Gnats or fruit flies

The insects themselves also look different once you know what to look at, and a third look-alike comes out of the drain.

A fungus gnat is dark gray to black, slim, long legged and delicate, closer to a tiny mosquito than to a fly. It flies slowly in a wobbly line and is easy to catch by hand.

A fruit fly is rounder, tan to brown, more solidly built, and most of them have obviously red eyes. They fly faster and more directly, and they hover in one place before darting off.

Size does not separate them well. Both are a few millimeters long, and a small fruit fly and a large gnat overlap.

The stuck insects on a yellow sticky card are the easiest place to compare. Once you have both under a magnifier the difference is obvious.

The females lay into the top layer of damp mix. The larvae are thin, translucent white with a black head and live in the top inch or two, where they eat decaying material and fine roots.

Established plants shrug that off. Seedlings, cuttings and fresh repots do not, and that is where the damage shows.

Every effective step goes at the wet surface they need. Let the top layer dry out between waterings, empty the saucer and the cachepot, and clear dead leaves off the mix.

Yellow sticky cards set low over the soil catch the flying females and show you whether the numbers are falling. They are a measurement as much as a treatment.

Nematodes watered into damp mix go after the larvae already in the pot. They are living organisms, so they need mix that stays damp for a while and they have a shelf life.

Expect this to take a few generations rather than a weekend. Adults keep emerging from eggs already laid after you change the watering.

Fruit flies need something fermenting. Ripe fruit on the counter, a splash of juice under an appliance, the trash, the recycling bin, a mop bucket or a wine glass left overnight all qualify.

They arrive on shop bought fruit as eggs, which is why a kitchen that was clear on Monday has flies by Thursday.

Finding the source is most of the job. Move the fruit to the fridge or into a covered bowl, empty the trash and the recycling, and look under and behind the appliances where something has spilled.

A cup of cider vinegar with a drop of dish soap in it catches the adults, since the soap breaks the surface and they sink. Cover it loosely with punctured wrap if the cup is in the way.

Clean rather than spray. Once the fermenting material is gone, the flies have nowhere to lay and the population ends within a couple of weeks.

Small dark insects that sit still on the wall or the tiles near a sink, with broad furry wings held over the body like a tiny moth, are drain flies. They breed in the film inside a drain rather than in soil or fruit.

They barely fly. They make short hops between surfaces and spend most of the time sitting, which is the giveaway against both of the others.

Clean the drain mechanically with a brush and hot water, since the film on the pipe wall is what they live in. Chemical drain cleaner does little about that film.

A trap that has dried out in a room nobody uses lets them through from the pipe. Running the tap for a moment refills it.

For the plants, the whole prevention is the top layer of mix. Water thoroughly, let the surface dry before the next watering, empty standing water and clear dead leaves off the soil.

Store opened bags of mix closed and dry. A bag that stood open or outdoors carries eggs and larvae, and a fresh repot is the classic start of a gnat problem.

Keep a new plant apart from the others for two or three weeks and watch the surface of its pot. That is when it will show.

For the kitchen, the routine is fruit in the fridge or covered, trash emptied, spills wiped under the appliances and the drain kept clean.

Neither insect bites, and neither carries a plant disease. What they cost is your patience and, in the case of the gnats, the seedlings.