Fungus gnats: what finally worked
The gnats showed up in June, a few tiny black flies drifting up from the pots whenever I watered, and by August there were dozens, mostly around the monstera and the two large pots of pothos. They are harmless to a healthy plant, as far as I can tell, but they are maddening, and the larvae in the soil can chew on fine roots. I tried four things over ten weeks and kept a rough count of adults on a yellow sticky card each Sunday.
Week one baseline: about 60 gnats on the card. First attempt, letting the top two inches of soil dry out completely between waterings. The larvae live in the top layer and need it damp. After two weeks the count dropped to around 35, which is real progress but not a solution, and the fern was unhappy about the regime. Second attempt, added to the first: a half-inch layer of coarse sand on top of every pot, with the idea that adults cannot lay eggs through it and larvae cannot climb out. Two weeks later the card had about 25. Better, but when I lifted the sand on the monstera the soil beneath was still crawling. The sand makes watering awkward too, because it channels water down the sides of the pot.
Third attempt: a drench with a biological larvicide, the kind based on a bacterium that is specific to fly larvae, mixed at the rate on the packet and watered into every pot on three consecutive weekends. This is what worked. Week one after the first drench, about 20 on the card. Week two, 8. Week three, 2. Week four, zero, and it has stayed at zero for three weeks since.
What I will keep doing:
- The dry top layer, because it costs nothing and the plants are fine with it once I adjusted the fern.
- A sticky card in the biggest pot year-round as an early warning.
- The larvicide drench as soon as the card shows more than a handful, rather than waiting until there are clouds of them.
I have taken the sand off. It helped a little but the drench did the job on its own, and I would rather see the soil. The cause, by the way, was almost certainly a bag of potting mix I bought in May and left open on the balcony. It is now sealed in a bin.