Shore flies: algae farmers in your propagation tray

Short answer. Shore flies (Scatella stagnalis, and in some greenhouses S. tenuicosta) are stout little black flies with pale-spotted wings that breed in algae on wet rockwool, trays, benches and floors ^[8][9]. Their larvae eat algae, not roots ^[8]. The risk is disease: larvae and adults carry Pythium, Thielaviopsis, Fusarium and Verticillium in their gut and droppings ^[5][6][7]. At 25 C S. stagnalis goes from egg to adult in 11.4 days and can double in under 3 days ^[1]. Kill the algae and the puddles first; Bti and soil mites do little, rove beetles and sticky traps help.

Shore flies at a glance: which numbers matter?

The shore fly numbers that matter are an 11-16 day generation at 20-25 C, about 300 eggs per female, and an r_m of 0.263/day at 25 C, more than twice the fungus gnat rate. Values are for Scatella stagnalis in rockwool unless stated ^[1].

What are shore flies and where do they live?

Shore flies are flies of the family Ephydridae, and Scatella stagnalis is the most common species in US greenhouses ^[11][12]. In the wild they live along wetland margins and feed on algae and microbes ^[4].

Shore fly larvae are trophic generalists on algae. On monocultures of 17 algae, 13 supported development to adult, with egg-to-adult times from 12.1 days on a diatom to 23.0 days on a blue-green alga ^[3].

In a grow, shore flies breed wherever algae grow on wet, lit, nutrient-rich surfaces: exposed rockwool cubes and slab tops, propagation trays, capillary mats, drippers and channels, under benches and on wet floors ^[8][9]. They thrive under moist conditions and are most common in propagation ^[8].

Shore flies often appear after fungus gnat numbers start to fall in a crop ^[12]. That timing tricks growers into thinking a gnat treatment failed.

No published study of shore flies on cannabis was found. Everything here comes from greenhouse ornamentals, vegetables and rockwool cucumber work and is extrapolated (Supported).

How do I tell shore flies from fungus gnats?

Tell shore flies from fungus gnats by body shape and wings. Shore flies are robust, house-fly-shaped, about 3 mm long, with short legs, short bristle-like antennae and dark wings with about five pale spots ^[8][9][14]. Fungus gnats are delicate and mosquito-like, with long legs, long beaded antennae and clear or grey wings with a Y-shaped vein ^[14].

Sources: ^{[8][9][11][14]}.

On a sticky card, shore flies look like tiny dark house flies and fungus gnats like thin mosquitoes. A 10x hand lens shows the wing spots ^[11].

The predatory hunter fly (Coenosia attenuata) is about twice the size of a shore fly, with clear unspotted wings, and perches on leaves to ambush prey ^[9]. Do not kill it.

What is the shore fly life cycle in numbers?

The shore fly life cycle runs egg, three larval instars, pupa and adult, and Scatella stagnalis completes it in 15.9 days at 20 C and 11.4 days at 25 C in rockwool ^[1][8].

How does temperature change shore fly development?

Shore fly development speeds up steadily with temperature, and a simple degree-day model fits the published data. These are the published values:

No development data at 15, 30 or 35 C were found for either species.

For S. stagnalis, 212.8 degree-days above 6.4 C ^[1] predict about 13.6 days at 22 C and 9.9 days at 28 C (calculated here; the model reproduces the measured 11.4 days at 25 C). S. tenuicosta needs about 154 degree-days ^[2]; its threshold was not given in the abstract.

Extension summaries agree: about 8 days at 78 F (26 C) and 14 days at 68 F (20 C) ^[9], or 15-20 days per generation depending on medium temperature ^[8][11].

How many eggs does a shore fly lay and how long do adults live?

A Scatella stagnalis female lays about 315 eggs at 25 C, about 20 per day, and lives about 15.5 days; males live about 22 days ^[1]. Extension sources give 300-500 eggs per female ^[9][11].

S. tenuicosta lays more when warm: 379 eggs at 20 C, 710 at 26 C and 477 at 28.5 C, with peak daily output of 33.5 eggs at 26 C ^[2]. The authors considered 26 C near optimal for reproduction ^[2].

Eggs are laid in algae or very wet areas and hatch in 2-3 days ^[9][11][13]. Larvae feed about 7-10 days, and adults emerge 4-5 days after pupation ^[11][13].

Females need about 3 days to mature before laying ^[9]. This short window is why adult sprays alone fail.

What are the shore fly life-table parameters?

