Fungus gnats: the watering problem with wings

Short answer. Small dark flies running over the medium and flitting up when you water are fungus gnats, usually Bradysia impatiens or B. coprophila ^[4]. Adults are harmless and live about six days. The larvae eat root hairs and young roots, which hurts seedlings and fresh clones, and in lab tests they carried Pythium into seedlings ^[7]. At 25 C a gnat population can double about every 6 days ^[3]. The fix is the substrate, not the air: dry the top few centimetres between irrigations, then use Bti, Steinernema feltiae nematodes, Stratiolaelaps scimitus mites or Dalotia coriaria rove beetles, with yellow cards and potato slices to count.

Fungus gnats at a glance: which numbers matter?

The fungus gnat numbers that matter most are a 17-22 day generation at 24-25 C, a population doubling time of about 6 days near 25 C, and a lower developmental limit of about 10 C. Values below are for Bradysia impatiens unless stated; all come from lab rearing on artificial diets, not cannabis.

What are fungus gnats and which species attack cannabis?

Fungus gnats are small flies in the family Sciaridae; the species that matter in greenhouses and grow rooms are Bradysia coprophila and Bradysia impatiens ^[4]. They are flies with a full metamorphosis: egg, four larval instars, pupa, adult ^[1][8].

Fungus gnat larvae feed mainly on fungi and decaying plant matter. They also eat healthy roots, tunnel into the stems of young cuttings and seedlings, and feed in the crowns of mature plants ^[4]. A fungal food source appears critical for good development and reproduction ^[5].

Fungus gnats are a propagation pest. Problems are mostly tied to excessive moisture, which is normal while cuttings and plugs root ^[4]. Losses of 20-40% of plants in propagation have been reported in ornamentals ^[23] (Supported outside cannabis).

No published study of fungus gnats on cannabis specifically was found. The biology here comes from ornamental crops and lab diets and is extrapolated to cannabis (Supported). Punja and colleagues recovered Pythium and Fusarium from roots of commercially grown cannabis, so anything that wounds roots in wet media matters ^[21].

How do I recognise fungus gnat adults and larvae?

Fungus gnat adults are delicate grey-black flies about 2-3 mm long, with long slender legs, beaded antennae longer than the head, and a Y-shaped vein in clear to light-grey wings ^[8]. They fly weakly, run over the medium and rest on pots and leaves.

Fungus gnat larvae are legless, translucent white to clear, up to about 6 mm long, with a shiny black head capsule ^[8]. They live in the top few centimetres of medium. In very wet, heavily infested pots they leave shiny slime trails on the surface ^[8].

Plant signs. Seedlings and clones in a wet medium wilt, stall, yellow or fail to root. Wilting with wet media is more often root rot or plain overwatering than gnats alone, so check roots ^[8]. See kb-diagnosis-drooping-overwatering-vs-underwatering.

Differential: shore flies. Shore flies (Scatella spp.) are stockier, like tiny house flies, with short bristle-like antennae and about five pale spots on each dark wing. Their larvae are yellow-brown with no black head and feed on algae ^[8]. See kb-pests-shore-flies.

Differential: root aphids. Winged root aphids on cards look like small aphids, not flies, and wingless ones sit on roots covered in white wax. See kb-pests-root-aphids.

What is the fungus gnat life cycle in numbers?

The fungus gnat life cycle (Bradysia impatiens) runs about 17-22 days from egg to adult at 24-25 C, slowing to about 49 days at 12.8 C and stopping below about 10 C ^[1][2][3][8]. Females start laying within a day of emerging if males are present ^[1].

How long does each stage last?

At about 24 C (75 F) on a ground-soybean diet, Wilkinson and Daugherty measured these mean stage durations for B. impatiens ^[1]:

Extension summaries give similar figures: eggs hatch in about 3 days at 75 F, larvae feed about 10 days and adults emerge about 4 days after pupation, for roughly 17 days egg to adult ^[8]. UConn gives 21-28 days egg to adult depending on temperature, with eggs hatching in 4-6 days and larvae feeding about 2 weeks at 72 F (22 C) ^[9].

Strains differ. Kennedy (1974) reported a B. impatiens strain that developed in 15.6 days at 25 C and was more fecund than the Wilkinson and Daugherty population (cited in ^[19]).

