Fungus Gnat Control: Monitoring Adults & Treating Larvae

Flying gnats near your pots are a signal to scout, not a complete diagnosis. An effective fungus-gnat program combines adult monitoring, growing-medium inspection, moisture management and treatments directed at the correct life stage.

Start here: use yellow sticky cards and monitoring supplies to track adults. If fungus gnat larvae are confirmed, compare product-specific directions in Bti Larvicides & Gnat Control Products.

Identify the pest before treating

Adult fungus gnats are small, mosquito-like flies with long legs and antennae. They are weak fliers and often rest on the growing medium or lower foliage. The larvae are translucent to white, legless and have a distinct black head capsule.

Four magnified views of a brown adult gnat showing long antennae, legs and clear veined wings
Magnified adult views show the long legs, antennae and clear wings to examine during identification. These examples are not a complete representation of fungus-gnat species or a stand-alone species-identification key.

Larvae feed on fungi and decaying organic matter, but can also damage roots, crowns and stems. Young plants and propagation crops are especially vulnerable. Poor growth or wilting alone does not confirm fungus gnats; inspect the root zone and consider other causes.

Monitor adults with yellow sticky cards

Place yellow sticky cards near the growing-medium surface or the edge of flats. Check them weekly, identify what is caught and record counts by location. Use consistent positions and checking intervals so changes in counts are meaningful.

Compare Catchmaster yellow sticky cards and Sensor yellow monitoring cards. Cards help detect adults and track activity; a catch does not show how many larvae remain in the medium. Moth traps and pheromone lures are not substitutes for fungus-gnat scouting.

Check the growing medium for larvae

Close-up of a translucent segmented larva with a dark head on moist dark growing medium
Look for the translucent body and distinct dark head capsule when inspecting suspected fungus-gnat larvae. Use magnification and the growing-site context; this example is not a complete identification key.

Inspect the medium and roots, particularly in propagation areas and persistently wet spots. Potato disks or wedges placed against the medium can help attract larvae for inspection. Cornell recommends checking after 48 hours, counting larvae and replacing old disks with fresh ones. Treat these as scouting aids, not a control treatment.

Reduce breeding sites

Avoid overwatering, correct drainage problems and prevent water from accumulating beneath benches. Remove old growing medium and plant debris from the growing area. Inspect incoming plugs for larvae and adults where practical.

Adjust irrigation to the needs of the crop and rooting stage rather than imposing a universal dry-down schedule. Sanitation and moisture management support treatment; neither replaces checking whether larvae are still present.

Target larvae with the right formulation

Gnatrol WDG

Gnatrol WDG Biological Larvicide contains Bacillus thuringiensis subsp. israelensis, commonly called Bti. Nufarm identifies it for fungus gnat larval control and as OMRI Listed for organic production. It does not control adult gnats.

Review the current label for your crop, use site, rate, treatment interval and application method. Nufarm describes growing-medium drench applications and repeat treatments for established infestations. Do not substitute a generic Bti rate or assume every Bti formulation has the same crop and application directions.

Nufarm cautions against injecting Gnatrol WDG in combinations with fertilizers or fungicides containing copper or chlorine because these may neutralize the active ingredient. Do not drench stressed plants or follow treatment with excessive water. Check the full label and compatibility directions before integrating it into an irrigation program.

Microbe-Life FGX: Microbe-Lift BMC Rebranded for Fungus Gnats

Know it as Microbe-Lift BMC? The same formulation has been rebranded as Microbe-Life FGX, Fungus Gnat Exterminator, with labeling specifically for fungus gnats. FGX uses Bti to target larvae in the root zone: larvae ingest the treatment, stopping the cycle before the next generation of adults emerges.

Browse our Bti larvicide selection for listings under the familiar Microbe-Lift BMC name. Follow the application rate, method and use directions on the package you purchase. Gnatrol WDG is a separate formulation with its own directions. Verify organic-production suitability for the exact product rather than assuming all Bti products share an organic listing.

Measure progress and follow up

Continue adult counts and larval checks after treatment. Overlapping generations mean one observation or application is not a reliable measure of control. Compare trends alongside irrigation changes, sanitation work and treatment records; use the product label to plan any repeat applications.

For certified organic production, confirm the exact formulation, intended use and applicable restrictions with your certifier. A shared ingredient or collection placement does not establish organic suitability.

Choose supplies for your next step

For adult scouting, browse Pest Monitoring & Identification. For confirmed larvae, compare Gnatrol WDG and the Microbe-Lift BMC listings in Bti Larvicides & Gnat Control Products. For other diagnosed problems, browse Pest & Disease Controls.

Technical references

Cornell Greenhouse Horticulture: insect management guidance
Nufarm: Gnatrol WDG product information and label resources
Microbe-Life Hydroponics: FGX Fungus Gnat Exterminator