At constant 25 C in rockwool, Scatella stagnalis has a net reproductive rate of 126.1 female eggs per female, a generation time of 18.4 days and an intrinsic rate of increase of 0.263/day ^[1]. The proportion of females was 0.521 ^[1].

The paper gives the doubling time as 5.3 days. ln2 / 0.263 is 2.6 days, and ln(126.1) / 18.4 confirms r_m = 0.263, so the 2.6-day figure is consistent with the other published parameters. Use 2.6 days for planning.

S. tenuicosta had its highest R0 (329.6) at 26 C and its highest r_m (0.430/day) at 28.5 C, with a generation time of 12.4 days and a doubling time of 1.6 days ^[2]. Stage-specific mortality stayed at or below 5.7% ^[2].

Heat costs S. stagnalis pupae. Pupal mortality rose from 7% at 20 C and 10% at 25 C to 29% at a fluctuating mean of 28.5 C ^[1].

Why do temperature and moisture matter for shore flies?

For shore flies, moisture and algae decide whether a population exists, and temperature decides how fast it grows. Every life stage is found on or near algae ^[9].

Temperature. Cannabis rooms at 22-28 C are where shore flies grow fastest. S. stagnalis develops in 11-14 days across that range ^[1] (calculated for 22 C), and S. tenuicosta reproduces best near 26 C ^[2]. Propagation heat mats under trays speed both the algae and the flies.

Lower limit. The S. stagnalis threshold of about 6.4 C ^[1] is far below grow-room temperatures. Cooling is not a control.

Surface moisture. Shore flies tolerate saturated and even submerged conditions that most predators cannot ^[8]. A surface that dries between irrigations stops algae and removes the habitat ^[9].

Light on wet surfaces. Algae need light, water and nutrients. Exposed rockwool, white trays and clear channels under strong LEDs grow algae fastest (Grower practice). Covering wet surfaces from light starves the larvae.

Fertiliser. Use the minimum fertiliser level that grows the crop well and avoid excess runoff, which feeds algal blooms ^[11].

Air humidity. Room RH is a minor factor. High RH and low VPD in propagation domes slow surface drying, so algae and shore flies build up there. See kb-term-vpd.

Diapause. No diapause has been described for greenhouse Scatella in the sources checked; indoors they breed continuously.

How fast can a shore fly population grow? (worked example)

Ten shore flies left unchecked at 25 C can become about 400 in 2 weeks and about 2,500 in 3 weeks. This uses N(t) = N0 × e^(r_m × t) with the published r_m values ^[1][2].

N0 = 10; values rounded. Example: 10 × e^(0.263 × 21) = 10 × e^5.52 ≈ 2,504. Fungus gnat r_m is from Ishihara and Tagami; see kb-pests-fungus-gnats.

This is an unchecked exponential upper bound: unlimited algae, no predators, a stable age structure. Real populations level off when algae run out. The point is speed: a shore fly problem can appear within two weeks of algae appearing.

How do shore flies spread plant diseases?

Shore fly larvae and adults carry soil-borne pathogens in their gut and frass, and adults move them to healthy plants. The evidence is from lab and greenhouse-crop studies.

Pythium. S. stagnalis larvae fed on cucumber roots colonised by Pythium aphanidermatum; 97% of first and second instars, 20% of third instars and pupae, and 10% of adults carried mature oospores in the gut ^[5]. Excreted oospores germinated, and both larvae and adults transmitted the pathogen to healthy cucumber plants ^[5].

Other soil-borne fungi. Adult shore flies were shown to be aerial vectors of Verticillium dahliae, Fusarium oxysporum f. sp. basilici and Thielaviopsis basicola ^[6]. Adults were attracted to sporulating cultures and infected tissue and picked up spores within 10-20 minutes ^[6].

Inoculum load. One adult deposited about 2.4 × 10^7 viable V. dahliae spores per day in frass, and spread the pathogen over about 2.2 cm² per hour ^[6].

Fusarium on lisianthus. Adult shore flies, fungus gnats and moth flies picked up Fusarium avenaceum spores from infected stems and carried them to healthy lisianthus; 75% of plants were infected within 4 days ^[7].

Field relevance. Extension sources note that disease transmission may be less likely under real greenhouse conditions than in the lab ^[8][9]. In cannabis propagation, where Pythium and Fusarium are already present in many facilities, treat a shore fly outbreak as a sanitation alarm (Supported). See kb-diseases-root-rot-pythium.

Where do shore flies hide and how do they get in?

