Fungus gnats
Fungus gnats (family Mycetophilidae, often associated with Sciaridae) are small, fragile-looking flies that are a common nuisance in indoor plant cultivation, greenhouses, and nursery operations. While the adults are mostly seen hovering near the soil surface, the immature stages pose a significant threat to plant health.
What the section contains
Fungus gnats
These pests primarily affect young plants, seedlings, and crops grown in high-humidity environments. They are frequently found in greenhouses growing cucumbers, tomatoes, and ornamental flowers. Because they thrive in decaying organic matter, soils rich in peat or compost are highly susceptible to infestations.
The life cycle begins with eggs laid in moist soil. The larvae, which are the main destructive stage, have a distinct black head and a translucent body. They feed primarily on fungi and decaying organic material, but when the population density is high, they turn their attention to healthy root hairs and the succulent roots of seedlings.
Damage caused by fungus gnats is often subtle at first. Infested plants show signs of stunted growth, unexpected wilting, and yellowing foliage. Severe root pruning by the larvae leaves the plants vulnerable to root rot diseases like Pythium and Fusarium, which can kill young seedlings rapidly.
Control strategies for fungus gnats must be multifaceted to be effective. The most crucial cultural practice is managing irrigation to allow the top layer of soil to dry out between waterings. For integrated pest management, growers should deploy yellow sticky cards to monitor adults and apply biological agents like Steinernema feltiae (beneficial nematodes) or Bacillus thuringiensis israelensis directly to the growing medium.