Beneficial nematodes are microscopic, non-segmented roundworms classified as entomopathogenic nematodes, meaning they are obligate parasites of insects. They serve as a natural biological control agent against numerous insect pests. They are harmless to plants, humans, pets, and earthworms, making them a safe alternative to chemical pesticides. Their primary function is to actively seek out and infect specific insect larvae, pupae, and sometimes adult stages that reside in the soil or other moist environments.
How Beneficial Nematodes Hunt and Kill
The nematodes responsible for pest control exist in the soil as infective juveniles. These juveniles navigate through soil moisture films, locating prey by sensing chemical cues like carbon dioxide and excretory products released by the insects. Once a host is located, the nematode enters the pest’s body cavity, usually through natural openings such as the mouth, anus, or spiracles.
Upon entering the host, the nematode releases millions of symbiotic bacteria (Xenorhabdus or Photorhabdus species) carried within its gut. These bacteria multiply rapidly, overcoming the insect’s immune system. The resulting bacterial infection, known as septicemia, typically kills the host within 24 to 48 hours.
The bacteria break down the host’s internal tissues, turning the insect into a nutrient source. The nematodes feed, mature, and reproduce inside the cadaver until a new generation of infective juveniles emerges. These new nematodes then leave the remains to search for new hosts, restarting the biological control cycle.
Primary Targets: Common Soil Pests
The greatest success with beneficial nematodes is seen against pests that spend their larval stage within the soil profile. White grubs, the larvae of various scarab beetles (Japanese beetle, May/June beetle, Oriental beetle), are a major target. These grubs feed on plant roots, damaging lawns and turf, and are effectively controlled by Heterorhabditis bacteriophora. This species uses a cruiser foraging strategy, actively seeking out and penetrating less mobile grubs deeper in the soil.
Fungus gnat larvae are another common target, especially in potted plants or greenhouses. These larvae feed on plant roots and organic matter, and are best controlled by Steinernema feltiae. S. feltiae prefers cooler soil temperatures and exhibits an ambush strategy, waiting near the soil surface for hosts.
Other general soil pests controlled include the larvae of cutworms and sod webworms. Flea larvae, which develop in sandy soil, turf, and carpets, are also highly susceptible. Steinernema carpocapsae is often recommended for these surface-dwelling pests due to its ambush behavior and effectiveness against highly mobile insect larvae.
Specialized Targets: Borers and Weevils
Certain nematode species target insects that bore into plant material or have unique life cycles. The Black Vine Weevil larva, a serious pest of ornamentals and berries, feeds on roots and girdles the crown of plants. Heterorhabditis bacteriophora is effective against these root-feeding weevil larvae.
Various borer species, such as the Dogwood borer and Peach tree borer, can be controlled by applying nematodes directly to entry holes in the tree bark or stem. This targets the larval stage as it tunnels within the plant tissue, where conventional insecticides cannot reach. Steinernema carpocapsae is often chosen for trunk and stem applications due to its ability to survive in environments with slightly lower moisture content.
Cabbage root maggots, the larvae of a fly that attacks the roots of brassicas, are also highly susceptible. Treatment involves applying the nematodes to the planting hole or drenching the soil around the base of the plant.
Maximizing Effectiveness: Application and Timing
Successful application depends on creating ideal environmental conditions for nematode survival and mobility. The soil must be thoroughly moist before and after application, as nematodes rely on water films to move and locate hosts. Immediately after application, the treated area should be watered again to wash the nematodes off foliage and into the soil.
Timing is critical, as the infective juveniles are vulnerable to ultraviolet light and desiccation. They should be applied during the cooler parts of the day, typically at dusk or on a cloudy day, to minimize direct sunlight exposure. Soil temperatures must be between 60°F and 90°F, as temperatures outside this range reduce nematode activity and survival.
Nematodes must be stored properly in a refrigerator between 35°F and 50°F and should never be frozen. The entire package should be used immediately upon mixing with water. The solution must be continuously agitated during application to prevent the organisms from settling. Maintaining soil moisture for at least a week following application ensures the nematodes have sufficient time to infect hosts and establish a new population.