How does temperature change fungus gnat development and growth?

Fungus gnat development speeds up with heat until about 29 C and then fails; the population growth rate peaks near 25 C. These are the constant-temperature results for B. impatiens:

Doubling times are ln2 / r_m, calculated here from the published r_m values. Development data at 15, 20 and 35 C exactly were not found.

Wilkinson and Daugherty put the lower limit for development at about 10 C and the upper limit at 32.2-35 C ^[2]. The optimum for each stage differed: about 29.4 C for eggs and pupae, 23.9 C for larvae, and 12.8 C for the longest adult life ^[2].

No published degree-day total for a B. impatiens generation was found. Do not apply the shore fly figure (about 213 degree-days above 6.4 C) to gnats.

How many eggs does a fungus gnat lay?

A B. impatiens female laid about 75 eggs on average (range 12-156) at 75 F in the original life-history study ^[1]. Fecundity was about the same from 12.8 to 29.4 C and fell at 32.2 C ^[2].

Other populations lay more. Mated Bradysia sp. nr. coprophila females laid 21-217 eggs, mean 141 ± 9.3, within 48 hours in soilless growing media ^[12]. Wisconsin extension gives up to 200 eggs per female, laid in clusters on moist media ^[22].

Adults are short-lived: about 5.9 days at 24 C ^[1] and about one week in greenhouse descriptions. Longevity increases as temperature drops ^[2].

Why do temperature and substrate moisture matter for fungus gnats?

For fungus gnats, temperature sets the speed of the cycle but substrate moisture decides whether the cycle happens at all. Every immature stage lives in the medium, where the air is close to saturated whatever the room RH, so air humidity matters far less than how wet the top few centimetres stay.

Why does grow-room temperature favour fungus gnats?

Cannabis grow-room temperatures sit right on the fungus gnat optimum. Veg rooms at about 24-28 C and flower rooms at about 22-26 C match the 20-25 C band where B. impatiens r_m is 0.103-0.111/day ^[3]. The 32-35 C needed to harm gnats ^[2] would harm the crop first.

Cool rooms slow gnats but do not stop them. At 16 C r_m is still 0.058/day, a doubling every 12 days ^[3]. A cold propagation area mainly slows your clones' rooting, which lengthens the wet, vulnerable window.

No diapause has been described for greenhouse Bradysia in the sources checked. Indoors they breed all year with overlapping generations ^[8][11]. Outdoors in inland California they peak in winter and spring, when soils are wetter ^[8].

How does a wet medium increase fungus gnat numbers?

A constantly wet medium raises fungus gnat survival. In a greenhouse trial, about 2.6 times as many adult Bradysia sp. nr. coprophila emerged from pots kept "constantly saturated" (9.9 ± 1.4 adults) as from pots only "initially saturated" (3.8 ± 1.0) ^[13].

The type of medium mattered as much as moisture. In the same trials, one peat-based mix produced 3-7 times fewer adults than another ^[13], and in a later study larval survival differed between two other commercial peat-based mixes ^[14]. The reasons (drainage, air-filled porosity, microbial load) were not separated.

Microbially active media attract egg-laying females. Composts, vermicompost, blood meal and kelp attract adults, and 2-5 times as many adults may emerge from microbially active media ^[11]. Immature composts less than a year old are a known source ^[11].

Females lay in media rather than on bare surfaces. In choice tests, B. sp. nr. coprophila laid far more eggs in three soilless mixes than on filter paper, with no clear preference among the mixes; the authors suggested moisture and volatiles drive the choice ^[12].

Does air humidity matter for fungus gnats?

Air humidity is a minor factor for fungus gnats. In the classic temperature study, chamber RH ran 50-90% while the rearing cells were close to 100% ^[2], and development still followed temperature. Room RH matters indirectly: high RH and low VPD slow how fast the medium surface dries.

Tie to set-points. A propagation dome at 80-90% RH keeps the plug surface wet all day. Once clones root, venting the dome and dropping to 60-70% RH in veg lets the top layer dry between irrigations (Grower practice). See kb-term-vpd and kb-term-dryback.