Shore flies hide as eggs, larvae and pupae in algae mats on wet media surfaces, under benches, in floor puddles, drains and capillary mats, and as adults on leaves and walls near those sites ^[8][9].

Infested media and plants. Large numbers move between greenhouses in infested growing medium ^[8]. Inspect incoming plugs and cubes for larvae and algae ^[11].

Flight. Adults are strong fliers but usually stay close to breeding sites ^[11]. They come in through doors and vents from wet outdoor areas.

Wet structures. Uneven floors that pool water, leaking emitters, open reservoirs and algae-covered trays keep a population going between crops ^[9].

How do I scout for shore flies?

Scout shore flies with yellow sticky cards checked weekly, and by looking for larvae and pupae in algae on wet surfaces.

Sticky cards. Place yellow cards near the media surface and just above the canopy, and count adults weekly with a 10x lens ^[8][11]. Wisconsin extension suggests blue cards for shore flies and about 10 traps per 1,000 sq ft (about 1 per 9 m²) ^[13].

Sticky tape. Yellow sticky tape along walls or above the crop, especially in propagation, both monitors and mass-traps adults ^[8][9].

Larvae and pupae. Look in green or brown algae films for yellow-brown larvae with forked rear tubes and dark, spindle-shaped pupae ^[9]. Potato slices attract fungus gnat larvae, not shore fly larvae; a clean potato slice with flies on the cards points to shore flies (Grower practice).

Frass. Black specks of droppings on leaves near wet areas are a shore fly sign ^[8][9].

Thresholds. No published action threshold exists for shore flies on any crop in the sources checked. Record counts per card per week, and act on a rising trend in propagation (Grower practice).

Which shore fly controls hit which life stage?

Shore fly controls that work target the algae habitat and the larvae; adult sprays fail because females lay eggs about 3 days after emerging and the puparium resists insecticides ^[9][11].

Re-application timing. Larvae feed for about 7-10 days and the generation turns over in 11-16 days at 20-25 C ^[1][11]. Any larval treatment needs repeating at least every 7-10 days through two generations, about 3-4 weeks, while algae control takes effect (Grower practice, derived from ^[1]).

Resistance. Shore flies breed fast (r_m 0.263/day ^[1]), so repeated use of one insecticide class invites resistance. Rotate by mode-of-action class only. Insect growth regulators such as cyromazine (IRAC 17) and diflubenzuron (IRAC 15) are listed as drenches for both shore flies and fungus gnats ^[13]. Registrations on cannabis vary: follow the label and local law.

Which biological controls work against shore flies?

Biological control of shore flies is weaker than for fungus gnats, because shore fly larvae live in saturated algae and even standing water where most natural enemies cannot survive ^[8]. Use biologicals preventively and together with algae control ^[9].

Rove beetles (Dalotia coriaria)

Dalotia coriaria (formerly Atheta coriaria) is the best-supported predator for shore flies. Adults and larvae feed on shore fly eggs and larvae, as well as fungus gnats and thrips pupae, in the growing medium ^[9][15]. Release adults in the evening; they are nocturnal ^[9].

Dalotia develops from egg to adult in about 17 days at 26 C (79 F), lays about 90 eggs per female, and does best at 18-27 C and 50-85% RH ^[15]. Release rates vary by supplier; follow the biocontrol supplier technical sheet.

Entomopathogenic nematodes (Steinernema)

Steinernema nematodes infect shore fly larvae but do not give sufficient control, because they are poorly adapted to the semi-aquatic breeding habitat ^[9]. Kansas State extension describes lab results as inconsistent and dependent on greenhouse conditions ^[8].

Some growers use repeated drenches of Steinernema carpocapsae at high rates against shore flies ^[9]. Do not mix nematodes with oxidising sanitisers.

In peat and rockwool trials against S. tenuicosta, both S. feltiae and S. carpocapsae were tested; in rockwool the best control was 83-84% in the second week after two applications, and control was lower than in peat because nematodes disappeared quickly from rockwool ^[16] (year and journal not confirmed)being reviewed.

Soil predatory mites (Stratiolaelaps scimitus)

Stratiolaelaps scimitus eats shore fly larvae ^[10][15], but Kansas State extension reports that soil mites, including S. scimitus and Hypoaspis aculeifer, are in most cases not effective at regulating shore fly populations ^[8]. An older extension note says the mite can control shore flies only where there is no standing water ^[11]. Use mites for fungus gnats and thrips pupae, not as the main shore fly tool.

