Mosquito Bits end a fungus gnat infestation and yellow sticky traps do not, because the two products work on different halves of the insect. Mosquito Bits carry Bacillus thuringiensis israelensis, a bacterium that kills fungus gnat larvae feeding in the wet top layer of the pot, where the next generation is being made. Sticky traps catch adults, and trapping them does not reach the larvae already in the pot. So the traps tell you whether you are winning, and the BTI is the thing that wins. Neither works while the pot stays wet.
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TL;DR
- The pot is the problem. Fungus gnat larvae need a moist organic top layer; letting it dry between waterings is the precondition for anything you buy.
- BTI kills larvae, never adults. Flies the day after a dose are expected; the visible drop lags about 4 to 5 weeks.
- Yellow sticky traps catch egg-laying adults. They keep score and never end an infestation on their own.
- Buy Mosquito Bits, dose them off the label, and buy traps as your gauge. Skip neem oil for this pest.
- Stop when the traps stay clean through one full lifecycle, not when the calendar says three weeks.
Why the pot is the problem, not the flies
Fungus gnats are not really a fly problem. They are a wet substrate problem that produces flies. The larvae live in the top 5 to 8 centimetres of the pot, feeding on fungus and decaying organic matter, and they need that layer to stay moist.
That is why the first move costs nothing and comes before any product. The fungus gnats guide on this site starts the routine with dry out the breeding layer, and the reason is mechanical: BTI has to reach larvae feeding in the soil, and traps only intercept the adults a wet pot keeps producing. Nothing you buy fixes a pot that stays wet.
What BTI does, and what it cannot do
Bacillus thuringiensis israelensis, BTI for short, is a soil bacterium that produces a protein toxic to the larvae of certain flies, fungus gnats among them. Larvae eat it while feeding in the wet top layer, and it kills them from the gut.
The limit is that adults never eat it. An adult fungus gnat does not feed in the soil, so a dose does nothing to the flies already in the air. The flies you see the day after are expected; they came from eggs laid before you dosed. The Cornell Cooperative Extension note in my sources is blunt about the timeline: expect 4 to 5 weeks before the population visibly drops, and reapply as new adults keep appearing. BTI also works best on young larvae. A heavy, established infestation takes longer to clear, and larvae already close to pupating will still emerge as adults, so start early and keep going. The same note calls BTI safe to use indoors.
What a sticky trap does, and what it cannot do
A yellow sticky trap is a card coated in glue that flying adults land on and cannot leave. The adults it catches are mostly egg-laying females, so every one stuck to the card is a female that will not lay again. That trims the next generation and cuts off further egg-laying.
What a trap cannot do is reach the soil. The larvae already in the pot keep maturing underneath, untouched. A trap catches the output of the infestation, not the source. The fungus gnats guide’s Step 3 treats traps as a way to trap and monitor the adults, not as a treatment. So a trap is a gauge and an interception tool, and the gauge is the only cheap way to see whether larvae below are still turning into adults.
Buy this: Mosquito Bits, dosed off the label
The product I would buy first for fungus gnats is Summit Mosquito BitsAmazon, a granular form of BTI. The label lists the active ingredient as Bacillus thuringiensis subspecies israelensis strain SUM-6218 at 2.86 percent, EPA Reg. No. 6218-86. The label owns the dose, and the way I read it, it also states the honest negative.
The fungus gnat directions on the Summit specimen label: mix 4 tablespoons of Mosquito Bits per 1 gallon of water, soak for 30 minutes, skim off the floating granules, and water it in. Then the sentence that matters: the treatment “controls fungus gnat larvae present in the soil at the time of application”. Eggs laid after a dose are untouched, which is why the label says repeat weekly for three weeks, and again if fungus gnats reappear. The label says nothing about how long it persists in a pot. What it claims is control of the larvae present in the soil at the time of application.
Here is the tension, stated plainly. The label says weekly. The fungus gnats guide on this site says dose with each normal watering and never water on a schedule to satisfy a label, because the extra water is what bred the gnats. Both are right: one describes how often the larvicide must be present, the other what watering a still-wet pot every seven days does. The reconciliation is to let the top layer dry as usual and make the water the pot genuinely needs into the BTI soak. Pots that dry weekly land on the label. Pots that take longer get dosed less often, and the traps tell you whether that is enough.
Two more honest notes. The 30 ounce container is far more than one shelf of houseplants needs. And there is no result to see while the pot stays wet, because a wet pot produces new larvae faster than a weekly soak removes them.
