Small dark flies that rise up when you water a houseplant: those are fungus gnats. The gnats themselves are annoying but harmless. The real problem sits in the soil, where the larvae feed on the fine roots.
So you need two tracks. Yellow sticky traps remove the adults so no new eggs follow, and they show you straight away how big the population is. The glue is water resistant, so it does not wash off when you water or when the air is humid. One trap lasts up to three months. Alongside that you need to deal with the larvae in the soil and let the top layer dry properly between waterings.
Fungus gnats or fruit flies?
The confusion is understandable, because at first glance both are small and dark. Look at the place and the behaviour. Fungus gnats rise out of the potting soil, move in jerky darts and stay near the plant or head for the window and the lamp. Fruit flies are yellow brown with red eyes, circle around fruit and the bin and fly far more steadily. If they come from the fruit, go to get rid of fruit flies.
Are fungus gnats harmful?
Not to people. They do not sting, do not bite and carry no disease. To your plants they are. With a large population the fine roots get damaged, the plant grows poorly, leaves yellow and young cuttings can die. On a mature plant the damage is usually limited; on seedlings and cuttings it is not.
Where do they come from?
Usually into the house with a new plant or a bag of potting compost, with the eggs already in the soil. Watering too often is the main driver, because the larvae need a permanently damp top layer. A plant that is allowed to dry out between waterings rarely becomes a problem.
Why are the sticky traps yellow?
Fungus gnats respond to colour, and bright yellow works on them like a magnet. There is no scent or lure in them, only that colour and the glue. It is also why they catch whitefly, aphids and other small flying insects alongside the gnats.
Where do you put the traps?
Push them into the pot, just above the soil, because that is where the gnats rise and where they lay their eggs. Hanging them high in the plant works far less well. With several plants side by side, one per pot, because they barely travel.
How long does it take?
Expect three to four weeks to break the cycle. The adults live only about a week, but the eggs and larvae in the soil need time. Catching only the adults is not enough, because the next generation is already in the soil.