How to identify fungus gnat pressure
Adult fungus gnats are small, 2 to 3mm, with long legs and antennae. They fly slowly and are often found near the substrate surface or on lower canopy leaves. Adults do not feed on plants; their presence indicates that larvae are active in the substrate below.
Larvae are white, legless, and translucent, with a black head capsule. They are typically found in the top 2 to 5 cm of growing media. Root damage appears as brown, stunted, or decayed root tips; in severe infestations, damping-off symptoms in seedlings and cuttings are common.
Yellow sticky traps placed horizontally near the substrate surface are the standard monitoring tool. More than a few adults per trap per week indicates a population that warrants intervention.
Why fungus gnats are a vector problem, not just a pest problem
Adult fungus gnats are a nuisance. Larvae are a root zone threat. The vector risk is what makes fungus gnat pressure a facility-wide contamination concern.
Fungus gnat larvae carry Pythium, Fusarium, and other soilborne pathogens on their bodies and in their gut. As they move through the substrate, they deposit pathogen propagules at healthy root tissue. A facility with baseline Pythium pressure in its water or substrate and an active fungus gnat population is running a mechanical inoculation system across every container the larvae inhabit.
This is why treating the visible adult population without addressing the substrate conditions that sustain larvae produces temporary improvement followed by recurrence. The adult population is the indicator. The root zone is the problem.
Conditions that drive fungus gnat pressure
Two conditions account for most fungus gnat pressure in cannabis facilities:
Moisture. Fungus gnat larvae require moist media to complete development. Overwatering — watering before the substrate surface has partially dried — maintains the conditions larvae need throughout the growing cycle. Allowing the top layer of substrate to dry between waterings significantly reduces larval survival. This applies across media types but is most impactful in peat- and coco-based substrates with high water retention.
Organic matter. Larvae feed on decomposing organic matter as readily as on roots. Substrates with high organic content, algae growth on container surfaces, and plant debris on the substrate surface all provide food sources that sustain populations independent of root feeding.