Nematodes, roundworms, and eelworms:Get to know these soil organisms

Estimated reading time: 6 minutes

Nematodes, also known as roundworms or eelworms, are a diverse group of aquatic, multicellular organisms. These organisms are mostly microscopic in size (most plant and soil nematodes range from 0.1 to 12 mm). However, some can reach 8 m in length (parasitic nematode of sperm whales) (Swart and Marais, 2008).

Nematodes occupy most terrestrial and aquatic habitats on earth including extreme environments such as caves, underground mines and thermal vents (Girgan et al., 2020). With an estimated species diversity of more than one million species, nematodes can be considered one of the most diverse groups of organisms globally, representing 80% of all multicellular organisms (Girgan et al., 2020). Apart from their diversity, nematodes are also known to be abundant with the highest densities reaching more than 50 million per m2 (Bongers and Bongers, 1998).

Nematodes are represented by various feeding groups which include plant-parasitic nematodes, free-living nematodes as well as animal parasitic nematodes (including humans). The most famous nematode groups are the animal- and plant-parasitic nematodes due to their economic importance. However, due to the diversity of nematodes and their unique roles in various ecosystems, the beneficial counterparts to the parasites are often used as biological indicators of ecosystem health.

Feeding groups

These nematodes are classified into feeding groups based on their method of feeding (Yeates et al., 1993).

1. Sedentary parasites include the endo-parasitic nematode groups that form feeding sites within plant parts, mostly the roots. These sites often cause physiological changes in the host plant and symptoms such as galls on the roots. These nematodes have an infective stage often found in the soil, which infects the plant and then starts to develop into the sedentary stage (mature stage) that form the feeding sites.

Examples of sedentary parasites include the root-knot nematodes (Meloigodyne spp.) and cyst nematodes (Heterodera spp. and Globodera spp.).

2. Migratory endoparasites are the nematode groups that also feed within plant parts; however, unlike sedentary parasites, they move through the plant or plant parts while feeding. Often this type of feeding causes the destruction of plant cells as the nematodes move and feed through the plant, often causing necrotic lesions. An example is the lesion nematodes (Pratylenchus spp.).

3. Semi-endoparasites are plant-parasitic nematodes that are partially imbedded in the plant while they feed, and most are able to migrate or move to other feeding sites. Members of the nematode family Hoplolaimidae are examples of this feeding group.

4. Ectoparasites are plant-parasitic nematodes that feed outside the plant roots normally with long feeding apparatus (stylets). They are able to move freely in the soil around the roots and feed on different sites on the plant root. Examples of these nematodes include the stubby-root nematodes (family Trichodoridae) and dagger nematodes (Xiphinema).

5. Epidermal cell and root hair feeders are plant feeding nematodes that are generally small in size with small stylets that feed on the surface of plant roots; these nematodes are rarely considered to be a threat to crop production. Examples are members of the family Tylenchidae.

6. Algal, lichen and moss feeders are, as the name states, nematodes that feed on algae, lichen (algal or fungal component) and moss through piercing. They are also not considered to be of any agricultural importance; and examples include Tylenchus.

Signs of nematode damage

Symptoms of plant-parasitic nematode damage may include lesions, galls or cracks in the below-ground plant parts. However, above-ground symptoms are seldomly seen and can often resemble other pathogen symptoms and even abiotic symptoms (Swart and Marais, 2008).

Therefore, the only way to truly diagnose a nematode infestation is through analyses of soil and plant samples by a nematology laboratory.

Cyst nematodes (Globodera spp. and Heterodera spp.), root-knot nematodes (Meloigodyne spp.) and lesion nematodes (Pratylenchus spp.) are considered the top three most economically important plant-parasitic nematodes globally. These nematode groups cause extensive crop loss globally and increase input costs due to the expensive chemical nematicides.

The extensive host range of these nematodes and phasing out of nematicides make the control of these organisms difficult. However, extensive research is done annually to improve existing control measures and investigate new solutions.

Free-living nematodes

Free-living nematodes or beneficial nematodes include all the nematodes that do not parasitise animals or plants. These nematodes also represent the largest proportion of nematodes found in terrestrial habitats.

Free-living nematodes play an important role in various soil systems and are involved in numerous levels of the soil food web as a food source to various other organisms and as predators. Due to their diversity and resilience in the soil ecosystem, free-living nematodes are one of the most often used and useful biological indicators of soil health.

Free-living nematodes, as with plant-parasitic nematodes, are characterised into groups based on their food source (Yeates et al., 1993). Bacterial feeding nematodes are probably the most diverse group of the free-living nematodes and feed on prokaryotic organisms also including the free-living life stages of animal parasites. Hyphal or fungal feeding nematodes feed on fungal hyphae. Predaceous nematodes feed on invertebrates such as protozoa, rotifers, enchytraeids and other nematodes. Omnivores, as the term suggests, feed on a wide range of organisms.

The last two groups are the substrates ingestors (non-selective deposit feeders) and the dispersal or infective stages of animal parasites (feeding on bacteria until a host is found). The population sizes and diversity of these feeding groups are used in various ecological indices to get a snapshot view of the soil health status.

Apart from their diversity and the role they play in the soil ecosystem, free-living nematodes are probably the most suitable biological indicators of soil health due to their rapid response to any environmental or anthropogenic changes in the soil environment (Swart et al., 2017). The use of nematodes as biological indicators has vastly increased in the last few decades and, for scientists and producers, are important organisms to consider.

Entomopathogenic nematodes

Entomopathogenic nematodes (EPNs) are a group of insect parasitic nematodes defined formally as “nematodes which are mutualistically associated with bacterial symbionts and in which all life stages of the nematode, except for the free-living third-stage infective juvenile (IJ), are found inside the insect host” (Malan and Ferreira, 2017).

These nematodes have great and proven potential as biological control agents against many insect pests. EPNs are produced and sold in several countries as biological control agents. Research on EPNs have increased in South Africa, especially on native species of EPNs with commercial potential. This may have a positive influence on the use of these nematodes as biological agents, as native species may have stronger survival abilities and be more affective in native soils.

Biological control agents

Nematodes are often disregarded due to their size; however, they are the most abundant and diverse organisms found in all environments on earth. And not only are they some of the most destructive pests in agriculture but they can also be used to monitor soil health and have great potential as biological control agents. – Chantelle Girgan, Australian Department of Energy, Environment and Climate Action, Mariette Marais, ARC-Plant Health and Protection, and Prabashnie Ramouthar, Farmers Agricare

For more information and references, send an email to ramoutharp@agricare.co.za