**Eelworms** is a common, older term for **Nematodes** (phylum Nematoda), particularly the plant-parasitic species that are slender and microscopic, resembling tiny eels. Plant-parasitic nematodes are highly destructive agricultural pests that attack plants in various ways: by feeding externally on roots, burrowing into roots, or feeding within stems and leaves. They cause stunting, root damage, wilting, and reduced crop yield, leading to massive economic losses globally.
Taxonomy and Classification
Plant-parasitic nematodes belong to the phylum Nematoda, which encompasses both free-living and parasitic species. Key genera include **Root-Knot Nematodes** (*Meloidogyne*), **Cyst Nematodes** (*Heterodera*), and **Lesion Nematodes** (*Pratylenchus*). They are not insects but unsegmented roundworms. The vast majority of species are beneficial or harmless, but the parasitic species are highly specialized pests.
Physical Description
Plant-parasitic nematodes are microscopic, typically measuring less than $2$ mm long. They are transparent, cylindrical (round in cross-section), and tapered at both ends. Their key identifying feature is the **stylet** (a sharp, spear-like mouthpart) used to puncture plant cells and suck out the contents.
Symptoms of nematode attack, which are often mistaken for nutrient deficiencies or water stress, include:
- **Root Knots/Galls:** Abnormal swelling on roots (Root-Knot Nematode).
- **Stunting and Yellowing (Chlorosis):** General lack of vigor above ground due to root damage.
- **Lesions:** Dark, dead spots on roots (Lesion Nematode).
Distribution and Habitat
Nematodes are found in virtually all terrestrial and aquatic environments. Plant-parasitic species are ubiquitous in agricultural soil. They require water to move, so populations are highest in moist, sandy soils. They generally live and move within the thin film of water surrounding soil particles, seeking out tender root tips for feeding.
Behavior and Life Cycle
The life cycle is relatively simple: egg, juvenile stages (J1–J4), and adult. It can be completed in as little as 3 weeks in warm soil. Nematodes are spread through movement of infested soil, contaminated equipment, water runoff, or infected planting material. Many species can survive harsh conditions by entering a cryptobiotic state (anhydrobiosis) in the soil or in cyst forms (Cyst Nematodes).
Their feeding damages root cells, diverting the plant’s resources to the nematode’s benefit (in the case of Root-Knot Nematodes, which induce giant feeding cells). This damage severely limits the root’s ability to absorb water and nutrients, leading to poor yield.
Damage and Economic Impact
Plant-parasitic nematodes are estimated to cause billions of dollars in crop losses annually.
- **Yield Loss:** Reduced plant health directly translates to reduced crop quantity and quality.
- **Disease Complex:** Nematode feeding wounds create easy entry points for secondary soilborne pathogens (bacteria and fungi), contributing to larger disease complexes.
- **Quarantine:** Some species (e.g., Potato Cyst Nematode) are quarantine pests, restricting trade and requiring expensive eradication or control programs.
Management and Prevention
Management is difficult due to their microscopic size and soil habitat.
- **Crop Rotation:** Rotating susceptible crops with non-host or resistant crops starves the nematodes in the soil.
- **Resistant Varieties:** Planting genetically resistant varieties is the most effective and sustainable long-term solution.
- **Nematicides:** Chemical fumigants and non-fumigant nematicides are used, but their use is restricted due to environmental concerns.
- **Biological Control:** Using antagonistic fungi or bacteria that parasitize the nematodes or their eggs is an emerging control method.
Conservation and Research
Nematodes are a central topic in plant pathology and soil science. Research is focused on precision agriculture techniques for mapping nematode infestations, developing new biocontrol methods, and understanding the genetics of resistance in host crops.