a Numbers are means of three replicates. Lack of a number indicates a combination that was not tested.

^b # indicates that estimated EC₅₀ level is greater than 2 g /10 milliliters water and may be impossible to prepare in aqueous solution.

c ## indicates that estimated EC₉₀ level is greater than 2 g/10 milliliters water and may be impossible to prepare in aqueous solution.

d EC levels greater than 100% of stock solution concentration are estimated by probit analysis and extrapolation.

The mode of action of most plants that reduce nematode numbers is not established, nor is the influence of these plants on nematode chemotaxis. The mechanisms whereby root exudates and plant extracts influence nematodes include allelochemics (nematoxic or nemastatic effects), anoxic rhizospheres, disruption of nematode taxis to roots, and disruption of male taxis to females. Christie (1960) hypothesized that root diffusates from marigolds might neutralize or mask host diffusates and render infection an inefficient chemokinetic event rather than a chemotactic response. However, toxic thiophenes have been recovered from marigold root extracts and from undisturbed rhizospheres. A variety of allelochemicals from certain plants may be directly toxic to nematodes, although their mode of action against plant-parasitic nematodes has not been clearly established. Allelochemic root exudates have potential use in nematode management programs if they can prevent active stages of nematodes from penetrating host roots.

Tea seed cake is a by-product of tea-oil production from camellia (Camellia oleifera). Production of similar Chinese traditional medicines results in residues of Paeonia rockii and Paeonia suffruticosa.  In controlled experiments, extracts from C. oleifera cake and P. rockii stems suppressed hatch and were nematotoxic to second-stage juveniles (J2) of both Heterodera glycines and Meloidogyne incognita (Wen et al., 2019). Extracts of Paeonia rockii  were more effective than those of P. suffruticosa in decreasing M. incognita hatch and J2 viability. In greenhouse trials with soybean (Glycine max ‘Essex’), powdered C. oleifera cake applied as a soil amendment suppressed H. glycines cysts/g root by up to 66% compared with nonamended controls. The extracts of Paeonia species and C. oleifera tea seed cake are candidates for further studies on management of these nematodes.

An approach related to selective breeding or genetic engineering of plants is not to alter the horticulturally or agronomically acceptable crop plant, but to develop effective trap plants which may be planted in rotation or interspersed with the main crop. The effectiveness of the trap crop may be expressed in stimulating egg hatch of the target nematode species, or by attraction of nematodes into root tissue which proves unfavorable for completion of their life cycle. Planting an additional crop in rotation, or as a winter cover, simply for nematode control may not be economical in most situations. In a larger systems context, however, there may be many other beneficial effects of the cover crop, including nitrogen fixation associated with legumes, enhanced water penetration during rainfall or irrigation, the creation of refugia for the natural enemies of other pests, and competitive weed management.

Chemotaxis

Repellants and Attractants.

Plant-root exudates are known to stimulate hatch of, and act as attractants for, certain nematode species, and certain inorganic ions may be attractive or repellent to particular nematode species. For example, cucumber roots have been shown to possess both attractive and repellent fractions for root-knot nematode juveniles. Nematode semiochemistry can play a dominant role in the development of new materials and methods for plant protection. It offers the possibility of obtaining environmentally safe attractants, repellants, or nematicides that are based upon the natural responses of infective stages to plant roots. Despite this enormous potential (Dusenbery, 1987), there is very little basic knowledge at a molecular or physiological level.

In 1925, Steiner proposed that plant-parasitic nematodes located their hosts by chemoreception. It is now well established that nematodes accumulate about the roots of host plants (Lownsbery and Viglierchio, 1960, 1961; Azmi and Jairajpuri, 1977; Prot, 1980; Prot and Van Gundy, 1981). Moreover, a range of plant parasites, including Heterodera schachtii (Viglierchio, 1961), M. hapla (Viglierchio, 1961), M. javanica (Riddle and Bird, 1985), Aphelenchoides besseyi (Lee and Evans, 1973), Hirschmanniella oryzae (Bilgrami et al., 1985), and others (Green, 1971), has been demonstrated to be attracted or repelled by plant roots and their emissions. Furthermore, an attractant for the leaf-gall nematode, Ditylenchus (Orrina) phyllobia, has been extracted from leaves of Solanum elaeagnifolium (Robinson and Saldana, 1989). However, none of the substances responsible for any of these interactions has been isolated or identified. While knowledge of the character of root emissions has increased (Schwab and Leonard, 1984; Thompson, 1985), no specific set or subset of compounds has been delineated that elicits chemotaxis of nematode parasites.

