Shore flies

In short

Shore flies are small, sturdy black flies that breed in algae growing on wet potting mix, pots, benches and floors. Neither the adults nor the larvae feed on plants; the problem is black fly specks on leaves, nuisance swarms and a possible role in spreading root diseases.

How it looks

Adults are about 3–5 mm long (about 1/8 inch), robust and housefly-like, with a black body, reddish eyes, short bristle-like antennae and dark wings marked with about five pale spots each. Larvae are small yellowish-brown to translucent maggots up to about 6 mm, with no dark head capsule and a forked breathing tube at the rear; they live in the green algae film on the mix surface. Pupae are dark brown and spindle-shaped, on or just under the surface.

Signs

Dark flies resting on the mix surface, pots and benches, often where green algae is visible. Small black droppings ("fly specks") on leaves are the main visible damage. There is no chewing or root damage from the shore flies themselves.

Life cycle

Females lay a few hundred eggs on algae-covered, wet surfaces. Development is fast — about 8 days at 26 °C (78 °F) or about two weeks at 20 °C (68 °F) — and adults live two to three weeks, so numbers can build quickly in warm conditions.

What favours it

Constantly wet media, puddles and wet benches where algae grows; heavy watering of slow-growing plants and propagation areas. Common in greenhouses and wherever pots stay saturated.

Prevention

• Water less often and let the mix surface dry so algae cannot establish.
• Avoid puddles and standing water on trays, benches and floors.
• Use a free-draining mix and pots sized to the plant.
• Clean algae off pots, trays and benches.

Treatment

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

1. Reduce moisture and remove algae; without algae the larvae have nothing to eat.
2. Use yellow sticky cards to catch adults and monitor numbers.
3. In greenhouses, rove beetles (Dalotia coriaria) feed on shore fly eggs and larvae; naturally occurring hunter flies and parasitoid wasps also help where sprays are avoided.
4. If still needed, insect growth regulators aimed at the larvae — follow the product label.

Pros and cons

Look-alikes

Fungus gnats

Delicate, mosquito-like, with long legs and long antennae and no wing spots; larvae have a shiny black head and feed on roots.

Fruit flies

Tan with red eyes; gather at fermenting fruit, bins and drains rather than algae.

Affected plants

Any plant kept in wet, algae-covered media; especially propagation trays and slow-growing plants.

Related

• Fungus gnats
• Overwatering
• Drainage in containers
• Root rot
• Rove beetles

Sources

1. Managing Shore Flies in the Greenhouse — UConn IPM
2. Fungus Gnats and Shore Flies (E-111) — Purdue Extension Entomology
3. Shore fly — Koppert

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