How fast can a fungus gnat population grow? (worked example)

Ten fungus gnats left unchecked at 25 C can become about 100 in 3 weeks and about 1,000 in 6 weeks. This uses N(t) = N0 × e^(r_m × t) with the B. impatiens r_m values from Ishihara and Tagami (rice bran, constant temperature) ^[3].

Starting number N0 = 10; values rounded. For example, at 25 C after 21 days: 10 × e^(0.111 × 21) = 10 × e^2.33 ≈ 103.

This is an unchecked exponential upper bound. It assumes unlimited food and moisture, no predators and a stable age structure. Real pots plateau, and a drying surface or a predator breaks the curve.

The table shows why 20-25 C is the danger band and why 30 C is not a cure: growth at 30 C is slower, but still about 13 times in 6 weeks.

Compare shore flies, which multiply far faster (r_m 0.263/day at 25 C for Scatella stagnalis); see kb-pests-shore-flies.

Where do fungus gnats hide, how do they spread and how do they get in?

Fungus gnats hide as eggs, larvae and pupae in the top few centimetres of moist medium, and as adults on the medium surface, pot rims and lower leaves. Most of the population at any moment is below the surface, not flying.

Bagged media. Commercial bagged soilless media and rooted plugs can arrive carrying fungus gnats (Cloyd and Zaborski, cited in ^[9]). Store bags closed, dry and off the floor ^[8][11].

Clones and plugs. Rooted cuttings and plugs move larvae in their root balls. Inspect incoming plugs for larvae and feeding damage ^[11].

Compost and organic amendments. Unfinished compost, worm castings, blood meal and kelp attract females and can carry eggs ^[8][11]. Pasteurise or age compost before use ^[8].

Debris and cull piles. Adults can emerge from discarded media and plant debris and migrate to the crop; keep waste in sealed bins or outside ^[4][11].

Wet floors, algae and spills. Puddles, algae under benches and spilled media are breeding sites ^[4][11].

Flight. Adults fly weakly and stay near pots, but they still spread locally and are attracted to light ^[8]. Open windows and doors let outdoor gnats in; screen and seal them ^[8].

How do I scout for fungus gnats?

Scout fungus gnats with two tools: yellow sticky cards for adults and raw potato pieces for larvae, both checked weekly.

Yellow sticky cards. Lay cards horizontally at the medium surface, or stand small pieces on skewers just above it; check and change weekly ^[9][11]. Wisconsin extension suggests about 10 traps per 1,000 sq ft (about 1 per 9 m²) for gnats and shore flies ^[22]. Use a 10x hand lens to confirm the Y-shaped wing vein and long antennae.

Potato slices or plugs. Press raw potato pieces, cut side down, onto the medium. UConn uses plugs at least 2.5 cm (1 in) across and checks after 48 hours; thin "French fry" strips suit small plugs ^[9]. Count larvae under each piece, then discard it. A potato-based larval monitoring technique was published for B. coprophila in floriculture ^[18].

Where to place them. Put cards and potato pieces in propagation first, then in a fixed sample of veg pots, and always in the wettest pots and near any algae.

Action thresholds. No research-based threshold exists for cannabis; growers set their own ^[11]. Supplier guidance used by growers is about 20 adults per card per week as a trigger for predatory mites, and about 50-75 to switch to nematodes (Grower practice). In propagation, any larvae under potato slices on rooting clones justify action, because larvae there can carry Pythium (Grower practice, based on ^[7]).

Trend, not snapshot. Record counts per card per week. A rising trend after a watering change means the source is somewhere else: a wetter bench, a compost-rich pot or a floor drain.

How do fungus gnats spread plant diseases?

Fungus gnat larvae can spread soil-borne pathogens, but adults are poor vectors of Pythium. Laboratory work shows a clear split by life stage.

Larvae and Pythium. B. impatiens larvae ingested Pythium aphanidermatum oospores and transmitted the pathogen to geranium seedlings in 65% of filter-paper assays, but in under 6% of plug cells with potting mix ^[7]. Strains with few oospores transmitted less, and strains without oospores were not transmitted ^[7].

Gut passage. Passing through a larval gut increased germination of P. aphanidermatum oospores ^[7]. Pythium DNA was detected in larvae and pupae but not in adults ^[6].