A potent attractant for the pinewood nematode, Bursaphelenchus xylophilus, is b-myrcene (Ishikawa et al., 1986), while simple inorganic salts and germinated host-plant seeds have been reported to attract Rotylenchulus reniformis (Riddle and Bird, 1985). In contrast, M. javanica was only attracted to tomato seeds. An apparently general attraction of nematodes to carbon dioxide (Klingler, 1965; Pline and Dusenbery, 1987) and oxygen (Dusenbery, 1983) has been noted. The response of male Heterodera glycines to attractants has been examined (Huettel and Jaffe, 1987). A substantial body of knowledge exists on the response to chemical stimuli of the free-living nematode, C. elegans (Dusenbery, 1983; Riddle and Georgi, 1990; Ward, 1973). Host or prey lectin-nematode carbohydrate interaction has been proposed as a general mechanism for recognition of host or prey (Dusenbery, 1983; Zuckerman and Jansson, 1984). Many exciting opportunities for research on repellants and attractants of plant-parasitic nematodes are suggested by this literature.

Recent development of a quantitative bioassay for chemotaxis (Castro et al., 1989) will facilitate research on attractants and repellants of nematodes. Within the last few years, several methods have been employed to assess the attractiveness of materials to nematodes. Simply counting the nematodes in zones of a Petri plate at various distances from a root fragment has been employed (Bilgrami et al., 1985), as has aggregation of nematodes under paper discs saturated with attractants (Robinson and Saldana, 1988). Other approaches include photographing the tracks of the animals on an agar Petri plate in relation to the position of an attractant (Riddle and Bird, 1985; Ward, 1973) or video monitoring of all nematode movements through time (Pline and Dusenbery, 1987; Dusenbery, 1983).

A possible utilization of attractants in nematode management is to combine the attractant with a nematicide in a pellet. This would render the nematicide more effective because the concentration of toxin would have to reach effective levels only in the vicinity of the pellet rather than in the entire soil volume. The pellets would be target-specific because stimulus chemicals appear to have greater specificity in their action than toxins. Consequently, adverse
environmental impacts of the nematicide would be reduced. This approach could well be combined with controlled release technology, which may be desirable for future nematicides (Feldmesser et al., 1985). Cost would depend on the price of the materials, the cost of application, the number of pellets required, the extent to which they must be incorporated into the soil, and their period of effectiveness (Dusenbery, 1987).

Root exudates have long been known to induce hatching of certain nematodes (Balam et al., 1949; Carroll, 1958; Hartnell et al., 1960; Jatala et al., 1977; Turner and Stone, 1981; Tanda, 1985). The chemistry of one hatching stimulus, glycinoecleptin, has been characterized (Fuzukawa et al., 1985). A variety of simple chemicals can also stimulate hatch of eggs of certain nematodes (Clarke and Shepherd, 1966; Jantzen, 1968; Okada, 1972; Greet, 1974; Clarke and Hennesey, 1983). Molting of certain nematode juvenile stages can be induced by carnation root diffusates (Rhoades and Linford, 1959) and other substances (Shepherd and Clarke, 1971). All of these responses to stimulants are potentially important in nematode control.

One approach to controlling nematode parasitism may be to alter the plant. Selective breeding or genetic engineering might be used to make a crop plant less attractive to nematodes. The plant could be altered to reduce its release of attractants, increase release of repellants, or modify internal stimuli. Very little is known of the mode of action of stimuli released from roots. So far, CO₂ is the only attractant stimulus produced by roots that has been clearly identified. Because it is a product of energy metabolism, reducing its production would require altering basic plant physiology which would probably be impossible without reduction of yield. Recent studies by Diez and Dusenbery (1986) have demonstrated that plant roots release repellants that can be manipulated chemically. If these chemicals can be identified, they may provide a basis for breeding or engineering plants that produce increased amounts of repellant (Dusenbery, 1987). Modified crop plants would provide an inexpensive management strategy and should have minimal environmental impact. However, the research and development necessary through selection and breeding, or genetic engineering, will be time-consuming and costly.

