Steinernema feltiae: entomopathogenic nematodes for fungus gnat larvae

Steinernema feltiae is a nematode you water into the media, not spray onto leaves: it hunts down fungus gnat larvae and thrips pupae and kills them from the inside, usually within 24 to 48 hours of finding them ^[1][2]. Most first-time failures with it have nothing to do with the biology. They come from a pack that sat in a warm van, or in direct light on a windowsill, for two hours before anyone opened it.

What it is, and how it kills a larva from the inside

Steinernema feltiae is a microscopic roundworm in the family Steinernematidae, sold commercially under trade names including Entonem (Koppert) and Nemasys (Biobest). The stage you buy and apply is the infective juvenile (IJ): a non-feeding, free-living form whose only job is to find a host and get inside it ^[2]. IJs move through the water film around soil and media particles, sense a nearby larva, and enter through a natural opening, the mouth, the anus or a breathing spiracle, or through a thin patch of cuticle ^[2].

Once inside, the IJ releases a symbiotic bacterium from its gut: a species of Xenorhabdus ^[2]. X. bovienii is the species most often reported for this nematode, though this article did not find a dedicated taxonomic source confirming that specific pairing, so treat it as commonly reported rather than confirmed. The bacterium multiplies rapidly, converts the host's tissue into food, and kills it, typically within 24 to 48 hours ^[1][2]. The nematodes feed and develop on the liquefying cadaver through two or three generations before a new wave of infective juveniles emerges, within a few days to about a week, to go looking for the next larva ^[2]. That reproduction is why an established application can keep working for longer than the single drench that started it, though it is not a substitute for repeat applications during an active outbreak (see below).

Schematic, each panel to its own scale. Infective juveniles are 0.4–1.5 mm long [2]; they can also get in through thin cuticle, not shown. Timings are for favourable conditions: moist media inside the working temperature range.

Source: Cornell CALS, Biological Control guide, entomopathogenic nematodes [2], accessed 26 Sep 2026

Applying it as a drench: rate, water and timing

Fungus gnat larvae live in the top few centimetres of media, where it stays consistently wet, so the nematode suspension has to reach that zone and the media has to stay moist long enough for the IJs to move through it. Manufacturer guidance for soil and media application is 250,000 to 500,000 infective juveniles per square metre of bench or floor area ^[1]. Scale that to your canopy area, not to plant count: a bench running four plants per square metre and a bench running one large plant per square metre get the same dose per square metre of media surface. Worked example: a 20 m² propagation bench needs 5 to 10 million infective juveniles for one full-rate application, and roughly double that if the outbreak calls for a second drench a week later (see below).

Mix the suspension in water at 15–20 °C; the tank or can should not exceed 25 °C once mixed, because warmer water shortens how long the nematodes stay viable before you apply them ^[1]. Agitate gently and continuously, the IJs settle out of still water within minutes, and if your delivery system depends on suspension staying even (a drip line rather than a watering can), use nozzles with an opening of at least 0.5 mm, take out any in-line filter finer than 0.3 mm, and keep line pressure under roughly 20 bar (290 psi); manufacturer datasheets flag all three as limits that will damage or filter out the nematodes ^[1]. A watering can, hose-end sprayer, backpack sprayer or the facility's own fertigation or drip system all work, provided the output goes to the media surface, not the canopy.

Before you open the pack

• Media is already moist, or lightly irrigated first if dry — nematodes cannot move through dry media
• Water for mixing is 15–20 °C and the tank stays under 25 °Cwarm water shortens viability before application
• Application is timed for evening or lights-off, to cut UV exposure during and after the drench
• The whole pack is used in one mixing session — do not store a mixed suspension for next time

Apply at lights-off or in the evening where that is practical: it cuts UV exposure during the drench itself and gives the media hours to stay wet before the lights and any heaters dry the surface again. Keep the media moist for several days afterwards; a dry surface stalls the nematodes before they reach a larva ^[1].

The temperature window that decides whether it works

Manufacturer data rates S. feltiae as active between about 10 and 31 °C, with the best results at 14–26 °C, and warns that temperatures below 5 °C or above 35 °C can kill the nematodes outright ^[1]. An extension source gives a broadly similar working range, roughly 15.5–32 °C (60–90 °F), in moist conditions ^[3].

The number that actually matters is media temperature, not air temperature or the room's thermostat reading. An unheated propagation bench in early spring, a media block sitting against an outside wall, or a floor-level bench under a cold slab can all read several degrees below the air a metre above them. Take the reading with a probe pushed into the media where the larvae are, in the top few centimetres, not a glance at the wall controller.

