What are those tiny, annoying little flies, swarming around my beautiful houseplant collection, and how did they get there?! It’s a question most houseplant enthusiasts will have asked themselves at some point. These microscopic winged menaces are fungus gnats and are possibly the most common houseplant pests. Luckily for us, despite being a nuisance, they are generally harmless healthy plants, although they should be dealt with swiftly to prevent their pesky population from exploding.
So if you’re fed up with swatting them away from your face and watching them swarm up as you tend your plants, here is our ultimate guide to getting rid of fungus gnats for good!
What are Fungus Gnats?
Fungus Gnats are tiny flying insects belonging to the Sciaroidea family. The adults measure between two and eight millimeters long with dark brown bodies and wings. They are often found crawling across the top layer of soil in plant pots or flying around nearby, although they are fairly weak fliers and tend not to stray far from host plants. The larvae look like microscopic maggots, with black heads and translucent, legless bodies. You won’t usually see the larvae, as they live beneath the soil where they feed on decaying matter, fungus, root hairs, and soft new plant growth. Fungus gnats are often viewed as a beneficial organism within a healthy ecosystem, as they help to break down decaying organic matter.
What Causes Fungus Gnats?
Fungus gnats love moisture! Plants that are overwatered are a paradise for these pests. The females lay their eggs below the surface of the soil where it is damp. Not only does an overwatered plant hold plenty of moisture, but roots also begin to rot when they are oversaturated, creating fungus for the gnats to eat.
Fungus gnats feast on all kinds of organic matter, particularly decaying matter and fungus. Potting mixtures and substrates with a high organic matter content are at higher risk of infestations because they provide all the nutrients that the larvae need to develop. Make sure that you store your houseplant compost in an airtight container to prevent them from infesting your soil, and later, your plants.
Plants with fungal diseases such as root rot or sooty mold are particularly susceptible to fungus gnat infestations. Treating fungal problems quickly will prevent gnats from taking hold and doing even more harm to the plant.
Indoors, fungus gnats can be active at any time during the year, due to the more consistent ambient temperatures.
How Can I Prevent Fungus Gnats?
As with all pest and disease prevention, it’s imperative to inspect any new plants for signs and symptoms before purchase. Even if nothing seems amiss, it’s good practice to isolate new plants for at least a month before introducing them to your collection as even the best-hidden problems will have become symptomatic by this time. When inspecting for fungus gnats, look for the flying adults, but also poke around just beneath the top layer of soil with your finger to look for any shiny larvae.
Aside from inspecting and isolating new plants, you should regularly check all of your plants for any signs of pests or disease so you can catch any problems early. Early detection generally means treatments are much more successful.
Monitor the moisture levels of all your plants and don’t overwater! It’s good practice to allow the top inch or two of soil to dry out between watering. Doing this not only deters moisture-loving gnats but also prevents waterlogged soil, root rot, and other fungal diseases. Make sure your potting mix contains plenty of drainage materials, and your pots have plenty of drainage holes.
What are the Symptoms of Fungus Gnats?
It is pretty easy to tell if you are dealing with fungus gnats, since they are plenty big enough to spot with the naked eye, and will often make their presence known by flying up in swarms from the soil if the plant or pot is disturbed. Adult gnats are attracted to light, so you may spot them congregating around windows and bright areas in your home. If the infestation is particularly heavy, you may even see slimy trails running across the top layer of soil, similar to slug or snail trails, which are left by the larvae.
The adults pose more of an annoyance to us humans than cause harm to our plants. They will often fly around faces, drinks, and food sporadically which makes them quite a nuisance! Mature, healthy plants will generally not be affected by a moderate fungus gnat infestation. It is the larvae of fungus gnats that pose a threat to plants.
Because the larvae feed on soft, new growth and young roots, they can cause extensive damage to seedlings. In young plants, you may notice stunted growth, yellowing, wilting foliage, and even death if the larvae have been feeding excessively from the root system. These symptoms, however, can be indicative of several other problems, such as root rot or under-watering, so make sure you give any symptomatic plant a full health check to determine the root cause.
Sometimes, fungus gnats can carry the pythium disease which causes ‘damping off’ of seedlings. Young plants affected by damping off will gradually break down and collapse, developing a layer of white fungus. There is no cure for damping off, and the seedlings will need to be disposed of.
If you think you may have a fungus gnat infestation, this simple potato trick will let you know for sure! Cut a raw potato into inch-wide chunks and bury them in the soil. The chunks will attract the larvae who attach themselves to the potato to feed. After a few days (or even hours), remove the potatoes and you will be able to see the larvae crawling on the potato if an infestation is present. Be sure to dispose of the potatoes!
