Fungus Gnats in Special Situations

Targeted advice for living soil, hydroponics, greenhouses, and indoor gardens

Home›Special Situations

Living Soil & Organic Growing

Living soil presents a unique challenge because the very conditions that support a healthy soil food web (consistent moisture, plenty of organic matter, active fungi) are exactly what fungus gnats thrive in. You cannot simply dry out the soil or sterilize it without destroying the biology you have worked so hard to build.

The Living Soil Strategy

  • Primary tool: Concentrated BTI soil drench. The EPA says BTI only affects the larvae of mosquitoes, black flies and fungus gnats, and Bt does not seem to hurt earthworms. That makes it the best fit I've found for living soil: it kills gnats without blasting everything else.
  • Secondary tools: Predatory mites (Stratiolaelaps scimitus) live in the top inch of the soil, and when prey is scarce they get by on plant debris and algae. One release can last a growing season. (UC IPM doubts they work in single houseplants, so this is for beds and bigger containers.)
  • Avoid: Hydrogen peroxide (a strong oxidizer that kills a lot of organisms), diatomaceous earth (no effect on fungus gnats in Kansas State's studies), and broad-spectrum insecticides.

Hydroponic & Soilless Systems

Fungus gnats do well in any moist medium with fungi or algae to feed on, and that includes soilless setups. Research has shown that they survive and reproduce just as well in coir as in peat. They can also live on algae and on damp, mossy surfaces.

Hydroponic-Specific Controls

  • Substrate choice: No medium is immune. Coir is not the gnat deterrent it was once sold as.
  • Control algae: Gnats can live on algae under benches and on damp surfaces, and growers with standing water and algae tend to have more gnats. Cover rockwool cubes and keep reservoirs light-proof so algae can't get started.
  • BTI on the medium: The labels I read describe BTI as a drench on the growing medium where larvae are. Neither the BMP 144 label (behind Microbe-Life FGX) nor the Gnatrol label gives a hydroponic reservoir rate, and the Gnatrol label warns that fertilizers containing copper or chlorine can neutralize it. So drench the cubes or coco directly, which is what the labels describe, rather than relying on the reservoir.
  • Top-dressings: Crushed-glass or sand layers are sold as barriers, but Kansas State found sand and diatomaceous earth layers did not stop egg-laying or emergence, so I wouldn't count on them.
  • Nematodes: S. feltiae can go through drip irrigation if the filters are removed, according to UMass.

Greenhouse Operations

Fungus gnats are a major pest in greenhouses, especially in propagation, where cuttings and plugs are kept wet while they root. Warmth and constant moisture let populations run all year.

Greenhouse IPM Program

  • Sanitation first: Eliminate weeds, algae, standing water, and decomposing debris from benches, floors, and underneath. Concrete flooring has fewer problems than soil floors.
  • Screen entry points: Fine mesh on vents, doors, and windows.
  • Growing media management: Avoid compost less than a year old. Pasteurize potting soil before use. Store bagged media dry and keep the bags intact; Kansas State notes gnats may come in with bagged soilless media.
  • Biological program: Release predatory mites preventively at planting, or rove beetles with breeder buckets under benches, but not both, since the beetles eat the mites. Apply S. feltiae nematodes as a drench two to three days after sticking cuttings or planting plugs.
  • BTI: Gnatrol WDG or Microbe-Life FGX drenches when larvae are present, with extra attention under benches, which the Gnatrol label calls one of the main breeding areas.
  • Chemical options (commercial): Kansas State lists the insect growth regulators cyromazine (Citation) and pyriproxyfen (Distance) as effective against larvae, and neonicotinoid drenches (imidacloprid, thiamethoxam, dinotefuran) as effective against most larval instars. Pyrethroids control adults but are generally harmful to natural enemies and can upset a biological program.

Indoor Houseplants

For most homeowners, gnats arrive on new plants, in bagged potting soil, or fly in through windows, especially in fall when outdoor plants are brought inside. Nuisance problems are most noticeable in late fall and winter.

Houseplant Protocol

  1. Keep new plants away from your collection for a few weeks and check them with a potato piece before moving them in.
  2. Use pasteurized potting mix, or solarize your own: moisten it, seal it in a plastic bag no more than about 8 inches deep, and leave it in the sun for 4–6 weeks.
  3. Water with BTI as a preventive every 2nd–3rd watering, at the label's light rate. Microbe-Life FGX is easy to dose with a teaspoon. Budget option: Mosquito Bits tea, per the Bits label: soak 4 tablespoons in a gallon of water for 30 minutes, skim off the granules, and water with the liquid.
  4. Place horizontal sticky traps at soil level in every pot.
  5. Improve drainage: increase perlite or sand in the mix, ensure drainage holes, and never let pots sit in standing water.
  6. Bottom-water when possible to keep the top layer drier. A dry surface is less attractive to egg-laying females, and eggs laid there don't hatch.
  7. Cover containers with clear plastic if starting seeds or cuttings to exclude adults.

Combustible & Consumable Crops

Extra caution is needed for any plant that will be eaten, smoked, or otherwise combusted. This is the main reason I went with BTI: the EPA says it has no toxicity to people, it is approved for organic farming, and Bt is exempt from food residue limits because of its low toxicity.

Edible and Combustible Crops

  • BTI on food crops: the Microbe-Life FGX and Gnatrol directions both list vegetables for fungus gnat drenches, and FGX lists herbs. Use a product whose label covers your crop.
  • Smoked crops: I couldn't find any study or label that looks at burning BTI residue. Its toxicity to people is very low, but I can't point you to data on smoking it, and rules for crops like cannabis vary by state.
  • Only use what the label allows on your crop. I don't use systemic neonicotinoids or pyrethroid sprays on anything I eat or smoke.
  • Beneficial nematodes and predatory mites are living organisms, not chemical residues.
← Previous: Biological Controls

Sources

  1. Bti for Mosquito Control, US EPA
  2. Bacillus thuringiensis (Bt) General Fact Sheet, National Pesticide Information Center
  3. Fungus Gnat Management in Greenhouses and Nurseries (MF2937), Kansas State University Research and Extension
  4. Stratiolaelaps scimitus: Biological Control Agent of Fungus Gnats and Western Flower Thrips (MF3632), Kansas State University Research and Extension
  5. Biological Control of Fungus Gnats, UConn Extension
  6. Managing Fungus Gnats in the Greenhouse, UConn Extension
  7. Managing Fungus Gnats, Thrips, Aphids and Mites, UMass Extension
  8. Pest Notes: Fungus Gnats, UC Statewide IPM Program
  9. Fungus Gnats, Colorado State University Agricultural Biology
  10. Darkwinged Fungus Gnats, Bradysia spp. (EENY-215), University of Florida IFAS Extension
  11. BMP 144 (2X) label (EPA Reg. No. 62637-1), Becker Microbial Products via US EPA
  12. VectoBac WDG / Gnatrol WDG label (EPA Reg. No. 73049-56), Valent BioSciences via US EPA
  13. Mosquito Bits label (EPA Reg. No. 6218-86), Summit Chemical via US EPA
  14. Hydrogen peroxide for fungus gnats, Ask Extension
  15. Microbe-Life FGX fungus gnat directions, Ecological Laboratories (Microbe Life Hydroponics)