Sticky Trap

Fungus gnats (or Sciarid flies) are tiny little black flies that live in decaying organic matter. If you keep a compost caddy indoors you may find a cloud of them rise when you raise the lid, but the fat-bodied slightly brown flies are the more disgusting fruit flies. Fungus gnats are more at home in your houseplants, and are problematic during early spring when we’re raising seedlings indoors.

On a human scale, fungus gnats are merely irritating. The females scurry over the surface of the compost, and the males flies off in search of new breeding partners. But they are rubbish at flying and flit aimlessly around the place until they lock on to a target that’s warm and moist and (preferably) producing carbon dioxide. In most houses this means humans, or nice warm cups of tea. They then proceed to fly right into your face or drown themselves into whatever tasty drink you’ve just made yourself. Leave the dregs out overnight and, in the middle of an infestation, you’ll probably be disposing of half a dozen corpses in the morning.

On a seedling scale, they are nastier beasts. The adults pose no threat to plant life, but their larval offspring tend to chomp into plant roots and cause seedlings to wilt and die. I mention them in The Peat-Free Diet.

Pete is currently practicing tolerance, understanding and acceptance as well as his ninja-like reflexes, because the windowsill mushroom kits have provoked a fungus gnat population surge. They always do, I should give up trying. Except the oyster mushroom kits, which don’t involve compost and hence don’t attract the gnats.

With houseplants the solution is careful watering, preferably from below to keep the surface of the compost dry. With seedlings it’s trickier, but their life indoors is usually limited and taking them outside solves most problems and yellow sticky traps control the population in the meantime – position them low over the offending compost for success and keep them away from plant leaves.

There is a biological control – brand name Gnat Off here in the UK – which is marketed to *cough* hydroponic growers. Apparently it is very effective, but it’s not a cheap option; no doubt it’s what the commercial mushroom growers use (iirc there are no pesticides sanctioned for use against fungus gnats on food crops) but for the windowsill mushroom grower it would make things distinctly uneconomic.

My best tip for surviving a fungus gnat infestation is to start collecting the clear plastic lids you get with big pots of yoghurt. They have absolutely no effect against the blasted gnats, but they do keep them from drowning in your drinks ;)