Mushroom Sciarid Fly

How to identify it

• The adult is a small black fly about 0.25 inch long with long antennae and gray wings folded over its back. It prefers to run rather than fly, but flies when disturbed.^[3][1]
• The larva is a worm-like maggot that reaches about 6 mm (0.25 inch) and feeds on mycelium in moist, dark parts of the compost and casing, so it is seldom seen on the casing surface.^[1]
• Damage shows as destroyed pins, tunnels in stems and caps, and frass; sections or whole mushrooms have to be trimmed and discarded.^[1]

Look-alikes

Where it lives

The sources describe it in commercial mushroom production: Penn State calls it the most devastating insect in the Pennsylvania mushroom industry, and the Pacific Northwest Insect Management Handbook lists it as the major insect pest of mushrooms.^[1][3]

US map: state where the cited source reports it as the mushroom industry's most damaging insect (the pacific northwest handbook also lists it, without naming states). View the map

Biology and life cycle

Eggs are laid singly or in groups of up to 15 and hatch in about 6 days at 65 F. The larvae move away from the hatching site to feed, usually on mycelium, for about 18 days at 65 F, growing to about 6 mm. They prefer moist areas and darkness.^[1]

The pupal stage lasts about 6 days at 65 F; the pupa works its way up toward the casing surface and the adult emerges with folded wings that expand within 30 to 60 minutes. Mating follows almost immediately. Males live about 7 days and females about 10 days at 65 F, feeding only on water and other liquids, not on mushrooms.^[1]

A generation averages 35 to 38 days at 65 F, shorter as the temperature rises, which changes how many generations fit in a crop. Penn State's monitoring guide figures one fungus gnat can lay 170 eggs every 18 to 22 days, so populations can get out of control quickly.^[1][2]

Adults prefer darkened areas and fly during limited hours, so growers generally do not see them; UV light traps with sticky paper in the growing rooms, counted at least weekly, are how farms track them.^[1][2]

Damage and signs

• Larvae feed on mycelium, destroy pins and burrow or tunnel into the stems and caps of maturing mushrooms, and they may introduce decay organisms along with their frass. The handbook lists compost, spawn, mycelia, pins, stems and caps as larval targets.^[1][3]
• Adults carry pyemotid (pigmy) mites, nematodes, mold spores and possibly other pathogens into the crop. Penn State estimates Pennsylvania growers lose 10 to 40 percent of the crop during peak fly populations (sciarids and phorids together).^[1][3][2]

Health risk

The sources describe it only as a pest of mushroom crops and a carrier of mites, nematodes and mold spores between growing rooms; they mention no bite, sting or direct effect on people.^[1][3]

Control

Prevention

Monitor with UV light traps and sticky paper in each growing room from the day a house is filled, counting at least weekly, and act on rising counts before the next generation emerges 21 to 26 days later.^[2]

Keep them out

Growing rooms should be sealed and properly screened, with attention to doors, fans, ceiling joints, drains and ductwork, and doors kept open as little as possible. Lights and traps near doorways may help.^[3]

Sanitation

Cultural control is the basis of mushroom culture: sanitation (washing and sanitizing), proper composting and pasteurization, shorter crop cycles and post-harvest steam cleaning all reduce fly populations. Equipment and workers should not move from older, infested rooms to newer, clean ones.^[3]

Biological control

Entomopathogenic nematodes work well in the moist growing media; Steinernema feltiae is effective against sciarids. Bacillus thuringiensis israelensis (Bti) is more effective against early-instar sciarid larvae.^[3]

Pesticides

The handbook's registered options for sciarids include azadirachtin, Bti and Beauveria bassiana incorporated into compost or casing soil, Isaria fumosorosea at spawning, casing and case hold, cyromazine in compost (spent treated compost cannot be used to grow food crops), S-methoprene against larvae and eggs, pyrethrin aerosols to knock down adults before pickers enter, and permethrin for adults only when no mushrooms are present. Pesticides must be mixed thoroughly into the compost to work. Some labels cover Agaricus mushrooms only; follow the label.^[3]

Related profiles

• Mushroom Phorid Fly
• Fungus Gnat
• Phorid Fly
• Getting rid of fungus gnats in houseplants

Common questions

How do I tell sciarids from phorids on a sticky trap?

Sciarids are about 0.25 inch long with long antennae and gray wings folded over the back; phorids are half that size, humpbacked, with tiny antennae, and move in erratic runs and hops.^[3]

Why don't I see the adults in the growing room?

Adults prefer dark areas and fly only during limited hours, and they would rather run than fly, so growers generally miss them without a light-and-sticky-paper monitor.^[1][2]

How fast can an infestation build?

At 65 F a generation takes 35 to 38 days, faster when warmer, and Penn State figures one fungus gnat can lay 170 eggs every 18 to 22 days.^[1][2]

Sources

1. Penn State Extension, “Sciarid Fly in Commercial Mushroom Production (D. M. Beyer; updated December 23, 2025).” extension.psu.edu/sciarid-fly-in-commercial-mushroom-production/ (accessed 2026-11-10)
2. Penn State Extension, “All About Monitoring Mushroom Flies (updated March 18, 2026).” extension.psu.edu/all-about-monitoring-mushroom-flies/ (accessed 2026-11-10)
3. Pacific Northwest Pest Management Handbooks (Oregon State University, Washington State University and University of Idaho), “Mushroom-Mushroom fly, Pacific Northwest Insect Management Handbook.” pnwhandbooks.org/insect/vegetable/mushroom/mushroom-mushroom-fly (accessed 2026-11-10)
4. iNaturalist, “Lycoriella mali taxon page (order and family).” www.inaturalist.org/taxa/85252 (accessed 2026-11-10)