Fungus Gnat (Sciarid Fly)

Stratiolaelaps and nematodes.

Sciarid larvae damage young roots. Beneficial mites reduce breeding sites...

About Fungus Gnats (Sciarid Fly)

Fungus gnats, often called sciarid flies, are persistent pests in propagation, nurseries and protected crops where moist substrates favour their development. Adults are small, dark flies that are mostly a nuisance, but the larvae feed on delicate roots and stems at the compost surface, causing check in growth, wilting and plant loss in severe cases. They also spread root diseases such as Pythium and Fusarium as they move through the growing media.

Symptoms of Fungus Gnat Damage

• Small black flies running over compost or flying when disturbed
• Seedlings collapsing (“damping off”) in propagation
• Soft, chewed root tips and slow establishment
• Wilted or stunted young plants
• Larvae visible in moist compost around the stem base

Why Biological Control Works

Sciarid larvae develop in the top 1–2 cm of growing media, making them ideal targets for soil-dwelling predators and entomopathogenic nematodes. Two key biological agents dominate UK horticulture: Stratiolaelaps scimitus (formerly Hypoaspis miles), which hunts larvae and pupae, and Steinernema feltiae, a nematode that rapidly infects and kills young larvae. When combined with good moisture and hygiene management, they provide strong and reliable long-term suppression.

Common UK Sciarid Species

• Bradysia paupera
• Bradysia impatiens
• Bradysia coprophila

IPM Recommendations

• Introduce Stratiolaelaps into propagation and young plant areas early
• Apply Steinernema feltiae drenches where larvae pressure is high
• Allow compost surface to dry slightly between irrigations
• Use yellow sticky cards to monitor adult emergence
• Remove algae, debris and standing water from benches and paths
• Quarantine incoming plant material to prevent re-introduction

Preventative vs Curative Strategy

Preventative: Introduce Stratiolaelaps scimitus at the start of propagation cycles or when fresh substrates are filled. Predators establish in the compost and provide long-term suppression of early larvae.

Curative: Apply Steinernema feltiae as a drench when larvae are present. Repeat at 7–10 day intervals depending on pressure and substrate temperature.

Environmental Notes

• Stratiolaelaps thrives in moist compost surfaces but tolerates drier conditions
• Steinernema feltiae requires moist substrate for movement and infection
• Cooler conditions slow sciarid development but also slow some predators
• Algae growth creates ideal breeding sites — improve hygiene to reduce hotspots