Scatella spp.

Shore fly


Shore flies of the genus Scatella are a common greenhouse pest found throughout the United States, Canada and northern Mexico. In Europe and Asia, the species Scatella stagnalis occurs widely. These small black flies can be found naturally at marshy, muddy lake shores and intertidal zones, and they breed readily in other moist environments. In greenhouses, adults feed on algae and their larvae on algae and decaying organic matter on planting substrate. Growers consider shore flies to be mostly a nuisance pest, but the insects can act as vectors for disease organisms, such as Fusarium, among both food crops and ornamentals.

Life cycle and appearance of the Shore fly

Adult Scatella shore flies are 4–5mm long with large eyes, thick bodies and short antennae, and their wings can have greyish brown patches. While shore flies are strong, quick fliers and can move throughout a greenhouse readily, they generally remain close to breeding sites. They breed in algae growing on soil, pots, benches, and other surfaces and the female disperses her tiny (0.1 mm) eggs on the surface of the plant growing medium.

Short, leathery, translucent-white larvae, around 0.5 mm long and with prominent rear breathing tubes, emerge from the eggs 2–3 days later. These maggots live in and just beneath the algal crust growing on top of the soil/medium, where they feed on algae, bacteria, yeasts and other microorganisms. In 3–6 days, they pass through three larval stages and then pupate inside the final larval skin (forming a protective puparium). The puparia can be found on or just below the soil surface, and new adults emerge after 4–5 days. Adult shore flies typically live between 2–3 weeks after eclosion.

Damage symptoms

Since all life stages of shore fly feed primarily on microscopic algae, bacteria and other unicellular organisms, they do not cause any direct damage to plant production. However, they are considered a nuisance pest because the flies:

  • Leave their feces (as black flecks) on the foliage of ornamentals or food crops, causing cosmetic damage.
  • Can develop quickly into dense swarms inside greenhouses, becoming an annoying deterrent to employees and browsing customers.
  • Are capable of transmitting pathogens such as Fusarium, Pythium, and other root diseases.

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