Nature’s tiny killers: How nematodes and bacteria team up to control pests P. Manju1, K. Sankari Meena2, Ch. Akhila3, M.V. Deborah3 and K. Pranahitha3 1. Kumaraguru Institute of Agriculture, Tamil Nadu 2. ICAR-Indian Institute of Oilseeds Research, Hyderabad 3. Professor Jayashankar Telangana Agricultural University, Hyderabad Introduction Nematodes are roundworms that are colorless, unsegmented, and lack appendages. They occur in a wide range of habitats and may be free-living, predaceous, or parasitic in nature. While many nematode species play beneficial roles in soil ecosystems, several parasitic forms are responsible for economically important diseases affecting humans, animals, and plants. Among parasitic nematodes, entomopathogenic nematodes (EPNs) are unique because of their ability to infect and kill insect pests, making them valuable tools in biological pest management. Entomopathogenic Nematodes: Classification and Habitat Entomopathogenic nematodes are soil-dwelling, insect-parasitizing nematodes mainly belong to the genera Steinernema, Heterorhabditis and Neosteinernema. Of these, Steinernema (family Steinernematidae) and Heterorhabditis (family Heterorhabditidae) are the most extensively studied and widely used in agriculture. These nematodes are naturally present in soils and actively seek out insect hosts, especially the soil-inhabiting insect pests. A distinctive feature of EPNs is their symbiotic association with specific bacteria, which plays a central role in insect pathogenicity and nematode survival. Steinernema species are associated with bacteria of the genus Xenorhabdus, while Heterorhabditis species are associated with Photorhabdus bacteria. Host seeking behaviour of Entomopathogenic NematodesThe infective juvenile (IJ) is the only free-living and host-seeking stage in the life cycle of entomopathogenic nematodes. Infective juveniles actively search for suitable insect hosts in the soil using chemical, physical, and biological cues. These cues include carbon dioxide released by insects, body heat, vibrations, and insect excreta. Based on their foraging strategy, EPNs are broadly classified into ambushers and cruisers. Many Steinernema species exhibit an ambush strategy, where infective juveniles wait near the soil surface and attack highly mobile insect hosts. Some Steinernema species also display cruising behavior by actively moving through the soil. In contrast, Heterorhabditis species are mainly cruisers and actively move through the soil profile in search of relatively sedentary insect hosts. This efficient host-seeking behavior enables EPNs to locate and infect insect pests effectively under natural soil conditions. Symbiosis between Steinernema and Xenorhabdus Xenorhabdus species form an obligate, mutualistic, and highly specific association with Steinernema nematodes. The bacteria are carried within a specialized region of the intestine of the infective juvenile (IJ) stage, which is the only free-living and infective stage of the nematode life cycle. When an infective juvenile locates a suitable insect host in the soil, it enters the host through natural openings such as the mouth, anus, or spiracles. After reaching the insect’s hemocoel, the nematode releases its symbiotic Xenorhabdus bacteria. These bacteria multiply rapidly and are primarily responsible for killing the insect host. They produce a wide range of toxins and hydrolytic enzymes, including proteases, lipases, and chitinases, which cause septicemia and toxemia, leading to insect death within 24–72 hours. Additionally, Xenorhabdus suppresses the insect immune system by inhibiting hemocyte activity and melanization responses. An important function of Xenorhabdus species is the protection of the insect cadaver from invasion by other microorganisms. The bacteria produce antibiotics and antimicrobial compounds that prevent the growth of competing bacteria and fungi. At the same time, Xenorhabdus degrades insect tissues and converts them into simpler nutrients, which serve as the primary food source for developing nematodes. Following insect death, Steinernema nematodes feed on the bacteria and degraded tissues, develop into adults, and reproduce sexually within the insect cadaver. Several generations may be completed inside a single host as long as sufficient nutrients are available. Symbiosis between Heterorhabditis and Photorhabdus Photorhabdus species form an obligate and highly specialized mutualistic association with Heterorhabditis nematodes. As in Steinernema, the bacteria are carried in the intestine of the infective juvenile stage. This association is essential for the pathogenicity, development, and survival of Heterorhabditis nematodes. Infective juveniles of Heterorhabditis can enter insect hosts through natural openings, but unlike Steinernema, they also possess the unique ability to directly penetrate the insect cuticle. Once reached inside the hemocoel, the nematodes release Photorhabdus bacteria, which rapidly multiply and kill the insect host, usually within 24–48 hours. Photorhabdus produces powerful toxins, extracellular enzymes, and secondary metabolites that disrupt cellular processes and overwhelm the insect immune system. A distinctive characteristic of Photorhabdus species is their ability to produce bioluminescence, although this trait does not directly contribute to insect mortality. More importantly, these bacteria produce strong antibiotics and antifungal compounds that suppress competing microorganisms and preserve the insect cadaver as a suitable environment for nematode development. The bacteria also convert insect tissues into nutrient-rich substrates that support nematode growth and reproduction. After insect death, Heterorhabditis nematodes feed on the bacteria and liquefied tissues and develop into adults. A unique feature of this genus is that the first generation is hermaphroditic, capable of self-fertilization, while later generations reproduce sexually. When nutrients are exhausted, new infective juveniles are formed, re-associate with Photorhabdus, and leave the cadaver to continue the life cycle in the soil. Importance of EPN in Biological Control The associations between Steinernema–Xenorhabdus and Heterorhabditis–Photorhabdus are obligatory and interdependent. Neither the nematode nor the bacterium can successfully complete its life cycle without the other. These highly specialized nematode–bacterium complexes form the biological foundation for the effectiveness of entomopathogenic nematodes as eco-friendly and sustainable biological control agents against a wide range of soil-dwelling insect pests. Conclusion Entomopathogenic nematodes represent a remarkable example of mutualism in nature, where nematodes and bacteria function together as a single biological unit to infect, kill, and utilize insect hosts. Their specificity, rapid action, and safety to non-target organisms make them invaluable components of integrated pest management programs. Understanding the biology and symbiotic relationships of EPNs enhances their effective use in sustainable agriculture and environmentally friendly pest control strategies. References: 1. Mallick, S., Chakkalakkal, G.J., Heryanto, C., Alqassar, J.D., Martin, A., Lazetic, V. and Eleftherianos, I. 2026. 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