Bti

Bti is not a shore fly tool. It is a larvicide used against fungus gnats ^[10][14], and no published trial showing good control of Scatella larvae was found. Extension shore fly guides from Kansas State and UConn list algae control, rove beetles, nematodes and insect growth regulators, not Bti ^[8][9]. The bacterial product tested against S. tenuicosta in the peat and rockwool trial was a different subspecies, B. thuringiensis var. thuringiensis ^[16].

Natural enemies that arrive on their own

The hunter fly Coenosia attenuata catches adult shore flies and fungus gnats in flight; it is not sold commercially but moves between greenhouses on plants ^[9]. The parasitoid wasp Hexacola neoscatellae (spelled Hexacloa neoscatella in the UConn sheet) can help regulate shore flies in unsprayed greenhouses ^[9].

How do I use the environment against shore flies?

The environmental weapon against shore flies is a grow with no algae: no standing water, no lit wet surfaces, and media that dry at the surface between irrigations ^[9][11].

Floors and benches. Level floors so water drains and does not pool; clean up standing water on and under benches and walks ^[9][11]. Landscape or weed fabric under benches reduces algae, and with it fungus gnats and shore flies ^[18].

Cover wet surfaces. Cover rockwool cube and slab tops, trays and channels from light with opaque film or covers (Grower practice). Keep reservoirs lidded and lightproof.

Irrigation. Avoid overwatering, especially early in a crop, so the upper surface dries between irrigations ^[9]. Fix leaking drippers.

Media. Choose free-draining media and do not compact it ^[9].

Disinfectants for algae. Use disinfectants in pre-crop clean-up on surfaces and walkways ^[9]. Hydrogen peroxide can reduce algal growth and so shore fly numbers, but some disinfectants can harm plants ^[8]. The effect of hydrogen peroxide on algae, seedlings and shore fly reproduction in rockwool was studied directly by Vänninen and Koskula ^[17]. See kb-term-hydrogen-peroxide.

Trade-off in flower. Removing algae and puddles has no downside for botrytis or powdery mildew; drier floors lower humidity around plant bases. The only risk is drying the root zone instead of the surface. See kb-diseases-botrytis-bud-rot.

What should I not do against shore flies in flower?

In flower, do not spray adult shore flies with insecticides. Adults are not the source, females lay within about 3 days of emerging, and residues on buds can fail compliance tests ^[9]. Follow the label and local law.

Do not pour strong oxidisers or algaecides onto the root zone of flowering plants. Treat floors, benches, trays and covers, not the medium around live roots ^[8].

Do not starve flowering plants of water to dry out algae. Cover and dry the surfaces instead.

Rove beetles, sticky tape and algae control are fine through flower.

What are the common myths about shore flies?

"Shore flies eat roots like fungus gnats." Myth. Shore fly larvae feed on algae and decaying matter and do not consume roots ^[8]. Their harm is pathogen spread and frass ^[5][6].

"Bti controls shore flies too." Contested. No trial showing good control was found, and extension shore fly guides do not list Bti ^[8][9].

"Shore flies are only cosmetic." Myth. Larvae and adults carry and transmit Pythium aphanidermatum, Thielaviopsis basicola, Fusarium and Verticillium in lab studies ^[5][6][7].

"Spraying adults clears shore flies." Myth. Insecticides do not effectively suppress shore flies because the puparium resists them and new adults lay within days ^[9][11].

"Soil predatory mites handle shore flies." Contested. Mites eat larvae but are usually not effective because they cannot live in standing water ^[8][11].

What are the common mistakes with shore flies?

• Treating shore flies as fungus gnats and using Bti or potato traps.
• Spraying adults instead of removing algae.
• Leaving rockwool tops, trays and channels uncovered under strong light.
• Ignoring puddles under benches and leaking emitters.
• Using algaecides or strong peroxide on live root zones.

Evidence

• Biology and life tables of S. stagnalis in rockwool and S. tenuicosta on algae: Established (lab and greenhouse conditions) ^[1][2][3].
• Larvae feed on algae, not roots: Established ^[3][8].
• Pathogen transmission by larvae and adults: Established in the lab ^[5][6][7]; importance in commercial greenhouses: Contested ^[8][9].
• Algae and moisture control as the main tactic: Established ^[8][9][11].
• Rove beetles: Supported; nematodes and soil mites: inconsistent ^[8][9].
• Bti against shore flies: Inconclusive (no trial found).
• All extrapolated to cannabis; no cannabis-specific study found.