Adults and Pythium. Adult B. impatiens did not pick up infectious Pythium from diseased plants and transmit it in any experiment ^[6]. Pythium produces no aerial spores for adults to carry ^[4].

Adults and fungi with aerial spores. Adult fungus gnats can carry spores of fungi that sporulate above ground, including Botrytis cinerea, Fusarium avenaceum, Thielaviopsis basicola and Verticillium spp. ^[4]. Adult gnats, shore flies and moth flies carried F. avenaceum macroconidia to healthy lisianthus, which then died ^[20].

Larvae and other fungi. Larvae have been shown to transmit Thielaviopsis basicola, Fusarium and Verticillium in lab work ^[4].

What this means in a grow room. Transmission may be less important in real greenhouses than in lab dishes, and it varies with the Pythium species ^[4][7]. Treat gnats in propagation as a disease risk, and fix the wet medium that favours both. See kb-diseases-root-rot-pythium and kb-diseases-fusarium.

Which fungus gnat controls hit which life stage?

Fungus gnat controls work almost entirely on larvae; nothing practical kills eggs or pupae in the medium. This is why every control is repeated on a schedule set by development time.

How often should I reapply fungus gnat treatments?

Reapply fungus gnat larvicides on a schedule that hits every new cohort while it is still feeding. At 24-25 C eggs hatch in 3-4 days, larvae feed for about 10-14 days, and pupae last 3.5-4 days ^[1][8][9].

Bti schedule. Bti stays toxic in the medium for about 2 days ^[11]. Applied every 5-7 days, every larval cohort is exposed at least once during its 10-14 day feeding window. Extension guidance is that 2-3 repeat applications may be needed ^[9][11]; three rounds over about 3 weeks cover one full generation (Grower practice, derived from the stage durations above).

Nematode schedule. S. feltiae stays active about 10-14 days ^[10], and UConn recommends repeat applications every two weeks ^[11]. Because larval feeding lasts 10-14 days, a 14-day interval keeps infective nematodes present through each cohort.

Cooler rooms stretch the schedule. At about 16-18 C development is roughly half as fast ^[2][3], so larval windows are longer. Spacing can stretch slightly, but keep at least three rounds.

Do fungus gnats become resistant to insecticides?

Fungus gnats can develop insecticide resistance with repeated use, and few products still work against adults because of resistance ^[4][11]. Their short generations (about 3 weeks) and high egg output favour it.

Rotate by mode-of-action class, not by product name. Classes used against fungus gnat larvae elsewhere include microbial disruptors of the insect midgut (Bti, IRAC 11A), and insect growth regulators such as cyromazine (IRAC 17), diflubenzuron (IRAC 15) and azadirachtin (IRAC UN) ^[11][22]. Registrations on cannabis vary by jurisdiction: follow the label and local law.

Which biological controls work against fungus gnats, and in what conditions?

Four biological controls are established against fungus gnats in greenhouse crops: Bti, the nematode Steinernema feltiae, the soil mite Stratiolaelaps scimitus and the rove beetle Dalotia coriaria ^[4][9][10]. All are used in moist media and work best preventively ^[11]. Their use on cannabis is extrapolated from ornamentals (Supported).

Bti (Bacillus thuringiensis subsp. israelensis)

Bti is a bacterial larvicide whose protein crystals must be eaten by fungus gnat larvae; larvae stop feeding and die ^[11]. It is most effective against young first-instar larvae and is toxic for only about 2 days ^[9][11]. It is toxic only to fly larvae such as mosquitoes, black flies and fungus gnats ^[8]. See kb-term-bti.

Bti does not affect eggs, pupae or adults. It is a curative tool on an established population, often combined with mites or nematodes ^[10]. Bti is not a reliable shore fly control; see kb-pests-shore-flies.

Steinernema feltiae (entomopathogenic nematode)

Steinernema feltiae infective juveniles enter fungus gnat larvae and kill them within 1-2 days ^[9]. They remain active in the medium for about 10-14 days ^[10], so they are reapplied every two weeks ^[11]. Apply as a drench to moist medium, ideally in low light or on overcast days ^[10][11]. See kb-term-steinernema-feltiae.