Apart from plant roots, certain nematophagous fungi are known to attract nematodes (Zuckerman and Jansson, 1984; Balan et al., 1976; Klink et al., 1970; Jansson and Norbring-Hertz, 1983). Infection by these fungi occurs specifically at the chemosensory organ (Jansson and Norbring-Hertz, 1983). No chemical structures have been identified, but a lectin-carbohydrate interaction is suspected (Zuckerman and Jansson, 1984).

The discovery that certain bacteria attract second-stage M.
incognita, while others repel them (Diez et al., 1986), raises some interesting new possibilities. Bacteria on the root surface are ideally located to intercept root-feeding nematodes. Seed might be coated with inoculum of a bacterial strain that would colonize the root surface and produce nematode repellants. Alternatively, bacteria that produce an attractant could be formulated into a nematicide pellet, thus increasing its effectiveness. The automated culturing and screening procedures for bacterial metabolites that are now implemented in state-of-the-art biopesticide laboratories provide an excellent vehicle for identification of effective organisms in naturally-occurring populations. Given the recent and potential advances in genetic engineering, it might even be possible to develop bacteria that produced both a nematicide and a nematode attractant.

Methods for inoculating bacteria into the rhizosphere have been described by Lynch (1982). The release of bacterial strains into the environment, especially those genetically engineered, will be subject to appropriate, stringent regulation (Dusenbery, 1987).

Sensory Confusion. 

There are various ways in which sensory responses might be exploited to control nematodes. The application of a chemical that inhibits nematode sensory responses would have an advantage over conventional nematicides because the chemical need not be inherently toxic to animal cells. Zuckerman and Jansson (1984) have suggested the use of lectins for this purpose. However, there are few examples of chemicals with inhibitory effects, even in mammals, and little effort has been expended in screening chemicals for inhibition of nematode sensory systems (Dusenbery, 1987).

An approach that has been employed for insect management is to flood the environment with a chemical stimulus so that gradients useful to the pest in locating host plants or mates are eliminated. Again, this has an advantage in that the chemical need not be toxic.  Major limitations are the amount of chemical that must be applied to flood a soil system and the potential for rapid microbial degradation of the material. Only very potent stimuli that are relatively inexpensive would be useful, unless they could be produced in the soil by augmentation of the metabolic activities of naturally occurring organisms. 

Amyl acetate has been identified as a potent stimulus for the free-living nematode Panagrellus redivivus (Balan, 1985), which responds at a concentration of 1 ppb. If we assume that a concentration 100-fold higher is needed in the soil solution to effectively confuse the nematode, and apply enough to provide the required concentration in soil containing the equivalent of 10 cm of water, 100 g is all that would be necessary to treat 1 hectare. The cost of materials would be less than $1/ha. This approach might be cost effective for management of plant-parasitic nematodes if efficacious stimuli of similar potency and cost could be identified (Dusenbery, 1987). An alternative approach may be to plant the crop into a "living mulch"-essentially an intercrop or cover crop of a different plant species. Stimuli released by such plants would flood the system and disrupt chemical gradients released by host plants. Flooding pots of soil with root-diffusates of several plants reduced infection of tomato roots by R. reniformis (Caswell, Tang, DeFrank and Apt).

Research Needs - Chemical Ecology

Research is needed on the effect of natural chemicals on the
nervous and reproductive systems of nematodes, understanding mechanisms of biological control and rhizosphere biology, and on the development of technology for applied biological control. Many of these research needs highlight the linkages to more basic areas of nematode biology.

The developmental biology of C. elegans has been detailed with extreme precision. The complete "wiring diagram" of the nervous system of this nematode can be drawn, as can the sequence of events in its development. Because C. elegans is a self-fertilizing hermaphrodite, clonal lines of mutants with aberrant behavior or movement patterns can be maintained. Study of the mutants has allowed understanding of the control of nervous and musculature system development, and provides a basis of understanding for nematode response to environmental stimuli. These advances in understanding of the fundamental biology of a small, yet complex, multicellular organism provide exciting opportunities for transfer of knowledge to the management of plant-parasitic nematodes.