Biocontrol suppliers commonly position S. feltiae as the more cold-active of the commercially sold entomopathogenic nematode species, which is part of why it is the one usually recommended for fungus gnats rather than, say, the warmer-adapted S. carpocapsae. That is supplier and grower consensus rather than a head-to-head trial result, but the practical floor it points to is real: below about 10 °C, expect the drench to under-perform even though the label says "apply as directed."

Why most first failures are UV and heat, not biology

Infective juveniles are killed rapidly by direct ultraviolet light and by desiccation ^[2]. A pack that travels in a warm delivery van, sits on a sunlit loading-bay shelf, or waits a day before refrigeration can lose a meaningful share of its viable nematodes before you have mixed a single litre. None of that shows up as a visible defect: the water still looks the same, and a grower who applies it exactly as instructed and sees no drop in gnat counts reasonably assumes the nematode "didn't work," when the actual cause was the two hours it spent somewhere hot and bright before it reached the media.

The practical response is handling discipline, not a different product:

• Order close to the planned application date rather than holding stock; shelf life is commonly 2–3 months refrigerated at 2–6 °C, shorter once opened ^[1].
• Keep the pack refrigerated until the moment you mix it, and transport it in a cool box if the facility and the nematode source are not in the same building.
• Mix only what you will apply that day. A suspension left standing loses viability faster than the unopened pack did.
• Apply at lights-off or in the evening, and irrigate the media lightly afterwards if it dries fast under your fixtures, to keep UV exposure and desiccation to a minimum during and after the drench.

Where a series of applications keeps under-performing despite correct rates and timing, product handling before it reached your facility is the first thing to check, not the biology or the rate.

One drench rarely clears an outbreak on its own

A fungus gnat generation runs about 17 days at 24 °C, egg to adult, with eggs hatching in around 3 days and the larval stage lasting roughly 10 days before pupation ^[3]. A single nematode drench only reaches the larvae present in the media on the day you apply it: eggs laid the day before hatch into a media that is no longer carrying a useful nematode population, because the IJs that didn't find a host have themselves died off or moved on.

For that reason, treat one application as the start of a series rather than the whole treatment. A short run of two to three drenches, roughly a week apart, catches overlapping generations far more reliably than a single application at the label rate, and matches the few-days-to-a-week cycle before new infective juveniles emerge from an infected larva ^[2]. Keep monitoring with yellow sticky cards at canopy base and media level through the series: a falling adult count on the cards is the signal the series is working, and a flat or rising count after two applications is the signal to check handling (see above) before assuming the biology has failed.

Where it sits next to a predatory mite and a rove beetle

Steinernema feltiae is generally compatible with the predatory mites and insects used elsewhere in a cannabis IPM programme, because it works in a different place and on a different life stage: in the media, against larvae and pupae, while most predatory mites and insects patrol the plant surface. That said, "generally compatible" is not a blank cheque. Always check a current compatibility source before tank-mixing the nematode suspension with anything else, since some fungicides, some water treatments and even some tap-water disinfection systems are toxic to nematodes even though they are harmless to plants see the compatibility guide.

Within this cluster, S. feltiae plays a different role to Stratiolaelaps scimitus and Dalotia coriaria. Both of those are media-dwelling predators that establish a standing population and keep fungus gnat and shore fly numbers suppressed over a whole cycle, as long as they are introduced early, before a population gets started. The nematode is the tool for a rapid knockdown once scouting shows an active outbreak: sticky-card counts already climbing, visible larvae in the top of the media, adults resting on the media surface in numbers. In practice, licensed facilities commonly run the mite or the beetle continuously as prevention and keep S. feltiae on hand as the reset drench for when prevention has already been outrun. None of the three replaces fungus gnat scouting and source control, drying the top few centimetres of media between waterings, covering media surfaces, and removing algae and standing water, which reduce the egg-laying sites the whole programme is fighting in the first place.

For facilities applying through a fixed drip or fertigation system rather than a watering can, confirm your emitters meet the 0.5 mm minimum opening, remove or check any filter finer than 0.3 mm, and stay under the 20 bar pressure limit before the first run, and dose a test zone before committing a whole bench; see release rates and timing for beneficials for how to plan a wider rollout once that test confirms good coverage.

Log the batch number, arrival date, media temperature at application and the sticky-card counts before and after each drench, alongside your regular IPM treatment record. When a series stops working, that log is usually the first place to look, and more often than not it points at a pack that spent too long somewhere warm and bright, not at the nematode itself.