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Nematodes are well known and more popular as pests infecting various crop plants. However, there is a group of nematodes, which never infect plants but are beneficial to soil and plants in several ways. Beneficial nematodes are often overlooked ‘Tiny Engineers of Plant Health’, which play a major role in ‘Soil Food Web’. They offer several ecosystem services that affect the nitrogen cycle in soil, soil organic matter decomposition capacity, nutrient and organic matter recycling, regulate the population of other soil microbiota including plant parasitic nematodes and also are capable of managing soil insect crop pests. Various nematode forms can also be used as biological models and bioindicators for investigations on ageing, soil nutritional status, environmental, water and metal pollution, soil health and ecological assessment, overall soil health bioindicators, cell fate determination studies, neuro- and developmental biological investigations etc., The course entitled ‘Beneficial Nematodes” has been included as a core course in PG degree programme (Plant Nematology) at various universities, with a common syllabus. Not a single text book covering the prescribed syllabus is available. Thus, a dire need of such a textbook has been felt for quite some time for the benefit of students specializing in the subject. This book, therefore, has been designed to cover the course outline of ‘Beneficial Nematodes”, prescribed by the Indian Council of Agricultural Research, New Delhi, India.

Nematodes offer several ecosystem services that affect the nitrogen cycle, decomposition capacity, and control of pests within soil systems. Firstly, the effects on the nitrogen cycle have generated a growing interest in the beneficial effects of nematodes to improve soil nitrogen content. Specifically, soil disruption, during activities such as tillage, increases nitrogen availability as well as the abundance of bacteria that compete for nitrogen (Ducker James, 2021). In response to more bacteria, bacterial feeding nematodes also increase in abundance, and this stabilizes nitrogen availability and increases nitrogen mineralization in soil once again, which is key for crop growth and soil fertility. Secondly, nematodes are involved in the decomposition of organic matter in the soil. Free-living nematodes are particularly essential as they recycle nutrients in the soil, which is matched by the indirect effect of bacteria and fungi-feeding nematodes that feed on the bacteria and fungi which decompose organic matter. Together, the feeding activity of these nematodes accelerates the decomposition process and makes minerals and nutrients accessible to growing plant roots. Thirdly, nematodes are valuable biological agents of pest control. Predatory nematodes regulate populations of other organisms in soil as they feed on organisms such as protozoa, insects, and other nematode species. As a result, nematodes moderate the population of other nematodes, and studies have shown that nematodes such as the Steinernematidae and Heterorhabditidae can kill soil-based insect pests within 48 hours. Finally, nematodes have become key biological indicators of soil health. The diversity and richness of nematodes communities are particularly insightful as they act as proxies to reveal information on the levels of nutrients, fungal availability, and bacteria, in soil. Altogether, nematode communities provide abundant information on soil health and generate a number of benefits for food production systems.

Nematodes and insects probably have influenced the shape of each other’s evolution from as early as the Devonian (409 million years ago). Over the course of their shared history, nematodes have established many different types of associations with insects. 2.1 Evolution and Types of Insect–Nematode Associations The majority of nematodes associated with insects (Rhabditida, in particular) are not true parasites, but have established only phoretic relationships. These phoretic nematodes usually are referred to as insect “associates.” Nematodes in phoretic relationships with insects usually rely on their host to transport them to food resources or improve their chances of finding a mate (or both) (Adams and Nguyen, 2008). Bacterial feeding soil nematodes often are found associated with insect cadavers and incorrectly inferred to be contributing factors to insect pathology. However, it has been proposed that nematode parasitism in some groups may have been initiated by necrogenous nematodes. Early stages in the evolution of insect parasitism may have involved necrogeny (feeding and reproducing on the bacteria of decaying insect cadavers). The association with decaying environments or insect cadavers may have become successively more intimate, as the nematodes began to utilize other saprophagous insects for transportation to new sources of food. Eventually, the nematodes could have changed from being passengers to parasites, invading and feeding on the tissues of their hosts.

Entomopathogenic / Entomophilic / Insecticidal/ Entomogenous nematodes can be found in the orders Aphelenchida, Ascaridida, Mermithida, Oxyurida, Rhabditida, Spirurida and Tylenchida. Phylogenetic relationships reveal that nematodes have established trophic associations with insects numerous times (Adams and Nguyen, 2008). Nematodes may have a proclivity (exaptation) for negotiating the natural defenses of insects and their associations with insects are tremendously fruitful (ecologically and evolutionarily). 3.1 Evolution of Parasitism in Insect-transmitted Plant Nematodes Nematode-insect associations have evolved many times in the phylum Nematoda, but these lineages involve plant parasitism only in the Secernentean orders Aphelenchida and Tylenchida. In the Aphelenchida (Aphelenchoidoidea), Bursaphelenchus xylophilus (Pine wood nematode), B. cocophilus (Red ring or Coconut palm nematode) (Parasitaphelenchidae), and the many potential host-specific species of Schistonchus (fig nematodes) (Aphelenchoididae) nematode-insect interactions probably evolved independently from dauerforming, mycophagous ancestors that were phoretically transmitted to breeding sites of their insect hosts in plants (Giblin-Davis et al., 2003). Mycophagy probably gave rise to facultative or obligate plant-parasitism because of opportunities due to insect host switches or peculiarities in host behavior. In the Tylenchida, there is one significant radiation of insect-associated plant parasites involving Fergusobia nematodes (Fergusobiinae: Neotylenchidae) and Fergusonina (Fergusoninidae) flies as mutualists that gall myrtaceous plant buds or leaves. These dicyclic nematodes have different phases that are parasitic in either the insect or the plant hosts. The evolutionary origin of this association is unclear.

