Sixty-three. That was the count on the yellow sticky trap beside my Pilea peperomioides the week I finally ordered a bag of Mosquito Bits. I had been drenching with hydrogen peroxide for a month, and the gnats kept coming back like they had a subscription.

Five weeks later the trap count was two. But the middle of that experiment surprised me twice, once in each direction, and the timing mistake I made in week one is the same one I now see in every frustrated forum thread about BTI "not working." So here is the whole log.

What Is BTI, and Why Doesn't It Touch the Adults?

BTI is Bacillus thuringiensis subspecies israelensis, a soil bacterium whose spores carry proteins that are lethal to the larvae of a narrow band of flies: mosquitoes, blackflies, and our enemy, fungus gnats. A larva eats the granule residue, the protein activates in its gut, and it stops feeding within hours. Mammals, birds, bees, and plants lack the gut chemistry that activates the toxin, which is why the EPA lists BTI among the reduced-risk biological options for home use.

The catch that the packaging undersells: BTI only works on something that eats it. Adult fungus gnats do not feed on soil. So the cloud of flyers around your face is untouchable by BTI, full stop. You are not killing the current generation; you are cancelling the next one.

That has a hard consequence for your expectations. Adult gnats live 7 to 10 days, and the pupae already in the soil when you start will still emerge. NC State's extension notes put the full egg-to-adult cycle at 3 to 4 weeks in warm indoor conditions. Do the arithmetic and it lands on the number that matters: BTI looks like it is failing for the first two weeks. It is not. The adults you see in week two hatched from larvae that pupated before your first drench.

How Do You Actually Dose Mosquito Bits?

The label suggests sprinkling granules on the soil surface. I tested both the sprinkle and the tea method, on six pots each, and the tea won convincingly.

The tea method: 4 tablespoons of Bits per 4 litres of room-temperature water, steeped 30 minutes, stirred once, then watered into the pot as a normal watering. The corn-cob granules float; strain them off or ignore them. The spores are in the water now. I re-dosed every 5 to 7 days, four rounds total.

Why sprinkling underperforms: larvae live in the top 3 to 5 cm of moist mix, and surface granules only release spores when water happens to run through them. In my sprinkled pots, gnat larvae were still visible in the top layer at day 10 (the potato-slice check from tiny white bugs in houseplant soil makes them easy to census). The tea pots were clean at the same check.

One more dosing note: BTI spores degrade in light and warmth, so brew the tea fresh each round. A jug that sat for three days is compost water, not treatment.

The Week-by-Week Trap Counts

I ran fresh yellow sticky traps each week, one per shelf, and logged the catch. Same room, same 21 to 23 °C, ten pots including the Pilea, two Alocasia, and a chronically damp Calathea that started the whole outbreak.

  • Week 0 (baseline): 63 adults trapped. Larvae visible on potato slices in 6 of 10 pots.
  • Week 1 (first drench): 58 trapped. No visible change. This is where people quit.
  • Week 2 (second drench): 41 trapped. Potato slices clean in 8 of 10 pots. The pipeline is emptying.
  • Week 3 (third drench): 14 trapped. First week the room felt normal.
  • Week 4 (fourth drench): 5 trapped.
  • Week 5 (no drench): 2 trapped, both on the trap nearest a window, possibly walk-ins.

The shape of that curve is the honest pitch for BTI: nothing, nothing, cliff. Every drench-only remedy I have tested, including the peroxide protocol in hydrogen peroxide for fungus gnats, knocks larvae down on contact day but leaves the eggs, so counts saw-tooth instead of falling. Peroxide also fizzes away any BTI in the soil, so pick one lane; do not combine them in the same week.

Traps did real work here beyond scorekeeping. Each trapped female is up to 200 eggs that never happen, and the count told me when to stop treating. The comparison in fungus gnat traps compared covers which traps earn their spot.

Why Do Gnats Come Back a Month After BTI?

Because BTI has no residual action worth counting on, and because the conditions that fed the first outbreak are still there. My relapse checklist, in the order I check it:

Moisture. Fungus gnat larvae eat the fungi and algae that grow in constantly damp mix. My Calathea pot, which never dried past the top centimetre, was patient zero. Letting the top 2 to 3 cm dry between waterings starves the nursery; a cheap probe from how to use a moisture meter settles the guesswork. Bottom-watering for a few weeks helps too, since eggs are laid in the top layer.

New arrivals. Half the reinfestations I have traced came home in a nursery pot. Bagged potting mix can carry eggs as well. Quarantine routines from how to quarantine new plants apply to soil bags just as much as to plants; I now store opened mix in a sealed tub.

Open drainage trays. Standing water plus organic crud equals a breeding site that never sees your BTI drench. Empty trays within 30 minutes of watering.

If gnats return, one or two fresh BTI rounds end the new cycle. Resistance is not a practical concern at houseplant scale; the multi-protein mode of action has kept BTI effective against mosquito larvae for decades of far heavier use.

Frequently Asked Questions

Q: I used Mosquito Bits for a week and still see gnats everywhere. Did I get a bad batch?

A: Almost certainly not. BTI cannot kill adults or pupae, so every gnat you see in week one was already past the larval stage when you started. Judge the product at day 21, not day 7. If potato slices still show larvae after three weekly drenches, then question the batch or the dose.

Q: Can I make BTI tea from Mosquito Dunks instead of Bits?

A: Yes, same bacterium in a compressed donut. Quarter of a dunk per 4 litres, soaked longer since it is denser, works out about the same. Bits just dissolve faster and dose small collections more precisely.

Q: Is BTI safe in pots my cat drinks from when I'm not looking?

A: BTI's toxin only activates in the alkaline gut of specific fly larvae; cats, dogs, and humans digest it as protein. The EPA's home-use guidance treats it as one of the lowest-risk options available. Keep the dry granules out of reach anyway, as with any product.

Q: Will BTI hurt the springtails in my terrarium?

A: No. Springtails are not flies, and BTI's proteins do not activate in them. In my terrarium test jar the springtail population was unchanged after three drenches. If you actually want fewer springtails, that is a moisture problem, not a pesticide one.

Q: Does BTI do anything against root aphids or thrips pupae in the soil?

A: Nothing at all, and this matters because root aphids are regularly misdiagnosed as gnat larvae. BTI is specific to the larvae of mosquitoes, fungus gnats, and blackflies. If your "gnats" survive a proper BTI schedule, re-identify the pest before spending another cent.

BTI is the rare houseplant remedy that does exactly what the fine print says: total larval kill, zero effect on adults, and a two-week lag that punishes the impatient. Brew the tea, run four rounds, trap the flyers, and fix the damp that invited them. For the drying-out half of the fix, our watering calculator keeps the top layer honest.