Fungus Gnat FAQ

No. Fungus gnats don't bite people. They are a nuisance, annoying to have flying around your face and your screen, but that's all. The real damage is done by the larvae feeding on plant roots in the soil.

It's about as low-risk as pest control gets, but it is still a pesticide. The EPA says BTI (Bacillus thuringiensis subspecies israelensis) has no toxicity to people and only affects the larvae of mosquitoes, black flies and fungus gnats; it is approved for organic farming. Its toxin needs the high-pH gut of a fly larva to switch on, and acidic mammal stomachs destroy it. Bt is exempt from food residue limits because of its low toxicity, and the Microbe-Life FGX and Gnatrol directions list vegetables (FGX also lists herbs) for fungus gnat drenches. The concentrate still carries a CAUTION signal word: the BMP 144 label (the registration behind Microbe-Lift BMC and FGX) says it is harmful if inhaled or absorbed through the skin, and the Gnatrol label says harmful if inhaled and causes moderate eye irritation. So avoid breathing the mist, keep it off your skin, and store it where kids and pets can't get it.

With concentrated BTI as a weekly soil drench, the Gnatrol label says larvae generally respond within 24 hours. For me, adults on the traps dropped off over the following week or two, and the infestation was gone in about 3–4 weeks. Expect to keep seeing some adults for a week or two after the larvae are dead: pupae already in the soil aren't affected by BTI and still hatch, and adults live 7–10 days. The Gnatrol label calls for three weekly applications at the heavy rate for an existing infestation, so treat that as the minimum.

Yes, as a drench on the growing medium (rockwool, coco and so on), which is where the larvae are and what the labels describe. Neither the BMP 144 label (behind Microbe-Lift BMC and FGX) nor the Gnatrol label gives a rate for dosing a hydroponic reservoir. Both say the product isn't known to harm plants but hasn't been tested on every species, so try it on a few plants first. The Gnatrol label also warns that fertilizers or fungicides containing copper or chlorine can neutralize BTI.

Vinegar traps are made for fruit flies, which are drawn to vinegar and other fermented liquids. If your vinegar trap is full of flies, check whether they're fruit flies rather than (or as well as) fungus gnats. Either way, a vinegar trap does nothing about larvae in the soil. Yellow sticky traps placed horizontally at the soil surface are the standard tool for catching and monitoring adult fungus gnats.

Watering management alone is not a complete solution. Fungus gnats normally live outdoors, in compost and damp soil, and come in through doors and windows. A single pregnant female is enough to start an infestation regardless of how carefully you water. Gnats can also come in with bagged potting soil or on plants brought indoors. Effective prevention requires a multi-pronged approach: screening windows and doors, quarantining new plants, and routine BTI drenches as a preventive measure.

Probably some of them. Hydrogen peroxide is a strong oxidizer that kills a lot of organisms, not just gnat larvae, and I couldn't find a university trial showing the usual 1:4 drench (1 part 3% peroxide to 4 parts water) controls fungus gnats. Extension advice is to try gentler controls first and keep it off seedlings. For living soil I use BTI instead: the EPA says it only affects the larvae of mosquitoes, black flies and fungus gnats.

Sticky-leaved carnivorous plants such as sundews (Drosera) catch small flies, gnats included. But like sticky traps, they only catch adults and do nothing about the larvae in the soil, which is where the real problem lies. Fun to have, not a control method.

Yes. Fungus gnats normally live outdoors, so they come back every year, and how active they are outdoors depends on the climate: in inland California they are most common in winter and spring, and in mild, moist places they can be around all year. Indoors, in heated homes and greenhouses, they can breed year-round. Indoor problems are often most noticeable in late fall and winter, partly because plants brought in from outside carry gnats in their soil.

Not really for single pots. UC IPM says predatory mites (Stratiolaelaps scimitus) are probably not effective in individual houseplants; they are used in greenhouses. For individual houseplants, BTI drenches are more practical. Beneficial nematodes (Steinernema feltiae) can work in pots, but they need moist soil and, per UC IPM, temperatures between 60 and 90°F.

Use the potato method: peel and cut a raw potato into roughly 1-inch chunks, then place them on the soil surface cut side down. After 48 hours, lift each chunk and examine the underside and the soil beneath it for small translucent worms with black heads. Kansas State reports that potato disks recovered 38% of the larvae in the media (carrot disks got 23%), and that 48 hours works better than 24. Throw out infested chunks and replace them every few days. You can also inspect root systems for blunt, damaged tips and check the soil surface for shiny slime trails left by larvae.

All three contain BTI (Bacillus thuringiensis subspecies israelensis), in different forms. Dunks are solid rings made for standing water and built to release BTI slowly, over 30 days or more. Bits are BTI on corn cob granules; they are labeled for fungus gnats, either sprinkled on the soil or soaked to make a tea (4 tablespoons per gallon for 30 minutes), repeated weekly for three weeks. Concentrates (Gnatrol, a water-dispersible granule sold in 4 oz jars as Magic Gardener Fungus Gnat Killer, and Microbe-Life FGX, a liquid) let you mix the labels' heavy rate for an active infestation straight into a watering can. In my experience, Dunks and Bits held an established infestation steady and a concentrate is what ended it.

Yes. UMass notes that beneficial nematodes are compatible with most pest control materials except carbamate and organophosphate insecticides, and BTI is neither. BTI kills larvae that eat it, while nematodes seek out and infect larvae through body openings, so the two work in different ways. I'd keep hydrogen peroxide away from soil with nematodes in it.