Fungus gnats: They’re loved in the garden for helping your compost, but they’re often just plain annoying in the house. In this post, you’ll learn not only how to recognize and prevent them but also how to fight fungus gnats.
Detect fungus gnats
Although fungus gnats (Sciaridae) are mosquitoes, they are entirely harmless to humans. They do not bite and do not transmit disease, at least not to humans.
While they are simply annoying as adults, the larvae of the fungus gnat can cause severe damage, especially to young plants.
In total, there are over 1,800 species of fungus gnats. In Europe, however, only about 600 of these species are found. The fungus gnat owes its name to its dark body and cloudy wings.
A fungus gnat grows up to 7 mm long. In addition, six relatively long legs mate.
That an infestation has set in is often recognized by the fact that suddenly small black flies fly through the flat. But also, when watering, an infestation is quickly noticed because droves of fungus gnats rise from the flower pots.
The symptoms and general signs
In many cases, you cannot detect a fungus gnat infestation on the plant itself. However, if it is already weakened, still a young plant, or if the infestation is extensive, weak growth and stunted shoots may result.
That is due to the larvae, which not only feed on dead tissue but also on its living roots.
If an infestation on seedlings and cuttings is not treated, it may mean their end.
Finding fungus gnats on the plant
Unlike most pests, fungus gnats do not sit directly on the plant. They romp in and on the substrate. While adults sit on the substrate and rise into the air when threatened, larvae live in the substrate. Therefore, they often go undetected.
Conditions that fungus gnats love
Fungus gnats and their larvae love moist substrate. Therefore, they are often a concomitant of root rot.
Room temperature and humidity provide a good foundation for the fungus gnat habitat.
The life cycle of a fungus gnat
With just 28 days on average, the fungus gnat has a short life. During this time, it goes through the seven stages of its development. Four of these stages take place as larvae.
The fungus gnat’s life starts with an egg on a moist substrate. The egg itself is about 0.2 mm long and 0.1 mm wide. It has a whitish-to-yellow coloration. After 4 to 6 days, a larva hatches from the egg.
- Larval stages 1-4
The newly hatched larva is transparent to whitish and paired with a black head. It makes its way towards the roots, eating and digesting not fully decomposed components of the substrate. For this reason, it is popular in the production of compost.
If the larva does not find plant debris or becomes dehydrated, it attacks the plant’s roots.
The larva goes through four stages. With each stage, it grows slightly longer and darker. In the end, it is about 5.5 mm long, and after the fourth stage, it begins to pupate. Between the hatching of the larva and the pupation, 12 and 14 days pass.
When ready to pupate, the larva migrates to the upper 3 cm of the substrate. That makes it easier for the adult to crawl out of the substrate. The pupa is about 2.5 mm long. Colour-wise, it closely resembles the substrate. After 3 to 6 days, an adult fungus gnat hatches from the pupa.
- Adult animal
As an adult, the fungus gnat lives just 4 to 5 days. It mates and lays up to 300 eggs in the preferably moist substrate during this time. That is how it initiates the new cycle.
Here you will find a video depicting the life cycle of a fungus gnat:
Fight fungus gnats
To fight fungus gnats, you can choose between three control categories. You can use home remedies if you don’t like chemical or ready-mixed control agents.
If you want to fight fungus gnats naturally, beneficial insects are your thing! If neither of these options appeals to you, you can use ready-made plant sprays and water agents.
Home remedies against fungus gnats
Yellow sticky boards
Yellow sticky boards are among the most popular control methods against fungus gnats. They are suitable not only for reducing colonies but also for checking whether there is an infestation in the home. Some plant lovers, therefore, use them all year round.
Yellow sticky traps* work by attracting the fungus gnat by their colour alone. It is not yet certain here why the colour yellow works so well. However, it is most likely that the fungus gnat thinks it is a yellow flower and, therefore, food or light for mating. If the fungus gnat lands on the yellow board, it will stick to the adhesive layer and die there.
Matches are also suitable as a home remedy. In their heads, they contain sulphur, which attacks the larvae. Therefore, stick matches with their heads first into the substrate.
With watering, the sulphur is distributed in the substrate.
A fly screen* can be used not only as a preventive measure. If you discover an infestation on only one plant, you can put the fly screen over the pot and thus limit the infestation to one plant. By the way, the same works with a lady’s stocking.
Mix 1 ml of neem oil* into every litre of water. A little washing-up liquid helps to combine the two substances. Then apply the solution to the plant and substrate with a spray bottle. Depending on the severity of the infestation, you can repeat the procedure after 14 days.
You can use coffee grounds for more than just fertilizer. Like sand, dried coffee grounds make it more difficult for eggs to lay. Spread the coffee grounds on the soil of the affected plant.
Although it may sound aggressive initially, you can also use 3% hydrogen peroxide* to control fungus gnat larvae. Mix 1 part hydrogen peroxide with four parts water and then water the affected plants with this mixture.
The hydrogen peroxide attacks the eggs, larvae, and pupae living in the substrate. The roots, on the other hand, will survive the hydrogen peroxide.
Repeat the application weekly until fungus gnats are no longer visible.
