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How To Control Fungus Gnats

Updated on July 6, 2013
The dreaded fungus gnat!
The dreaded fungus gnat! | Source

This article will show you how to deal with fungus gnats if they have infested your houseplants as well as show you ways that you can help to prevent further problems with these unwanted visitors.

What are fungus gnats?

Fungus gnats are small dark-coloured flies, typically 2-5mm in length, which thrive in, amongst other places, the compost of indoor houseplants.

Although the lifespan of the fungus gnat is relatively short at just 7-10 days, each female can lay up to 200 eggs. The larvae feed on plant roots and fungi in the compost and become fully grown in 2-3 weeks.

Although fungus gnats do not generally affect the health of a plant (unless it is a seedling) they can be a real nuisance. If you have a number of houseplants in your home the chances are that you will at some stage have to deal with fungus gnats.

Fungus gnat control: the options

Here is a short list of measures you can take to deal with fungus gnat infestations as well as prevent further problems.

  1. Allow the compost to completely dry out in between watering. Fungus gnats like moist conditions and their presence can sometimes be the result of overwatering. Some plants will be able to tolerate a longer period without being watered than other plants will. Although it is usually not recommended to wait until a plant starts to wilt before you water it, allowing the compost or soil to dry out in between watering will help control fungus gnats in some situations.
  2. Re-pot the plant. Take the plant outside, remove all the compost, wash the roots and re-pot into fresh compost which is not infested with fungus gnats. Sometimes this works, sometimes it doesn’t. The key seems to be washing out all the larvae from the root system and making sure that no fungus gnats find their way into the compost while you are re-potting.
  3. Add a layer of sand to the top of the compost. This is usually best done immediately after re-potting but can also be done with plants that are not being re-potted. All you do is place a layer of sand on top of the compost so that no compost is visible. Fungus gnats find it difficult to penetrate the sand so the adults are unable to get back into the compost and the newborn gnats are unable to escape from the soil. My experience is that this works very well with some plants and not so well with others. Where it works best is with pots that have recessed drain holes. If your pot has drain holes right at the bottom of the pot then fungus gnats can sometimes squeeze in and out of them. If the drain holes are recessed then this becomes more difficult for them to do.
  4. Water plants from the bottom. This is something that I have used in conjunction with other measures. With some plants I water them by adding water to the tray whereas with other plants I have inserted a tube into the compost and placed a cap on top of the tube – I then water the plant by pouring water into the tube (having first removed the cap and replacing it afterwards - the cap stops fungus gnats from getting in or out). Both these measures ensure that the top of the compost is kept dry. This works on the basis that fungus gnats like wet soil and that they are less likely to take up residence if the compost they first encounter is dry.
  5. Use sticky fly traps. If you have got problems with fungus gnats you’ll soon find out just how big the problem is by placing some yellow sticky fly traps near to your plants. These yellow sticky fly traps are commonly available and usually inexpensive. They are the kind of traps that are commonly used to control pests in greenhouses. Sticky fly traps are a great way to control the numbers of adult fungus gnats as the fungus gnats are attracted by the yellow colour, fly straight to them and get stuck. Using sticky fly traps on their own will only be part of the solution as they will only work above ground – they will not do anything to prevent the ones that escape from laying more eggs.
  6. Spray the plant with neem oil or washing up liquid. Many people swear by this treatment as the only fungus gnat control they ever use. They simply make a weak solution of washing up liquid or neem oil in water and spray the plants with the solution. In my experience this has little effect and has the unfortunate side-effect of making the compost wetter, which can in turn attract more fungus gnats.
  7. Nematodes. I have heard that you can use nematodes to deal with fungus gnats but have not tried this myself. If you do buy nematodes you will need to apply them in solution very soon after purchasing otherwise the nematodes will have died before you use them.
  8. Fungus Gnat Off. Fungus Gnat Off is a product that is readily available in the UK and presumably similar products are available elsewhere. According to the manufacturers: “Fungus Gnat Off is made using natural organic plant extracts that are suspended in a naturally derived liquid. Fungus Gnat Off contains the bacterium Bacillus Thuringiensis, which produces a toxin that prevents the larvae from feeding. This results in their death and subsequent eradication of the problem.”

    Basically you mix up a small amount of Fungus Gnat Off in water (1ml per litre so it goes a long way) and then thoroughly water your plants. The fungus gnats ingest the bacterium in the solution and are then unable to feed and so they die.

I hope you find these suggestions useful. I currently use a combination of these measures to keep fungus gnat numbers to a minimum. Using the Fungus Gnat Off has been a great help. Since I started using it the numbers have reduced dramatically. I still have yellow sticky fly traps in place but they are catching fewer fungus gnats each day as the numbers dwindle. Of course these things should not become an excuse for poor plant care. We also need to take care that we do not overwater plants.

If you get one plant that becomes infested it can be a good idea to try to quarantine it by placing it in an isolated location, either a spare room or a conservatory where the doors are usually closed and, crucially, in a place where you keep no other plants. By isolating the plant and not watering it for a while you stand a chance of treating the infestation before it spreads. But bear in mind, it only takes one female to get loose and find its way into another pot and you could end up with hundreds of the little pests!

Also, one final tip: if you do decide to quarantine an infested plant in a garden shed, make sure you remove your bags of compost first, otherwise all your fresh compost will become infested as well.

What have you found to be the most effective way of dealing with fungus gnats?

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