Every spring and autumn, many houseplant owners are annoyed by the presence of tiny flies that appear around their plants.  These are fungus gnats, sometimes called sciarid flies.  They are harmless, but very annoying - more so because they are attracted to carbon dioxide, so they tend to hover around your face and seem even more numerous than they really are.

Fungus gnats should not be confused with fruit flies or drain flies - both are slightly larger and are not attracted to plants.

Here are a few facts about them and tips to deal with them:

  • Adult flies emerge from the soil, where they have spent the previous several months as larvae, feeding on algae and other organic matter in the soil.  The adults seek mates, breed, lay their eggs in the soil and then die.  They are around all year, but most common in autumn and spring when they appear in significant numbers.
  • The adult flies DO NOT BITE!  They have simple mouthparts capable only of sucking up some moisture.  They are much smaller than mosquitoes (also quite common this time of year), but they are silent and do not suck blood.
  • They bury their eggs in the soil, but can’t dig deeper than a couple of centimetres.  The eggs need moisture to hatch.
  • If the top layer of soil is constantly dry, then the chances of the eggs hatching is much reduced, so water from the bottom if possible.  If your plants are in a foli8 coir pot, add water to the decorative pot and let it soak through the coir pot into the soil - the surface will always remain dry.
  • Add a layer of a decorative top dressing to the soil to make it harder for the flies to get to the soil surface.  A layer of horticultural grit, fine gravel or other lightweight product will do the job.

  • Yellow sticky cards will catch a lot of the adult flies, so place them near to the pots and catch as many as possible before they have a chance to mate.  Unfortunately, these cards are rather unsightly, but you will only need them for a week or two.
  • If you have lots of plants, consider getting some biological controls.  Nematodes (microscopic eelworms) are very effective and are non-toxic. They are added to water and watered into the plant pot every few weeks, where they destroy the gnat larvae by getting inside them and releasing enzymes that break down their tissues.

By Kenneth Freeman

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