagonists-a procedure that has failed thus far due to lack of basic biological information. The basic information may also lead us to specific cultural practices which enhance the activity of native and introduced agents. Such cultural practices could include crop rotations, organic amendments, and pH adjustment.
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