How to Get Rid of Fungus Gnats
Exterminating fungus gnats is much more successful when you tackle both the larvae and adults, as this disrupts the life cycle and reproductive process. As with any treatment for diseased or infested plants, the first step is to isolate the host plant to prevent the problem from spreading.
Placing a layer of sand on the surface of the soil can help eradicate excess moisture and discourage gnats. The sand will need to be around three-quarters of an inch deep to work properly, and will prevent the gnats from accessing the soil to lay their eggs, and confuses them into thinking the soil is dry.
Hydrogen peroxide is a very effective treatment against eggs and larvae, killing them on contact. It also stops any gnat-enticing fungus from growing in the soil, preventing future attacks. Hydrogen peroxide is sold cheaply in most drug stores and won’t harm your plants. It’s a naturally occurring chemical compound that is found in rain.
Make a mixture of one part 3% hydrogen peroxide and four parts water, pour liberally over the soil, and allow to soak in. The mixture will make a soft fizzing sound when it hits the soil but don’t worry, this is normal!
Yellow sticky traps can be placed in the vicinity of affected plants. The yellow color will attract the bugs, and they will become stuck to the tacky surface when they fly too close to it. This method will only kill the adults though, not the soil-dwelling larvae. You can use sticky traps in combination with a soil treatment to exterminate both the adults and the larvae.
Apple Cider Vinegar
Cider vinegar traps work in a similar way to sticky traps and are easy to make at home. In a shallow dish, mix a solution of equal parts water and apple cider vinegar, adding a few drops of dish soap and mixing thoroughly. Fill the container at least a quarter-inch full and cover with cling wrap, piercing it several times. The gnats will fly in through the holes and become trapped under the cling wrap, whilst the dish soap clings to their exoskeleton, suffocating them as they drown in the vinegar. You should empty and refresh the mixture every few days.
As with sticky traps, this method is only effective in killing adults so should be used in combination with a soil treatment.
Cinnamon and Chamomile
Both cinnamon and chamomile are efficient natural fungicides, so whilst they won’t kill gnats, they will prevent the growth of the mold which attracts them in the first place. You can brew a strong chamomile tea with boiling water, leave it to cool, and add the tea mix to your normal watering routine. Repeat regularly to prevent mold from regrowing.
Ground cinnamon can simply be sprinkled liberally over the soil to keep fungus at bay, with the added bonus of making your home smell delicious!
Diatomaceous Earth (DE) is a really effective, if somewhat gruesome way to deal with fungus gnats. Organic and non-toxic, DE is a powder-like substance that is formed from the crushed up fossilized remains of microscopic prehistoric creatures called diatoms. To humans, DE feels chalky or sandy, but to the tiny flies, it has the texture of millions of shards of broken glass.
DE can be mixed into the potting soil, as well as sprinkled over the top layer. When the gnats travel through it, the microscopic shards will pierce their exoskeleton causing them to leak bodily fluids until they become dehydrated and die.
Don’t apply DE to wet soil, as moisture reduces its effectiveness. When using DE as a pest treatment, always buy food-grade, rather than pool-grade, as this contains highly toxic silica.
Sometimes, simply amending your plants’ environment will be sufficient to clear up fungus gnats. Withhold water for short periods and allow the soil to dry if your plant will tolerate it. You could also reduce the amount of organic matter in potting mix, eliminating the gnats’ primary food source. You may even want to repot the plant in totally fresh soil.
None of these treatments are likely to eradicate all of the eggs, larva, and adults with one application, so you should continue to repeat the process even as the population dwindles. What’s more, the larvae and the adults respond differently to different treatments, so you will usually need to employ a combination of methods to target both the juveniles and adults simultaneously.
Frequently Asked Questions
Here are a few of the most common questions about getting rid of fungus gnats:
Are Fungus Gnats Harmful to Humans and Animals?
Fungus gnats aren’t harmful to animals or humans. Aside from their annoying flying habits, they cannot bite, they aren’t toxic, and they pose no danger to us at all.
How Long Do Fungus Gnats Live?
Fungus gnats have a very short life cycle consisting of four stages: egg, larva, pupa, and adult. At normal household temperatures the eggs hatch in around three days. The larvae stage lasts for around ten days before the gnat pupates and finally emerges as an adult around four days later.
Adults usually only live for a week, just long enough to deposit their eggs in the soil, and can lay around 200 eggs at a time. The whole life cycle can be completed and repeated in around three weeks, depending on the ambient temperature. Warmer temperatures will speed up the reproduction and development process. Because they can reproduce so quickly, it’s important to disrupt the life cycle at the first sign of infestation to stop the population from growing exponentially.