Entomopathogenic nematodes, commonly known as EPNs, are minuscule organisms that play a vital role in protecting our crops from pests. They are employed in biological pest control strategies and are so tiny that they are invisible to the naked eye. Despite their size, these tiny worm-like creatures are highly effective in eradicating the larvae of various harmful insect species. They exhibit highly selective behavior, posing no harm to the environment, crops, humans, fish, or other animals. Their use has shown promising results in the field and is considered an excellent alternative to chemical pesticides.
In this article, we aim to understand what entomopathogenic nematodes are, how they operate, and how these unique macroorganisms are used.
What Are Entomopathogenic Nematodes?
Nematode entomopathogens under the microscope
In nature, nematodes belong to the animal kingdom and are classified as a phylum, comprising over 20,000 species. These microscopic “worms”, visible only under an electron microscope, typically behave as parasites. Many of them are harmful parasites, affecting animals, humans, and plants. We have previously discussed crop nematodes, which cause severe damage to the root systems of plants. However, there are specific genera of nematodes, known as entomopathogenic nematodes, that parasitize insects considered pests of crops. In agricultural terms, they are “beneficial” parasites, suitable for biological control and completely harmless to humans and the environment. The genera commonly used in agriculture are Steinernema and Heterorhabditis, specifically the species Steinernema feltiae, S. carpocapsae, S. kraussei, Heterorahabditis bacteriophora, and H. megidis.
Which Pests Are Controlled by Entomopathogenic Nematodes?
Entomopathogenic nematodes are obligate parasites of insect larvae, including beetles, moths, flies, and wasps. They cannot parasitize adult or pupal stages of insects. Many of the insect species that are of agricultural interest and are effectively controlled by these nematodes spend their larval stages inside plants (or in the soil). Here, they create galleries and feed, causing severe damage to the affected crops. Some of these pests include:
- Black vine weevil
- Capnodis tenebrionis
- Tomato leafminer (Tuta absoluta)
- Tomato cutworm
- Western flower thrips (Frankliniella occidentalis)
- Yellow and red-legged borer
- Red palm weevil
- Japanese beetle (Popillia japonica)
- Codling moth (Carpocapsa)
- May beetle
How Do Entomopathogenic Nematodes Work?
Entomopathogenic nematodes harbor bacteria of the genera Xenorhabdus and Photorhabdus inside their bodies. These bacteria, in turn, parasitize the larvae of harmful insects. In practice, the nematode is a tiny worm-like creature that penetrates the target insect. Once inside, it releases these bacteria, which rapidly multiply and kill the host within 24-72 hours of penetration. The entomopathogenic nematode continues to survive, seeking out other larvae to parasitize.
Conditions Necessary for Their Effectiveness
Entomopathogenic nematodes require a highly humid environment to be effective. Low temperatures or exposure to direct sunlight can lead to their demise. Therefore, they are applied to crops and soil in combination with water, maintaining consistently high levels of humidity.
Where to Find Entomopathogenic Nematodes
Entomopathogenic nematodes are readily available in the market (for example, here) and can be purchased without requiring a license, as they are not classified as pesticides. Hence, they are permitted in organic farming. Typically, producing companies distribute them in specially designed refrigerated polystyrene containers to ensure the survival of these macroorganisms, which are sensitive to high temperatures. These nematodes are sold in formulations consisting of a dehydrated soft mass within an inert substrate (such as clay, alginate, or gel). These substrates contain billions of entomopathogenic nematodes that are reactivated upon contact with water. In practice, they are added to water, and the solution is mixed slowly and continuously. This solution is then distributed onto the soil or vegetation.
How to Use Them
In the Soil
When applied to the soil, entomopathogenic nematodes can reach the larvae of insects that spend part of their development in the soil or plant root systems. This includes beetles like the yellow and red-legged borer, Capnodis tenebrionis, or the black vine weevil. Nematodes utilize the same entry points created by insects, such as galleries in the underground parts of trees. Another entry point from the soil is through fallen berry fruits, such as apples infested with the codling moth.
On Vegetation
A similar mechanism is employed when treating vegetation with nematodes. They can be applied to tree trunks, and in this case, they can reach overwintering insect larvae that nest in the bark. However, on vegetable crops, nematodes do not directly penetrate the plant’s tissues; they rely on damage already caused by insects, such as the galleries created by leaf-mining pests like the tomato leafminer. For optimal results, it is essential to thoroughly wet the vegetation until it drips.
Application Tips
Both soil and foliage treatments should be carried out during the cooler hours of the day when maintaining high humidity levels is easier. For foliar applications, a shoulder-mounted sprayer (such as this one), is used to evenly wet the plants. For soil drenches, a simple watering can suffices. In this case, it is advisable to keep the soil moist even after nematode distribution by watering regularly. This ensures the nematodes’ survival.
As it’s evident, targeting insect pests in their larval stages (preferably when they are young) requires a good understanding of their life cycle, which isn’t always straightforward. Often, we only notice an insect when it reaches the adult stage, even though it’s the larvae that cause the damage. A classic example is the red palm weevil.
