Dalotia coriaria
Rove beetle, sold as Atheta- Rate
- 0.1–2 /sq ft (about 1–22 /m²); suppliers differ twentyfold
- Temp
- 65–80 °F (18–27 °C) best; suppliers quote 55–95 °F
- Best for
- Larvae in media and algae patches; breeds on site
Pest
Shore flies
Scatella spp.
The short answer
Shore flies are the stout, dark little flies with pale-spotted wings that swarm over wet benches, floors and algae-covered compost in greenhouses and propagation. Adults and larvae feed on algae and decaying matter, not on healthy roots, so the plants are not eaten. The harm is indirect: large numbers make a crop unsaleable, and adult flies have been shown to carry and spread root- and stem-rot pathogens. Control starts with the algae: Purdue says removing it will greatly reduce shore fly numbers.
hero macro · adult on an algae-covered compost surface
Adult Scatella shore fly, showing the pale wing spots and short antennae. Photo: pending.
Identification
Adults look like tiny houseflies: about 1/8 inch (3 mm) long, stout and black, with reddish eyes, antennae shorter than the head, and five or more pale spots on each dark wing. They are strong, fast fliers, unlike fungus gnats, which are delicate, long-legged and long-antennaed and drift weakly off the compost. On a yellow card the wing spots are the quickest tell.
The larvae are maggots up to 1/4 inch (6 mm) long, opaque and yellowish to tan-brown, with no dark head capsule and a breathing tube with two dark spiracles at the rear. A fungus gnat larva, by contrast, is translucent with a shiny black head. Shore fly larvae feed at the surface in algae, often with the body in the medium and the head in the algae. Purdue names Scatella stagnalis as the commonest greenhouse species; the laboratory figures below are for S. tenuicosta.
Easily confused with
- Fungus gnatsAlso a pestSlender, mosquito-like, weak fliers with long antennae and clear wings; shore flies are robust, fast fliers with very short antennae and pale-spotted dark wings.
- Fungus gnat larvaeAlso a pestWhitish and legless with a shiny black head capsule; shore fly larvae are an opaque tannish brown and have no head capsule.
Damage symptoms
Shore flies do not feed on healthy plant tissue. Purdue notes that large numbers reduce a crop's marketability, and the more serious concern is disease. In a 1999 study, about 95% of adult shore flies collected beside infected corn-salad plants carried Thielaviopsis basicola internally, and flies that fed on infected plants passed live propagules to healthy seedlings in their droppings. Adults also spread Fusarium avenaceum to lisianthus, and oospores of Pythium aphanidermatum stayed viable after passing through adult flies (other Pythium and Phytophthora spores tested did not).
- Clouds of small dark flies over wet benches, floors and compost surfaces
- Green algae on the compost, capillary matting, benches or floor
- Tan maggots with no dark head in the algae at the surface
- Root or stem rot spreading through a crop where the flies are numerous
- Flies on yellow sticky cards with pale spots on dark wings
Lifecycle & degree days
The whole cycle runs in algae, and it is fast. Eggs are laid singly on algae, the larvae graze it, and they pupate at its edge. Development speeds up sharply with warmth; the figures for S. tenuicosta below come from one laboratory study (Ugine et al. 2007), and Purdue's stage lengths are for the genus in greenhouses.
- Egg to adult
- 14.5 d at 68 °F (20 °C); 8.2 d at 79 °F (26 °C); 7 d at 83 °F (28.5 °C)
- Eggs per female
- 300–500 (Purdue); 379–710 in the lab
- Adult life
- About 3–4 weeks (Purdue); females 15–22 d in the lab
- Breeds in
- Algae on media, benches, floors
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Host crops
Shore flies are tied to algae and standing water, not to a plant. Any crop grown under glass on wet media, capillary matting or benches can host them. Purdue names pots, propagation areas, benches and the space under benches as the places to control algae. Each crop hub below carries the setting-specific program.
Monitoring & action thresholds
Action threshold
No published threshold
No extension source gives a numeric threshold for shore flies. Use weekly yellow-card counts to see whether numbers are rising, and act on the algae as soon as you see it rather than waiting for a count.
Use yellow sticky cards and check them weekly with a 10x hand lens, sorting shore flies from fungus gnats by the wing spots and antennae. UMass suggests 1–4 cards of 3 × 5 inches per 1,000 sq ft (about 93 m²); Purdue places them 1–2 inches above the canopy, UMass at the base of the plant at soil level. Walk the benches, floors and matting for green algae, which is where the larvae will be.
Beneficial
Biological control
Biological control is harder than for fungus gnats because shore flies breed in semi-aquatic spots the predators cannot reach. UMass says the nematodes and predatory mites used for fungus gnats do not appear to work as well against shore flies for this reason. The rove beetle Dalotia coriaria is sold for shore flies and hunts larvae in the media, but not in standing water. The soil mite Stratiolaelaps scimitus attacks the larvae but cannot survive standing water either. Steinernema feltiae is named for shore fly larvae only in supplier material; UMass says a related nematode, S. carpocapsae, infects the larvae but may not give enough control in commercial greenhouses. UConn also notes the wild hunter fly Coenosia attenuata, which catches adults in flight and whose larvae prey on shore flies in the media.
Stratiolaelaps scimitus
Hypoaspis miles, soil predatory mite- Rate
- 100–500 /m² per release (Koppert); Biobest 50–500, up to 1,000 at high pressure
- Temp
- Media above 15 °C (Biobest: active from 10 °C); compost moist, not wet
- Best for
- Larvae in the top of the media; not in standing water
Steinernema feltiae
Entomopathogenic nematode- Rate
- 250k–500k /m²
- Temp
- 10–30 °C
- Best for
- Supplier claim only; weakest evidence
Cultural controls
Algae is the lever. Purdue says removing it will greatly reduce shore fly populations, and both Purdue and UMass put algae and water control first.
- Remove algae from benches, floors, matting and the compost surface, especially in propagation and under benches
- Avoid overwatering, and fix low spots where water stands
- Limit fertiliser run-off, which feeds the algae
- Keep the house weed-free and clear standing water
Chemical, last resort
Algae control comes first. Purdue says the best long-term chemical control targets the larvae, and Gnatrol (Bti) does not control shore fliesCheck any product before it goes on. Open the compatibility matrix →
Rating: the effect on this pest's natural enemies if a spray hits them or they feed on treated plants. REI is the re-entry interval on farm and greenhouse labels; home labels usually give none, so follow the label.
Sources
Purdue Extension E-111, Fungus Gnats and Shore Flies (Sadof, 2021)
UMass Extension, Fungus Gnats and Shore Flies (T. Smith)
UConn Extension IPM, Biological Control of Fungus Gnats (Pundt, rev. 2024)
UC IPM Pest Notes 7448, Fungus Gnats (rev. 2013)
Stanghellini et al., Aerial transmission of Thielaviopsis basicola by shore flies, Phytopathology 1999
El-Hamalawi & Stanghellini, Fusarium avenaceum spread by shore flies, Plant Disease 2005
Hyder et al., Oomycete propagules after ingestion by fungus gnats and shore flies, Plant Disease 2009
Ugine et al., Development times and life tables for Scatella tenuicosta, Environ. Entomol. 2007
Page status
Drafted from the sources below, then checked claim by claim against those sources on 23 September 2026.
Suggest a correction →Shore flies by crop
The same pest behaves differently on different hosts. Each page below gives the host-specific control, rate and threshold.