Yellow sticky traps: worth buying, but for a different job than BTI
The second thing worth buying is a pack of yellow sticky traps for indoor plantsAmazon, and the reason is not to kill the gnats. It is to know when to stop treating. My notes name no trap brand; any dual-sided yellow trap laid on the soil or stood in it does the job described here.
Set one trap per infested pot at the start and watch the count of fresh catches. Fresh catches mean adults are still emerging, so larvae are still maturing below; keep dosing. The honest negative is the same as before: on their own, traps never end it.
The stop criterion lives here, and it is the trap, not the calendar. The fungus gnats guide’s section on the fungus gnat lifecycle puts the egg-to-adult cycle at about 3 to 4 weeks at room temperature. So dose with each watering through at least one full cycle, and stop only when fresh catches on the traps stay at zero. Stop early and the survivors rebuild.
Skip neem oil for fungus gnats
Neem oil is what most people reach for first, and for a soil-based pest it is the wrong tool. Neem and horticultural oil are short-persisting contact insecticides. On foliage they can knock down a few adults they physically wet, and that is where their reach ends. They do not reliably get to larvae feeding inside a moist substrate, and they need constant reapplication.
The clearest way to see it is the mechanism my notes record for soap spray, the nearest analog: contact only, the spray has to wet the pest, and what it misses it does not kill. The fungus gnats guide on this site lists neem oil under what does not work, and this guide follows that recorded position. There is no product link in this section on purpose; one here would contradict everything above.
When you do not need to buy anything
A mild case, a few gnats around one or two pots, often needs no purchase. The Cornell Cooperative Extension note gives a hydrogen peroxide drench: mix 1 part 3 percent hydrogen peroxide with 4 parts water, about 0.6 percent, and pour it over the soil. It fizzes as it hits the larvae and kills on contact, and then the soil must dry. Note the starting concentration. A ratio is meaningless without it. Peroxide has no persistence, so it shares BTI’s limit: eggs laid after the drench are untouched.
The longer fix is the substrate itself. The fungus gnats guide’s Step 4 covers how to block re-infestation at the substrate, and the one grower’s account in my notes on this pest is a switch to a chunkier, faster-drying mix, followed by no infestations for over two years. Store-bought potting soil is the usual source of the next round. If your pots are slow to dry on top, read the signs your soil mix is wrong before you buy another bag of granules.
FAQ
Will yellow sticky traps alone get rid of fungus gnats?
No. Yellow sticky traps catch flying adult fungus gnats, mostly egg-laying females. The larvae in the wet top layer keep maturing underneath. Traps show whether the infestation is still active; ending it takes a drying pot and a larvicide such as BTI.
Why are there still adult gnats the day after I used Mosquito Bits?
Because Mosquito Bits kill fungus gnat larvae, not adults. The active ingredient, Bacillus thuringiensis israelensis, has to be eaten by larvae feeding in the soil, and adult fungus gnats do not feed there. The flies you see the day after a dose hatched from eggs laid before you treated; the visible drop lags about 4 to 5 weeks.
How often should I apply Mosquito Bits, and can I use them with every watering?
The Summit Mosquito Bits label says repeat weekly for three weeks and again if fungus gnats reappear, because a dose only controls larvae present at the time of application. Using the soak with every watering is the cleaner way to hit that, as long as the pot drying drives the watering, not the label’s calendar. Top watering carries the BTI through the top layer where the larvae feed; if you normally use bottom watering, the soak still needs to reach that top layer to do anything.
Does neem oil kill fungus gnats?
Not in a way that ends an infestation. Neem oil is a short-persisting contact insecticide, so it can knock down a few adult fungus gnats it physically wets on foliage, and it needs constant reapplication. It does not reliably reach fungus gnat larvae inside moist soil, which is where the infestation is made.
What is the difference between Mosquito Bits and Mosquito Dunks?
Mosquito Bits and Mosquito Dunks both carry Bacillus thuringiensis israelensis. Mosquito Bits are small granules that steep into water in about 30 minutes and get watered into a pot, which is the form the fungus gnat directions on the label are written for. Mosquito Dunks are the compressed ring form made for standing water such as rain barrels, where the label positions them as the more residual, longer-term option. For houseplant pots, Bits are the form to buy.
Is BTI safe for pets, people and beneficial insects?
BTI, Bacillus thuringiensis israelensis, is a larvicide that has to be eaten by the larvae of certain flies, fungus gnats among them, to have any effect, and the Cornell Cooperative Extension note in my sources calls it safe to use indoors. Insects that do not feed as larvae in the treated soil or water do not ingest it. Read the product label and follow the stated dose.