S. feltiae is a cool-adapted nematode. In lab assays it infected hosts over roughly 8-30 C, a narrower range than other steinernematids ^[17]. Biocontrol supplier technical sheets give activity at about 10-31 C with best results at about 14-26 C. Root-zone temperatures above about 30 C, common in hot pots under strong light, reduce its performance.

Rate, timing, potting medium and host plant all changed S. feltiae efficacy against B. coprophila in floriculture trials (Jagdale et al. 2004, cited in ^[4]). Do not mix nematodes with oxidising sanitisers such as hydrogen peroxide or chlorine.

Stratiolaelaps scimitus (soil predatory mite)

Stratiolaelaps scimitus (formerly Hypoaspis miles) is a soil-dwelling mite about 0.4 mm long that eats fungus gnat larvae and thrips prepupae and pupae, up to about 30 prey per day ^[16]. It lives in the top 2.5 cm (1 in) of the medium ^[16]. See kb-term-stratiolaelaps-scimitus.

Temperature window. Development takes 10 days at 28 C, 12 days at 24 C, 18 days at 20 C and 34 days at 15 C ^[16]. The optimum for development and reproduction is 15-30 C, and activity drops below about 14 C ^[16]. UConn puts the minimum media temperature at about 15-16 C (60 F) ^[10].

Reproduction. Females lay up to three eggs per day; populations are 50-70% female; unfertilised females produce only males ^[16]. Adults survive 3-4 weeks without food and eat algae and debris when prey is scarce ^[16].

How to use it. Release early, when gnat numbers are low; one release can establish for a whole crop cycle ^[16]. S. scimitus alone usually does not manage an established gnat population, so pair it with watering changes ^[16]. Release rates vary by supplier; follow the biocontrol supplier technical sheet.

Dalotia coriaria (rove beetle)

Dalotia coriaria (formerly Atheta coriaria) is a generalist rove beetle about 3 mm long whose adults and larvae eat fungus gnat larvae, shore fly larvae and thrips pupae in the medium ^[10][11].

Life history. In growing medium in the lab, egg, larva and pupa took 2.2, 7.1 and 7.8 days, 17.0 days egg to adult; females laid 90.2 eggs on average; females lived 47.8 days and males 60.3; sex ratio 1:1 ^[15]. UConn gives the optimum as 18-27 C (65-80 F) and 50-85% RH ^[10].

Moisture and medium. In the lab, one or two beetles per pot halved gnat emergence (2.7 and 2.3 adults vs 5.4 in controls), but in the greenhouse the beetle effect was not significant, while moisture and medium were ^[13]. In later greenhouse trials, 10 adult beetles per 15.2 cm pot (2 L medium) regulated larval numbers ^[14].

Compatibility. UConn describes S. scimitus, S. feltiae and D. coriaria as mutually compatible ^[11]. Kansas State notes that D. coriaria will eat S. scimitus and advises against releasing both at once ^[16] (Contested). Beetles are nocturnal; release in the evening ^[9].

What else eats fungus gnats?

The hunter fly Coenosia attenuata catches adult gnats and shore flies in flight, and its larvae prey on larvae in the medium ^[10]. It is not sold commercially but turns up in unsprayed greenhouses ^[10].

How do I use the environment against fungus gnats?

The environmental weapon against fungus gnats is a dry top layer of medium, not a room set-point. A constantly saturated medium produced about 2.6 times as many adults as one allowed to dry back ^[13].

Soil and living soil. Water less often and more completely, and let the top 2-5 cm (1-2 in) dry between waterings (Grower practice, consistent with ^[8][11]). Lift pots: a light pot is ready for water. See kb-guides-watering-how-much-how-often.

Coco coir and rockwool. Coco and rockwool are run wet by design with frequent fertigation, so drought is not an option. Rely on biologicals, and keep the surface drier with drip emitters under a cover or by increasing dry-back between the last and first shots (Grower practice). See kb-term-coco-coir and kb-term-rockwool.

Bottom watering. Watering from below keeps the top layer drier than top watering (Grower practice). See kb-term-bottom-watering.

Media choice. Use well-drained media and do not compact it ^[4][11]. Add perlite to improve drainage ^[8]. Avoid immature compost and heavy surface dressings of blood meal, kelp or worm castings in propagation ^[11].

