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You may have heard of nematodes. Do you think of nematodes as harmful to your plants? Or do you think of them as beneficial worms that attack pests in the garden?
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To some it may seem that all nematodes are pests. But did you know that, of the 10 million estimated nematode species—20,000 known species—the vast majority are acknowledged to be beneficial?
What Are Nematodes?
Nematodes, as members of the phylum Nemata (or Nematoda), are tiny roundworms that live in humid environments. Most nematodes cannot be easily seen with the naked eye; some species can only be seen under a microscope. The smallest species known are only 82 µm — 0.2 mm (e. g. Sphaeronema minutissimum, Bunonema hessi). On the other hand, the largest nematode species lives in the sperm whale (Placentonema gigantissima, over 8 m long). Soil-dwelling species sizes ranged between 0.2 and 12 mm.
Several different types of nematodes exist and these can either be parasites or free living. Parasitic nematodes depend on hosts such as plants, animals or humans for their food acquisition. Nematodes are usually specific to a host and those that attack plants or insects cannot harm humans.
Nematodes As Foes
Certain species can attack plant roots and cause damage that is often difficult to diagnose. Such nematodes, which harm our plants, are known as plant-parasitic nematodes. They live in the soil and require high soil moisture. Depending on the species, they attack the roots, stems or leaves, penetrating the plants through injuries or stomata. Some feed in or on roots by inserting a needlelike structure, called a stylet, into plant cells. For instance, root-knot nematodes penetrate the roots and fine hairs of their host plants and suck out the sap there. Because they suck out the cell contents, plants may suffer from a lack of water or nutrients. While piercing plant tissues with their stylet, root-knot nematodes inject saliva secretions, which cause some changes in the tissues known as galls, mostly in vegetables.
Sometimes, nematode infestation leads to a change in the area of cultivation showing patches of irregular growth. Furthermore, piercing root cells and sucking out the contents can also transmit bacteria and viruses causing even greater damage. Affected plants, for example, show spots, irregular growth and deformities on roots and leaves. Sometimes nematodes even form so-called cysts on the roots that contain nematode eggs, which can survive long periods in soil. The offspring of nematodes only hatch when the conditions are favourable. For example, soybean cyst nematodes are feared in soybean cultivation areas. A proven measure against nematodes in soybean cultivation areas is a crop rotation with a wide range of crops (nematode-resistant crops). Most nematodes prefer plants that encourage their reproduction.
—Prevention and management.
Nematodes are mostly permanent residents of a soil. The goal is to keep plant nematode populations below the damaging levels to avoid potential yield loss. In gardens, some control measures include:
Crop rotation – Rotating crops that are not related to each other – from different crop families
Reducing plant stress – Proper irrigation, aeration and amendments (organic matter such as compost)
Planting the correct cover crops – Alternative poor or nonhost plants
Removal and destruction of infected plants.
Nematodes As Bioindicators of Soil Health
There are beneficial nematodes that live freely in the soil. They are called free-living nematodes. They feed on algae, bacteria and fungi as well as decomposing organic matter. Some can also have a stylet. They contribute to the natural balance of a soil food web and to cycle nutrients, enhancing nutrient availability to plants. Furthermore, because they are very sensitive to soil disturbance, the proportions of their trophic groups—bacterivores, fungivores, predators and omnivores—in the soil nematode community can be used to indicate disturbed soils, much as the canary was used to warn miners of unsafe health conditions in a coal mine.
Soil nematodes, especially bacterivores and fungivores, can contribute to maintaining adequate levels of plant-available nitrogen in farming systems relying on organic sources of fertility. Agricultural systems with fewer physical and chemical disturbances, such as pastures, hay fields, and orchards, tend to support larger populations of these nematodes than more frequently disturbed systems like vegetable- and row-crop fields. Agricultural management may increase the abundance of soil nematodes, primarily through an increase in the abundance of bacterivores associated with the incorporation of organic residues.
Nematodes As Natural Enemies of Pests in the Garden
Certain types of nematodes infect insect and slug pests. While natural/organic, they are one of the best broad-spectrum pest controls. They attack soil-dwelling stages of pests. Additionally, they are also known to combat diverse aboveground stages of pests. Some species of the genera Phasmarhabditis, Steinernema and Heterorhabditis are used as beneficial organisms for pest control. Steinernema and Heterorhabditis species are used against insect pests while Phasmarhabditis species are used against slug pests.
Beneficial nematode species are very host-specific in their choice of food sources or prey, although, they will attack and infect numerous insect pests. One type of nematode only parasitizes certain pests. Consequently, it is very important to choose the right species when using these beneficial nematodes. The exact identification of the pest and the associated correct selection of the suitable nematode species is therefore crucial for the subsequent success of the control. The larvae and pupae of the pests are the most attacked stages.
Nematode products can be easily dissolved in water (ready-to-use products) and applied with a watering can for small areas. However, the area should then be kept moist for several days or weeks so that the nematodes can move towards their prey. In addition, the soil temperature should be above 12 ° C. Pests are controlled naturally and biologically. Nematodes are safe to be used around humans, animals, pets and plants. No residues remain in the soil or in the plants. There are no side effects such as waiting times or the development of resistance. The treated areas are available for use and access at all times.
Do you have pests such as the leatherjacket found on your lawn or fungus gnats found in your garden? They usually actively seek out, penetrate through body orifices and infect insects (larvae, pupae and adults) with deadly/toxic bacteria. Within the insect host, nematodes release the bacteria, which kill the insect within 48 hours. The insect cadaver serves the nematodes as a source of food and a place of reproduction. When the food source is consumed, they search for other insect hosts to invade and multiply. This leads to more invading nematodes for pest control.
Beneficial nematodes for gardening insect control may be used to control pests such as, and not limited to, armyworms, beetles, billbugs, crane flies, crown borers, cutworms, fleas, fungus gnats, root weevils, rootworms and stem borers.
Learn more about plant-parasitic nematodes here, read more about nematodes for soil health here, or find out more about the use of beneficial nematodes for pest control here.