Diptera : Sciaridae
Bradysia spp., Lycoriella spp.

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Have you ever wondered what those pesky mosquito-like flies are hovering around your indoor plants? They are almost certainly the flies known as Fungus Gnats. There are several families of flies which are called ‘Fungus Gnats’, but the black Fungus Gnats of the family Sciaridae are the ones you are likely to encounter in the home.

Fungus Gnats are also pests in commercial greenhouses and plant nurseries where they are attracted to potting media, especially media with high moisture and high organic matter content. The adult Fungus Gnats are just annoying, but their larvae breed in the potting media and may damage plant roots.

Best Treatment for Fungus Gnats

The best way to protect your indoor plants is to prevent adult Fungus Gnats laying eggs in the potting media. Try Yates Gnat Barrier to help control Fungus Gnats in your indoor plants. Yates Gnat Barrier is a natural, non-chemical, physical barrier made from abrasive pumice granules.

The secret to pest control is to keep an eye on your plants so that you can spot pest incursions early. For example you could keep an eye out (see Monitor) while you are watering your plants – check for Fungus Gnats flying around your plants, or walking on the potting media surface. For this to work you will need to know what Fungus Gnats look like (see Description), so that you know they are actually Fungus Gnats and not something else.

Prevention is always better than cure. As well as trying Yates Gnat Barrier, there are a couple of other things that you can do to prevent incursions from Fungus Gnats (see How to Prevent Fungus Gnats Appearing).

Regular monitoring of your house plants will ensure that a major infestation of Fungus Gnats doesn’t occur ‘overnight’. Managing Fungus Gnats is much easier if you can catch an infestation in its early stages.

What are Fungus Gnats &
How to Get Rid of Them

Fungus Gnats are tiny insects that are part of the fly order Diptera. They are somewhat mosquito-like in appearance and they can be a real nuisance in the home. Major infestations may result in damage to seedlings in greenhouses.

Description

Common Fungus Gnat adults are small, dark mosquito-like flies about 4mm long, with small heads and long legs. They are often seen running on the soil surface. They are weak fliers and tend to just hover around house plants. They have a single pair of wings with a distinct Y-shaped vein at the tips.

Eggs are tiny (0.2 mm), oval, and translucent-whitish, and laid in crevices in the soil surface.

Larvae are pale, translucent, legless maggots, which grow to about 5 – 7 mm in length and have shiny black heads. They are usually found in the top 25 to 50 mm of potting media.

Pupae are brown, about 3 mm long, and found just under the soil surface.

Life Cycle

Female Fungus Gnats can lay between 100 and 200 eggs over their short life of about a week. The eggs are laid in the soil, and the larvae hatch and grow through four moults over a period of two weeks. Pupation occurs in a silken chamber near the soil surface. Adults emerge from the pupae about five days later. The entire life-cycle may be completed in about four weeks.

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What Plant are Impacted by Fungus Gnats

Fungus Gnats are pests of:

  • house plants.
  • pot plants in greenhouses and sheltered outdoor areas.

Symptoms of Fungus Gnats

  • Fungus Gnats are usually only a ‘nuisance’ in the home if the infestation is minor.
  • Fungus Gnat larvae usually only feed on organic matter in the potting media of house plants when the infestation is minor, but large populations of Fungus Gnat larvae may cause damage to the roots of house plants. In extreme cases plant growth may be stunted.
  • Larvae may tunnel into the stems of seedlings at or below the soil level causing the seedlings to collapse.
  • Fungus Gnat larvae can transmit soil-borne fungal diseases such as Pythium and Fusarium, which may result in ‘damping off’ of seedlings.
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How to Prevent Fungus Gnats Appearing

Monitor

  • Watch for small mosquito-like insects hovering near houseplants.
  • If you see a few adult Fungus Gnats hovering around your house plants check the top 25 mm of potting media for larvae.
  • An easy method to check for Fungus Gnat larvae. Place a 10 mm slice of potato on the surface of the growing media of each pot. Fungus Gnat larvae will migrate up to the potato slices to feed in a day or two. Turn over the potato slices to check for larvae on the underside.
  • Seedlings in greenhouses are susceptible to Fungus Gnat damage. Watch for seedlings that lack vigour or have leaves turning yellow. if symptoms occur check seedling mix for Fungus Gnat larvae.
  • Check that your house plants and other pots, and seedling trays aren’t overwatered.

Prevent

  • Try Yates Gnat Barrier.
  • Fungus Gnat larvae prefer feeding in very moist soil. If Fungus Gnats are a frequent problem at your place allow the top 25 to 50 mm of potting media to dry out before watering again.
  • Don’t allow pots to sit in trays full of water.
  • Always use clean sterile potting mix. If reusing potting mix you may need to sterilise it.
  • Potting mixes containing high levels of peat are known to be attractive to Fungus Gnat adults. Purchase potting media with low levels of peat, or if mixing your own try leaving peat out of the mix.

Natural Enemies

  • Treat pots with beneficial nematodes. Nematodes are roundworms which live in soil and kill Fungus Gnat larvae by penetrating their bodies and breeding inside them. Beneficial nematodes are available commercially to treat Fungus Gnat larvae and can simply be watered into pots.

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