Physical covers. Sand and diatomaceous earth on the surface did not reduce emergence or egg laying in tests ^[4]. A thick layer of coarse recycled-glass aggregate reduced adult emergence over time, depending on thickness ^[4]. Fitted collars or discs over the medium are Grower practice.

Trade-off in flower. Drying the top layer has no downside for botrytis or powdery mildew; it lowers local humidity at the base of the plant. The risk is overcorrecting into drought stress in mid-flower, when water demand peaks. Dry the surface, not the root ball. See kb-diseases-botrytis-bud-rot.

Temperature is not a lever. Room temperatures that suit cannabis (22-28 C) suit gnats ^[3]. Root-zone cooling below about 18 C slows gnats but also slows roots.

What should I not do against fungus gnats in flower?

In flower, do not drench residual insecticides into the root zone. Most are not registered on cannabis, and residues can show up in compliance tests. Follow the label and local law.

Do not starve flowering plants of water to kill gnats. Drought stress in weeks 3-7 of flower costs more yield than gnats do on established plants. Dry only the top few centimetres.

Do not use strong hydrogen peroxide or chlorine drenches as a gnat cure. They are poorly supported against gnats, can damage roots, and kill the nematodes and soil microbes you may be relying on (Grower practice / Contested). See kb-term-hydrogen-peroxide.

Biological drenches (Bti, nematodes) and soil predators act in the medium, not on buds, and are generally fine through flower. Check that what you use is allowed on cannabis where you live.

How do I know the fungus gnat treatment worked?

A fungus gnat treatment has worked when weekly sticky-card counts fall for 2-3 consecutive weeks and stay low, potato pieces show no larvae, and seedlings and clones root normally.

Expect a lag. Adults already present, and pupae that Bti and nematodes do not reach, keep appearing on cards for about one generation (2-3 weeks at 24-25 C) after larvae are controlled ^[1][8].

What are the common myths about fungus gnats?

"Sand or diatomaceous earth on top stops gnats." Myth. In tests, neither reduced adult emergence or egg laying, because larvae and females use the gaps ^[4]. Only a thick coarse aggregate layer helped ^[4].

"Adult gnats spread Pythium." Myth. Adults did not transmit Pythium and carried no Pythium DNA; larvae can, mainly in lab assays ^[6][7]. Adults can carry fungi with aerial spores, such as Fusarium and Botrytis ^[4][20] (Established).

"Gnats always mean overwatering." Contested. Wet media are the main driver ^[4][13], but microbially active media can produce 2-5 times as many adults ^[11]. Coco and rockwool are wet by design.

"Hydrogen peroxide drenches cure gnats." Grower practice / Contested. No controlled trial against Bradysia was found.

"Sticky cards control gnats." Myth. Adults live about 6 days ^[1]; most of the population is in the medium. Cards are for counting.

"Fabric softener sheets repel gnats." Supported (lab only). Dryer sheets repelled adults in a lab study, linked to volatiles such as linalool, but they kill nothing and adults move elsewhere ^[4].

What are the common mistakes with fungus gnats?

• Killing adults with cards and ignoring the larvae in the medium.
• One Bti or nematode application, then stopping before the next cohort hatches.
• Not changing the watering habit, or the compost-rich top dressing, that created the problem.
• Treating gnats while ignoring the root rot they signal.
• Releasing predators into a medium that never dries and expecting them to fix it alone.
• Mistaking shore flies for gnats and using Bti against them.

Evidence

• Biology, life-stage durations and temperature limits of B. impatiens: Established (lab rearing on soybean and rice-bran diets) ^[1][2][3].
• Damage to seedlings and cuttings, association with wet and microbially active media: Established in greenhouse crops ^[4][11][13]; extrapolated to cannabis (Supported).
• Larval transmission of Pythium: Established in the lab, weaker in potting mix ^[6][7]; adult Pythium vectoring: Myth ^[6].
• Bti, S. feltiae, S. scimitus and D. coriaria as controls: Established in greenhouse ornamentals ^{[4][9][10][16]}; on cannabis Supported (extrapolated).
• Re-application intervals derived from stage durations: Grower practice built on published development times.
• Action thresholds: Grower practice; no published cannabis threshold.