Fungus Gnats. Tiny pests which have been the bane of every indoor-plant owner at some point in their gardening lives. If you're yet to experience them, kudos to you! They're tiny flying insects which look slightly similar to but quite a lot smaller than a fruit fly and breed in the top layer of soil in your house plants. The larvae can be particularly annoying as they eat your plant's tiny new roots. These pesky gnats are difficult to get rid of, and the best cure is prevention.
Fungus Gnat Life Cycle
In order to cure an infestation, it's important to understand the lifecycle of the gnat.
Eggs can be killed by saturating the soil in a diluted hydrogen peroxide mixture, larva can be found by using the potato test, and killed by spraying pyrethrum, and the adults can be caught in a number of different traps, discouraged with natural remedies, or prevented from hatching or laying eggs by covering the soil.
When dealing with a large infestation of Fungus Gnats, understanding the lifecycle of the insect can help you to deal with each aspect of the infestation. Many methods will have to be repeated every two days for two weeks to ensure all of the Gnats have been killed and the lifecycle does not continue.
Here are some methods to help you deal with Fungus Gnats:
1. Let your soil dry out before watering
In the cool winter months, the soil in your indoor plants takes longer to dry between waterings. If you routinely water your plants without checking the soil first, then you could easily be overwatering your plants through winter. Keeping the top of the soil moist for prolonged periods of time is like an open invitation for Fungus Gnats, giving them the perfect breeding ground to move in to.
The easiest thing you can do to prevent this is to make sure your soil is dry before watering by sticking your finger in down to your knuckle, or by using a water checker.
2. Check everything that comes in the house
It's quite common for pests to come home with your new plant or bag of soil from the nursery. Fungus Gnats can be seen hanging around soil, and often on the outside or rim of the infected pot. You can coax these guys out by gently blowing on the top of the soil and keeping your eyes on the surface to spot any movement. They may move in response to the wind, or be attracted to the CO2 in your breath. Fungus Gnats can also be brought into your home inside soil bags. While this may be hard to check at the nursery, a good thing to do is keep your eyes pealed for any tiny flying insects in the isles near the soil bags.
The best way to prevent this is to check the soil and leaves of your new plants for any signs of pests before buying, and keeping your eye out for any tiny flying insects hanging around the soil isle.
3. Not sure if you have Gnats? Do a Potato Test
If you think you may have Gnats but you can't manage to spot them, or you're not quite sure which plants are infected, the potato test is an easy solution. All you need for the potato test is a sliced up raw potato, and a light on your phone.
Put a slice of raw potato (can be as thin as you like) down flat on the top of the soil, and leave it for at least 4 hours. This can be done to every plant in the room or house to make sure you know exactly what to treat for an infestation. After 4 hours, have your phone light out and ready, then use tweezers (or fingers, up to you) to pick up the slice of potato and check the side that was face down for any movement.
An infected plant will have tiny translucent larvae crawling through the soil, and they will be attracted to the potato. After 4 hours, the potato should become a smorgasbord for the larvae, and when checked, should reveal lots of tiny moving dots.
Discard the potatoes carefully so as to not spread any of the larvae to other plants, and quarantine the infected plants in a separate room.
This method can be repeated as much as you like to check this larval stage of the Fungus Gnat life cycle.
If you're seeing adult Fungus Gnats hovering around your soil, and are looking for a quick solution, you can spray the top of your soil with pyrethrum until saturated. This will kill the adults and the larvae on contact. Be sure not to water your plant again until the top soil has dried for maximum effect. This can be repeated every 2 days for 2 weeks to ensure the lifecycle ends.
5. Hydrogen Peroxide
This common household product can be used to treat a Fungus Gnat infestation, and won't harm your plant! Dilute 1 part hydrogen peroxide with 4 parts water and saturate the soil of all your infected plants. This will kill the larvae and eggs on contact.
If you don't want to use chemicals and you'd like to target the adult Gnats, then traps could be your best option. There are a few kinds of traps you could use; sticky traps, old beer/wine, and the classic apple cider vinegar.
Sticky traps, while effective in trapping the adult Gnats, pose their own risks to your household. These yellow, highly sticky, often double-sided traps can be found in most gardening or home stores. I've found they work best when only one side is exposed, by laying the un-peeled, un-sticky side on the tops of soil or pots and waiting for the Gnats to walk across the top. These traps should not be left within reach of children or animals, should not be used outdoors, and should be disposed of by ensuring the sticky sides are both covered. They pose a very real risk to animals and children if they get stuck over the nose or mouth.
Old beer or wine is an incredibly easy trap to make. Simply add a few drops of dishwashing detergent to a small shot glass of leftover beer or wine and leave it near infected plants. The Gnats will be attracted to the CO2 being released by the alcohol and the soap will break the surface tension of the liquid so that when they go to drink, they won't be able to get out.
The same principle is used for the apple cider vinegar trap, simply add some apple cider vinegar to a bowl or cup, and mix with a few drops of detergent. Place the trap near plants to trap the adult Gnats.
7. Covering the soil
If you prefer a less direct approach to Fungus Gnat eradication, covering the soil with sand or rocks can also work. This method is best suited to those with only a handful of plants, or who like the look of rocks on-top of their indoor-plant soil. Simply cover the entire top of the soil in either a few centimetres of sand or small rocks. The sand prevents the newly-hatched adult Gnats from reaching the surface and breeding, and similarly, the rocks prevent adult Gnats from reaching the soil to lay eggs.