Fungus Gnats

Sciaridae ssp.

Contents

1. Top Tips & Info
2. Symptoms
3. Susceptible Plants & Parts
4. Actions Needed (Includes Details on Diatomaceous Earth)
5. Other Ways to Alleviate an Infestation
6. Ways to Prevent an Attack in the Future

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Top Tips & Info

• Eradication Difficulty - Moderate
• Life Cycle - Up to thirty days.
• The female can lay up to thirty eggs a day - two hundred in total.
• Although the adults are harmless, their larvae may begin to attack the roots when a large infestation is present.
• Removing all of the Gnats is complicated as they're airborne and can quickly fly away when harassed - windows, soil & light sources are all popular destinations.
• Crush each bug when seen. If you've got a small number of houseplants, bring them all into one room and work on removing airborne flies in every other place.
• Before placing back into their original location, remove the top few centimetres of soil for a fresh batch of the appropriate potting mix. Spray the new soil with garlic water - boil a clove for ten minutes and use once the fluid cools down. 
• Place a thin layer of Diatomaceous Earth ('D.E.') over the soil to prevent any adult females from laying their eggs. It's essential to block every inch of soil on every plant, as the bugs will scavenge for hours to check out new ways of accessing the compost below.
• Maintain the golden rule of killing any Gnats as you go, at least for the next few weeks. Remove the grit from the soil once there aren't any signs of an infestation.
• Chemical pesticides aren't really necessary; however, a quick spray into the new soil's top layer may deter the laying of eggs, just before sprinkling the 'D.E.'.

Symptoms

Over time you'll start to notice an increasingly large amount of small black flies appear all over your home. They'll parade on the foliage, windows or light shades and will fly away when under threat. Although a large proportion of them are actually trying to escape, others will begin to deposit small eggs in the top layers of the soil, slowly developing into white larvae within a few days. These pesky critters are what will cause the damage to the plants - digesting the root systems, spreading diseases (like Pythium or Botrytis blight) and wreaking havoc around the home.

Fungus Gnat Larvae

Where Do They Come From?

Although most active during the colder months, they can attack throughout the year. Fungus Gnats are in search of a dark, moist breeding ground to lay their eggs. There are three main ways for the Gnats to enter your home - most will enter the home through open windows or doors, but a few will be in the soil already. When purchasing a new plant from a store, always be sure to replace the top 3cm (2 inches) with a fresh batch of compost to eliminate introducing new critters. The final way is via a transplant when compost is used from a poorly-stored bag. When hoarding soil for later use, keep the lid tightly enclosed in a dark, dry location - a great example would be in a garage or shed.

Susceptible Plants & Parts

Although the plants may be seen on the foliage, their larvae will only attack the top layer of moist soil, especially if it's made up of organic matter (like bark, coir, etc.). Although smaller infestations won't affect established plants, large numbers in one pot may cause a decline of health with seedlings. 

Plants - Most plants will be affected, especially with those located in dark settings. A continuation of moist soil will also promote an attack, as they thrive best in dark, humid environments with poor soil aeration. They'll rarely attack plants that require dry soil, for example, cacti or succulents. In some cases, they'll also feed off the nectar of flowering plants, spreading potential diseases across your plant collection.

Actions Needed

There are several methods of eradicating the Gnats and their larvae. The tips that ukhouseplants will provide have worked a treat over the last few years, so don't worry if there are other methods online.

Similarly to Vine Weevils, there are two stages to eliminate Fungus Gnats. The first stage is to remove the larvae and THEN the airborne specimens. There's little point in only removing the adults, as the eggs or larvae will only pick up where the predecessors left off. 

1. Prepare a room (ideally small) where there aren't any Gnats or plants present. Providing a 'neutral' zone where you can confidently say that there aren't bugs will aid pest regulation and the eradication process. Keep the doors shut and regularly check for any Gnats that may be hovering around the windows or lampshades.

2. Take all of your plants outside and replace the top 2cm (1 inch) of soil with the appropriate potting mix. Click on the link before continuing with this article if you're unsure as to which compost is right for your plant.

3. Repotting your plants all at the same time will reduce the chance of larvae hatching at different times, thus causing a resurgence of Gnats. Sprinkle a fine layer of Diatomaceous Earth (also known as D.E.) across the new soil to prevent the females from laying their eggs. This powder-like substance is made up of grounded diatomic mantels (think loosely of powdered seashells) that'll act as a sharp, abrasive texture that aggravates and weakens the pregnant females before they lay. Their bodies begin to be scratch and become punctured by the powder, causing dehydration and a slow death to halt another generation of these Gnats. Sprinkle a fine layer onto the soil of every plant and leave it on for several weeks. Remember that D.E. isn't harmful to plants, pets or humans, so there are no health risks to consider. Remove the powder once you feel the infestation has elapsed, which usually take three to four weeks. 

4. Place all of the plants into the prepared room. Keep them here for at least a week or two to check if any more Gnats are present. The larvae will develop into adults within ten days, so keeping the plants under a watchful eye will eventually show up any live bugs.

5. Crush each airborne pest that crosses your path throughout the house, as well as in the quarantined room. Once the two week period is over, you can safely re-introduce them back into their original locations around the home. 

6. Always keep an eye out for more Gnats and kill or hoover each one that you see. This can be a long process to eradicate, so doing small hunts each day will make a big difference long term. If another infestation arises, try replacing the top two inches of the soil for the second time and consider purchasing Diatomaceous Earth if you haven't already. Always remember that the Gnats are indeed harmless, so never get too worked up about your plants' overall health!

Other Tips to Alleviate an Infestation

• Introduce sticky pads onto the windows or near to the soil.
• Leave small bowls of water on windowsills and nearby plants to drown unfortunate Gnats.
• Spray the foliage and top layer of the soil with another organic pesticide - this won't kill the pests, but may deter them from laying the eggs. An excellent example of this is Neem Oil.
• Hydrogen Peroxide - Now, before you rush to your local garden centre, this chemical is mostly found online, so have a shop around for the best deal on this link. For every three irrigation, incorporate a splash of the peroxide to administer the pesticide on its eggs. You'll have to do this for at least three times to stop the chance of a relapse thoroughly. Always follow the manufacturer's recommended dose, as this chemical may become detrimental to the plant when over-used.
• The final tip is the plant's location. As Fungus Gnats hate dry and bright areas, keep the plant on the drier side to life, allowing the top layer to remain soil as long as possible. This can be done by using the bottom-up method of submersion, however, be wary of over-watering the lower portion of the rootball - especially in the winter months.

Ways to Prevent an Attack in the Future

At the local plant shop or garden centre, regulate and inspect any plants that you wish to buy. Check around the hard-hit areas, for instance, the soil and lower stems, before considering the purchase. Most Gnats will come from already-affected plants, so always keep this in mind when increasing your plant-collection.

Change the top layer of the soil when bringing any new plant from a shop. Replacing the top layer of the compost will remove any eggs & larvae that may have fallen in, or been deposited by a mature adult. We do this religiously, each time a new plant is purchased!

Regularly check for pests on your own plants. Although this may sound patronising, many gardeners forget to inspect their indoor specimens. As soon as you see a symptom, keep it quarantined and follow the steps above!

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