Fungus gnats in cannabis cultivation

The so-called fungus gnats in marijuana cultivation almost always correspond to mosquitoes of the Sciaridae family, known in English as fungus gnats (genus Bradysia and related). They are a typical pest in potted crops and moist substrate systems, including cannabis, especially in indoor spaces and grow rooms with high humidity.

Identification of the fungus gnat

Adults

Adults are small dark-colored mosquitoes (3–4 mm), with slender bodies, long legs and antennae, and transparent wings. They behave as weak fliers: they tend to walk over the substrate and make short flights around pots and light sources.

Larvae

The main problem for plants is not the adults but the larvae. These look like thin, translucent or whitish worms, up to 5–8 mm long, with a very visible black head capsule. They live in the top few centimeters of the substrate, where they feed on fungi, decomposing organic matter, and fine roots.

Life cycle and conditions that favor fungus gnats

The biological cycle of fungus gnats includes four phases: egg, four larval stages, pupa, and adult. In greenhouse or warm grow room conditions, a complete generation can be completed in approximately 2–4 weeks, depending on substrate temperature.

• Egg laying: females deposit their eggs in the top few centimeters of moist substrate rich in organic matter.
• Larvae: they emerge after a few days and feed on fungi and tender roots. This is the most damaging stage for cannabis due to the direct impact on the root system.
• Pupa: it occurs in the substrate itself; after a few days adults emerge.
• Adults: they live 7 to 10 days, during which they can lay around 100–300 eggs per female, favoring population explosions if humidity is not controlled.

Typical conditions in a marijuana grow that favor the pest include:

• Frequent watering without substrate drying periods
• Substrate mixes with high content of fresh or poorly composted organic matter
• Accumulation of roots, leaf debris, and residues in trays and grow tables
• Lack of ventilation around the pots

Numerous studies in commercial greenhouses have shown that sciarids develop best in moist substrates with high microbial activity and abundant organic matter.

Impact on cannabis cultivation

In cannabis crops, the presence of fungus gnats is often interpreted as a symptom of excess moisture in the root system.

Direct damage

• Larvae consume fine roots and root hairs, reducing the plant’s ability to absorb water and nutrients
• Seedlings and rooting clones are especially sensitive; they may collapse or wilt suddenly even with moderate populations
• In young plants, root loss results in growth delay and poor shoot development

Indirect damage in cannabis cultivation

• Root wounds favor the entry of collar and root pathogens (e.g., Pythium spp.), increasing the incidence of root rot
• Root stress makes the crop more vulnerable to other factors (salinity, nutritional imbalances, water stress)

Specific reports in cannabis cultivation confirm that, although they are rarely the most devastating pest, fungus gnats can compromise crop uniformity and yield if not managed preventively.

Monitoring and diagnosis in the grow room

Yellow sticky traps

The best tool for monitoring adults, and one of the essential items in any grow, is the use of yellow sticky traps placed at substrate level or slightly above it. They allow you to:

• Detect the pest early
• Estimate population pressure (counting adults per trap per week)
• Differentiate sciarids from other small flies

This technique is widely recommended in integrated pest management programs for greenhouses.

Integrated management of fungus gnats in cannabis

Effective control requires combining irrigation and humidity management with biological control measures. The use of broad-spectrum chemical insecticides is considered a last resort in technical horticulture and is undesirable in crops intended for consumption.

Irrigation and humidity control measures

• Adjust irrigation: avoid keeping the substrate constantly saturated. Allowing the surface layer to dry between waterings drastically reduces egg and larval viability.
• Improve drainage: use containers with proper drainage and aerated substrates, avoiding waterlogging in trays and tables.
• Grow area hygiene: remove leaf debris, roots, algae, and spilled substrate, which act as breeding sites.
• Ambient humidity control: indoors, adjust ventilation and dehumidification to avoid permanently moist areas around pots.
• Substrate management: use quality mixes, preferably pasteurized, and avoid prolonged storage of opened bags in warm and humid conditions.

How to water marijuana plants in soil

Knowing how to water your marijuana plants may seem a simple matter, but we have noticed that many people still have questions regarding how to do it properly. For this reason we would like to tell you about the different watering methods in different substrates. In this post, we are talking about watering in soil.

Biological control

The literature agrees that biological control is the central tool for managing sciarids in intensive systems. Among the most studied agents are:

Entomopathogenic nematodes (Steinernema feltiae)

• Applied through irrigation to the substrate
• They penetrate larvae and release symbiotic bacteria that cause death
• They are effective in moderate temperature ranges and in moist but not saturated substrate
• Comparative trials show 70–90% efficacy in reducing larvae and emerging adults in various crops

Bacterium Bacillus thuringiensis var. israelensis (Bti)

• Produces toxins specific to the larvae of certain dipterans (mosquitoes, black flies, and sciarids)
• Applied to the substrate following label instructions
• Various studies show significant reductions in larvae and adults when applied repeatedly at 5–7 day intervals

Physical traps and barriers

• Yellow sticky traps: help partially reduce the adult population and, above all, monitor their dynamics.
• Substrate coverings: surface layers of coarse sand, perlite, or inert materials can hinder egg laying and adult emergence, as long as root aeration is not compromised.

References

1. University of California IPM. “Fungus Gnats.”
2. University of Connecticut, Greenhouse IPM. “Biological Control of Fungus Gnats.”
3. Kansas State University. “Fungus Gnat: Management in Greenhouses and Nurseries (MF2937).”
4. Van Tol, R.W.H.M. et al. “Microbial Control Agents for Fungus Gnats (Diptera: Sciaridae).” Insects, 2021.
5. GrowerTalks. “A Constant Scourge: Fungus Gnats.”
6. AHDB Horticulture. “Cultural and Biological Control of Sciarid and Shore Flies in Protected Ornamentals.”
7. Southern European and Latin American horticultural blogs and guides on fungus gnat management in pot substrates (Verdecora, Jardinería On, Hortalan, Plantasmanía, Promesse de Fleurs, VerdeEdén, etc.).
8. Technical cannabis-specific articles and guides on fungus gnat management in marijuana cultivation.