Entomopathogenic nematodes (EPNs) are obligate parasites that infect and kill insects. Their short life cycles, simple rearing requirements, and straightforward molecular manipulations render them ideal to study host– parasite interactions. Additionally, their efficacy in reducing herbivore damage in the field contributed to their use as biological control agents in agriculture (Xi Zhang et al., 2021). EPNs comprise three genera, Heterorhabditis, Steinernema, and Oscheius. Infective juveniles (IJs) are third-stage free-living nematode larvae (iL3) that locate, select, and infect a host by entering through a natural orifice or by penetrating through the cuticle. Upon infection, IJs release venom proteins and regurgitate an endosymbiotic entomopathogenic bacterium, leading to a fatal toxemia and septicemia of the host. Juveniles then undergo a transition from free-living to parasitic lifestyle and resume growth and development by feeding on the infected flesh. Adult nematodes reproduce inside the host generating several new generations of nematodes. Resource depletion and elevated nematode densities induce the production of ascaroside C11 ethanolamine, an ascaroside triggering the production of next generation IJs, often through Endotokia matricida. The newly hatched IJs emerge from the resource-depleted host and search to infect new hosts. Juvenile host-seeking strategies are typically classified along a gradient ranging from ambushing to cruising (Xi Zhang et al., 2021). Ambusher nematodes are stationary and infect mobile hosts. Attachment to a mobile host can be achieved through nictation, which corresponds to the nematode standing on its tail, curling, and propelling itself in the air. Cruiser nematodes disperse in the soil and locate sedentary hosts. Nematodes with intermediate strategies can ambush or disperse depending on the soil matrix and host presence. Additionally, recent studies highlighted that EPNs can attract insects to infected cadavers just before emergence (Zhang et al., 2019). Being the first step of host–parasite interactions, host-seeking is critical in determining the success of a parasite.

5.1 Mass Multiplication Techniques Entomopathogenic nematodes (EPNs) are currently produced by different methods either in vivo or in vitro (solid and liquid culture) (Friedman, 1990). Each approach has its advantages and disadvantages relative to production cost, technical know-how required, economy of scale, and product quality, and each approach has the potential to be improved. Following production, a variety of formulation and application alternatives are also available (Shapiro- Ilan et al., 2012). Mass multiplication of EPNs can be achieved through in vivo (using insect hosts) and in vitro (using artificial media) methods, with the latter being more efficient for large-scale production. 1. In Vivo Multiplication Method: Involves rearing insect hosts (like Galleria mellonella or Corcyra cephalonica) and infecting them with EPNs, allowing the nematodes to multiply within the host’s cadaver. Process • Rearing of insect hosts. • Inoculation of host larvae with EPNs. • Collection of dead, infected larvae. • Extraction of infective juveniles (IJs) from the cadavers using methods like the White trap.

6.1 Nematodes as Biological Models A model organism can be a non-human species (e.g., microbe, plant, or animal) with extensive genomic research data. These organisms are used to study and understand specific biological phenomena, with the expectation that discoveries made in these models will provide new insights into “big biological questions” (Semprucci et al., 2025). Model organisms are widely employed to explore potential causes and treatments for human diseases when direct experimentation on humans is unfeasible or ethically problematic. This approach is supported by the common descent of all living organisms, as well as the conservation of metabolic and developmental pathways and genetic material over the course of evolution. The use of model organisms is not confined to biomedical research. Significant advancements in developmental biology, eco-toxicology, fertilization processes, speciation, symbiosis, and sexual selection have been achieved using these models. Furthermore, emerging animal models—species with less neurological complexity than those protected by legislation—are increasingly recognized as a valid application of the Three Rs principle (Replacement, Reduction, and Refinement). To support this approach, databases of invertebrate species currently used or with potential applications in biomedical research are continually being developed and updated. Nematodes, particularly Caenorhabditis elegans, are widely used as model organisms in biological research due to their simplicity, ease of study and shared biological processes with other animals, including humans. C. elegans, a small, free-living nematode, has a short life cycle, a known number of cells and a well-mapped genome, making it an ideal system for studying developmental biology, genetics and other aspects of animal biology.