Pesticides against fungus gnats
Home remedies don’t appeal to you, and the idea of having beneficial insects in your home doesn’t appeal to you? Then pesticides and plant protection products can be a solution for you. Again, you can choose between different methods:
Plant sprays are ready-made mixtures that can be used against various pests. Often a No products found. is effective against more than one pest.
Spray it according to package directions on the plant or substrate.
In addition to sprays such as plant sprays, you can also use water agents. These are mixed into the water and put directly into the substrate. There it attacks the larvae and pupae.
Popular agents are No products found. and the all-time favourite No products found..
A somewhat atypical approach that I was privileged to learn from a rarity grower is to use ARDAP Vermin Spray*. He applied this to the substrate in the event of an infestation. Due to the long-term effect of up to 6 weeks, only one application is often necessary. Since I tested the spray, it has been my go-to remedy for pests.
By the way, ARDAP also has a non-chemical version of the spray* in its range. Unfortunately, I could not test this yet.
Beneficial insects against fungus gnats
The use of beneficial insects is probably the most natural control method. By introducing predators, fungus gnat populations are decimated. Nematodes and predatory mites can be used for fungus gnats.
Nematodes against fungus gnats
If you want to use nematodes against fungus gnats, only so-called No products found. (Steinernema feltiae) are suitable. These 0.5 mm long nematodes are added to the substrate via the irrigation water. There they parasitically infest the fungus gnat larvae.
Predatory mites against fungus gnats
Both Macrocheles robustulus and Hypoaspis miles are suitable against fungus gnats. The predatory mites are scattered on the substrate of the affected plant. There they then begin their food search.
The prey of Macrocheles robustulus includes eggs, larvae, and pupae. Hypoaspis miles, however, only fights larvae. In return, they last longer than Macrocheles robustulus, with a lifespan of up to four weeks.
Beneficial insects against fungus gnats – an overview
|SF Nematodes||Steinernema feltiae||min. 12 °C||Eggs, larvae|
|MR Predatory mite||Macrocheles robustulus||15 °C-30 °C||Eggs, larvae, pupae|
|HM Predatory mite||Hypoaspis miles||15 °C-30 °C||Larvae|
Prevent fungus gnats
Even better than fighting fungus gnats is preventing them from reproducing in the home in the first place. Therefore, here are some helpful methods to avoid fungus gnats.
Allow substrate to dry superficially
The easiest way to prevent fungus gnats is to let the substrate dry out between watering. That makes the substrate less interesting for the sciarid flies, making it more difficult for them to lay their eggs.
A layer of sand prevents the fungus gnats from effectively laying eggs. They do not manage to get through the dense sand to the substrate. This also works the other way around. Thus, a 1 to 2 cm layer of sand can make it difficult for adult fungus flies to ascend from the substrate.
Another way of prevention is not to let the fungus gnats get to the plant and into the home in the first place. This is best accomplished with fly screens*. They prevent insects from entering the house. Depending on the model, they are also suitable for households with cats.
Check potting soil
Fungus gnats are often brought into the home via potting soil. If the potting soil is stored outside the manufacturer’s or vendor’s facility, fungus gnats may take the opportunity to lay their eggs.
The larvae live in the soil and usually pupate indoors. So check for pupae and larvae before using the potting mix. To be safe, you can also treat the potting mix with a pesticide before using it.
Quarantine new plants
In addition to potting soil, new plants are also a carrier for pests. Therefore, I can only advise quarantining new houseplants at first. If no diseases or pests appear after one or two weeks, you can put them in their final location.
What to do in case of recurring fungus gnat infestations?
If fungus gnats appear at regular intervals, likely their cause has not been treated, but rather its only symptoms. Therefore, try to find out where the sciarid flies are coming from.
Is there perhaps open potting soil near the home? Are you making compost near a frequently open window? Do you treat one plant at a time for fungus gnat infestations?
Causes are often individual, so they take a bit of research to find. For example, stick yellow tags in all flower pots to identify the source and other infested plants.
Fungus gnats, or sciarid flies, are among the least dangerous pests. They usually cause minor damage but are all the more annoying. Their control helps beginners to gain their first experience in pest control.
- Leaf lice (Nematodes)
- Leaf miner
- Lily Leaf Beetle (Lilioceris lilii)
- Root mites
- Scale insects
- Spider mites
- Thick-mouthed weevil
- Thrips (Thunderbug)
Let me answer your questions!
How do fungus gnats get to houseplants?
Fungus gnats are often carried into the home via new potting soil or freshly purchased houseplants. However, they can also fly into the flat by themselves during the airing.
(2022, June 21). Trauermücken. Wikipedia. Retrieved March 16, 2022, from https://de.wikipedia.org/wiki/Trauerm%C3%BCcken
(2022, September 13). Fungus gnat. Wikipedia. Retrieved March 16, 2022, from https://en.wikipedia.org/wiki/Fungus_gnat
(n.d.). Fungus Gnats. CFREC. Retrieved March 16, 2022, from https://mrec.ifas.ufl.edu/lso/entomol/ncstate/fly1.htm
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