Our understanding of the nervous system and sensilla of plant-parasitic nematodes can be summarized rather briefly. A "nerve ring," and associated ganglia, surround the isthmus region of the esophagus; neurons run longitudinally through the dorsal and ventral hypodermal chords; and various sensilla have associated neurons. The sensilla of plant-parasitic nematodes include pouch-like amphids opening on both sides of the anterior end of the nematode; phasmids, whose fine-structure has been determined only recently (Carta and Baldwin, 1990) in the posterior lateral fields; deirids in the anterior lateral field; and innervated setae, sensory pegs, and papillae. The first three of these sensilla are considered chemoreceptors, are generally recessed, and often contain a mucoid material. The amphids are usually the most elaborately developed of the structures, especially in marine nematodes. The setae, pegs, and papillae are probably tactile receptors. In most plant-parasitic nematodes, these tactile receptors are restricted to the head region or associated with the copulatory organs. There is some speculation, but a minimum of experimental evidence, on the functioning of the sensilla. Understanding their role and function in host-finding and response to attractant and repellent stimuli is a necessary component of real application of chemical ecology to nematode management.

Some of the research priorities in the area of allelopathy include identification of plant species that are capable of decreasing nematode numbers (and the specificity of such capabilities); determination of how these plants reduce nematode numbers, i.e., by chemical means or by acting as trap crops; determination of how effectively these plants reduce nematode numbers in annual and perennial field situations when the plants are exposed to polyspecific nematode communities; and determination of the influence of these plants on nematode population dynamics over long periods of time. Additional research is also needed to develop annual and perennial cropping systems for particular crop-nematode combinations that effectively utilize the potential of nematode-suppressive cover crops to reduce nematode numbers below the economic threshold.

The most promising area for future research appears to be the identification of chemical stimuli released by plant roots. In spite of little previous progress, the enormously increased capabilities of analytical instrumentation in recent years greatly improves the chances of success. An array of both attractants and repellants surely awaits discovery (Dusenbery, 1987). Dr. Michael McKenry of the U.C. Riverside Department of Nematology conducted research which suggests that, although marigold extract is nematicidal, it is also phytotoxic and can damage adjacent and subsequent crops. These setbacks, however, suggest a "better mousetrap" research opportunity. The challenge is to select or breed a plant that produces nematicidal agents, but does not have a phytotoxic or competitive effect on crops. There are many possible candidates; certain legumes are of particular interest.

Exploitation of chemical stimuli will depend on the characteristics and function of the chemicals. If they are involved in primary metabolism, altering their release by a plant, without decreasing metabolic efficiency, may be difficult. Plant-nematode interaction is the result of co-evolution in which nematodes develop adaptations to locate the host, and the host adapts to avoid nematode damage. A logical hypothesis is that once nematodes exploit a plant product to locate the host, there would be selection for reduced release of that chemical by the plant. However, a plant cannot easily reduce CO₂ release; this may be the reason that CO₂ seems to be a common attractant. A disadvantage of CO₂ as an attractant for the nematodes is that it is released by many soil organisms in addition to host plants. 

There may be potential for using the attraction of nematodes to CO₂ in a disruptive sense, or even for trapping and measuring nematode populations, but these possibilities will require further exploration. More specific attractants, as yet unidentified, are probably also important. The release of chemicals that repel nematodes would have an adaptive advantage for a plant. The repellant would continue to be effective only if there were a selective advantage to the nematode in avoiding the chemical. This might be the case if the chemical were toxic or were also released by fungal parasites of nematodes (Dusenbery, 1987).

The production of stimulus chemicals by bacteria is a promising area for further research. Initial steps will involve screening, selection, and characterization of candidate bacteria, and identifying their chemical stimuli. Further necessary research will include determining the effect of the stimuli on nematode behavior in the soil and understanding the ecology of the bacteria in the rhizosphere. Basic information regarding the range and effectiveness of chemical stimuli in the rhizosphere is also needed. (Dusenbery, 1987).

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References:

• Ferris et al (1992) Beyond Pesticides - Biological Approaches to Management in California.
• Ferris and Zheng

• Wen, Y. S.L.F. Meyer, M.H. MacDonald, L. Zheng, C. Jing, D.J. Chitwood. 2019. Nematotoxicity of Paeonia spp. Extracts and Camellia oleifera Tea Seed Cake and Extracts to Heterodera glycines and Meloidogyne incognita. Plant Disease 103:2191-2198