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Beneficial nematodes

Also known as Entomopathogenic nematodes, Insect-parasitic nematodes, Steinernema feltiae, Steinernema carpocapsae, Heterorhabditis bacteriophora

Beneficial or harmlessBeneficial
Steinernema feltiae, a nematode used against fungus gnat larvae Photo: Tony Coulson · CC BY 2.0

In short

Beneficial nematodes are microscopic roundworms that hunt soil-dwelling insects, enter them and release bacteria that kill the host. Sold as live products and watered in, they are used against fungus gnat larvae, vine weevil grubs, cutworms and other soil pests. They do not harm plants, people or animals.

How it looks

Too small to see clearly without magnification: a thin, transparent worm roughly 0.4–1.5 mm long. The applied stage is the infective juvenile. Under magnification, live nematodes are curved and move; straight ones that do not move when prodded are likely dead.

Signs

No plant damage. After application, soil pests such as fungus gnat larvae or grubs decline over the following weeks.

Life cycle

Infective juveniles find a host in the soil and enter it, releasing symbiotic bacteria that kill the insect. The nematodes feed and reproduce inside the cadaver, then new infective juveniles leave to search for more hosts.

What favours it

Moist soil or potting mix kept moist for at least two weeks after application, suitable soil temperatures (S. carpocapsae about 21–29 °C; S. feltiae about 13–29 °C) and protection from sunlight, which kills them quickly.

Prevention

  • Store the product as the supplier directs and use it before its expiry date.
  • Check a sample under magnification on arrival: if more than about a fifth are dead, contact the supplier.

Treatment

Start with the first step and move down only if the problem persists.

  1. Mix with water and apply as a drench to moist mix in early morning or evening.
  2. Match the species to the pest named on the product: S. feltiae for fungus gnat larvae, S. carpocapsae for caterpillars such as cutworms and armyworms; other species are sold for grubs and weevil larvae.
  3. Keep the mix moist, not waterlogged, for at least two weeks afterwards.
  4. Check the supplier's compatibility information before combining with other pesticides or fertilisers.

Pros and cons

Pros

  • Reach pests inside the soil that sprays cannot.
  • Do not harm plants or animals and have little impact on non-target arthropods.

Cons

  • Living products with a short shelf life that need cool storage.
  • Quickly killed by sunlight, drying out and unsuitable temperatures.
  • Work best on larvae; eggs and adults of many pests are not affected.

Look-alikes

Root-knot nematodes
Plant-parasitic nematodes that live inside roots and cause swellings (galls); beneficial nematodes attack insects, not plants.

Where you find it

Used in pots, nurseries, turf, orchards and vegetable beds against soil-dwelling larvae.

Sources

  1. Steinernema carpocapsae, beneficial nematode (Sc) — Cornell NYSIPM Biocontrol Fact Sheet
  2. Steinernema feltiae, beneficial nematode (Sf) — Cornell NYSIPM Biocontrol Fact Sheet
  3. Insect-Parasitic Nematodes for the Management of Soil-Dwelling Insect Pests — eOrganic

Written in our own words from these sources. Check a product label before using any treatment.