Fungus gnats are found inside homes and are commonly associated with overwatered houseplants. The adults are short-lived and harmless. But while the larvae develop, they can damage the roots of plants.
Adult fungus gnats are tiny, flying insects that are often mistaken for small flies or fruit flies. They belong to the Sciaridae family and go through multiple generations every year. They are more of a nuisance than anything else, though they may spread Pythium, a plant pathogen that often kills large numbers of emerging seedlings.
What is a fungus gnat?
A fungus gnat is a small fly that infests potting mix and other media used to grow houseplants in containers. They are tiny, only about ⅛ of an inch long, and have delicate bodies with long legs and antennae.
If you look carefully with a magnifying glass, you will see a distinct rounded Y-shaped pattern near the tips of their front wings. While they can be a nuisance when they multiply, adult fungus gnats are harmless and don’t bite or feed. They may, though, drink water that settles on plant leaves or on the surface of potting mix.
The larvae of fungus gnats are translucent and wormlike, with black, capsule-like heads. They feed on fungi, algae, and plant roots growing in containers.
Diana Alfuth and P.J. Liesch from the University of Wisconsin-Madison Division of Extension explain the basics in an article, Fungus Gnats on Houseplants.
Fungus gnat females lay as many as 200 eggs in clusters on the surface of moist potting soil where potted plants are growing. They usually hatch within 3-6 days and can develop from an egg to adult fungus gnats in 3-4 weeks.
The fungus gnat larvae go through four instars or stages before they pupate near the surface of the soil or other organic matter 1-2 weeks later. It only takes another 4-5 days for the adult fungus gnats houseplants have harbored to emerge.
The adults-only live for about a week, giving the females just enough time to lay hundreds more eggs. If your potting mix is moist and contains lots of peat moss, this will attract the females.
What causes fungus gnats?
The most common cause of fungus gnats is over-watering indoor plants. However, they may also infest homes with leaky pipes and other moisture problems where algae and fungus are present.
In a fact sheet, Fungus Gnats as Houseplant and Indoor Pests, entomology specialists, W.S. Cransaw of the Colorado State University Extension and R.A. Cloyd from Kansas State University, talk about the moisture levels in growing media that attract fungus gnats.
They say that as the growing medium or potting mix ages or degrades it retains more moisture. This attracts the adult gnats that then lay the eggs that produce fungus gnat larvae.
At the same time, when temperatures cool down and the days get shorter in the fall and in winter, plant growth slows down. This means that you need to water your plants less.
If you don’t change your watering practices during the fall and winter your potting mix will stay moist. It stands to reason, then, that conditions will improve for fungus gnats to thrive.
Alfuth and Liesch warn that fungus gnats can be more of a problem if your containers have been outdoors during the spring and summer months.
When you bring them back indoors for the winter, it can take 3-4 weeks to get them in check. But usually, modified watering will do the trick.
If you suddenly have fungus gnats after buying a new pot plant, there’s a good chance that it was infected when you bought it. In many ways, that’s going to be more irritating than the gnats themselves!
How to identify fungus gnats
Fungus gnats look very similar to fruit flies and any other sorts of gnats. They tend to fly more slowly than fruit flies.
They are usually about ⅛ inch in length, sometimes smaller. Fungus gnat larvae are about the same length.
You can monitor for fungus gnat larvae quite easily. All you do is place slices of potato on the surface of your potting mix.
If there are larvae in the soil, they will come up to the surface to feed. If they are there, you will find maggots (the larvae) after 3-4 days.
Also, remember that certain houseplants are more susceptible to infestation by fungus gnats than others. They include African violets, geraniums, inch plants, peace lilies, and spider plants.
How to get rid of fungus gnats
The best way to control and ultimately rid fungus gnats from houseplants is to ensure that your growing medium dries out totally between watering. This is, in any case, good practice for many plants, including succulents.
Additionally, you can add a ½-1 inch layer of fine gravel or sand on top of your growing media. This will help to keep the mix dry and make your potting mix less attractive for laying fungus gnat eggs.
Traps to catch fungus gnats
Non-chemical sticky traps can be effective while the fungus gnats are still around. By catching and killing the adults, you’ll ensure they won’t be around to lay more eggs.
But any that escape will be sure to lay their eggs. And you’ll still need to treat your soil.
Apple cider vinegar traps made with equal parts of cider and water, plus a few drops of liquid soap, work well for fruit flies. Some say they don’t work for fungus gnats, but it’s worth a try. You don’t need much, just about ¼ inches in a small container.
Also, you’ll need to keep changing the mixture to make it effective. And, like the sticky traps, they only work by trapping adults.
Insecticides and biological control agents to kill fungus gnats
You can use specific biological control agents or insecticides to control fungus gnats. A caveat from Cransaw and Cloyd is that different approaches are required depending on the life stage of your fungus gnats.
For example, biological products formulated to control fungus gnat larvae in the soil won’t kill eggs, the pupae, or the adult fungus gnats.
A diluted hydrogen peroxide soak will kill the larvae. Just mix the hydrogen peroxide 1:3 or 1:4 with water and drench your pot.
But there are two problems. Hydrogen peroxide will kill all the beneficial bacteria and microorganisms in your soil. And you’re making your soil very, very wet. Say no more!
Generally, it’s best to avoid chemical insecticides to control fungus gnat infections. Some that are pyrethroid-based may be used on houseplants. But short-persisting contact insecticides that contain pyrethrins, oils, neem, and soaps won’t control fungus gnats long term.
Also, if you don’t control watering, they will not be effective long term.
How to prevent fungus gnats from coming back
A good management program will usually prevent fungus gnats from returning once you are rid of fungus and algae off them. As mentioned above, let your potting mix or other growing medium dry out between watering.
Do the finger test by pushing your finger into the top 1-2 inches of soil. It must be scorched before you water it again.
Also, check your drip tray. If this is full of water, be sure to empty it.
This is important because a dry growing medium will decrease the number of eggs that survive and hatch into fungus gnat larvae. It also makes the potting mix less attractive to females looking for somewhere to lay their eggs.
It is also a good idea to re-pot your plants now and then. This is particularly important when the soil or growing medium breaks down and retains more moisture.
Also, if you find containers that have decaying roots or bulbs in them, throw them out. Decaying bulbs and plant roots are a great food source for the gnat larvae.
Are fungus gnats harmful to humans?
No, adult fungus gnats do not harm humans in any way – except to irritate them! So, if you’ve got a bad temper, take steps to get rid of them immediately.
But they don’t bite like mosquitoes and some other insects. Another important factor is that they don’t spread disease.
Basically, they are harmless.
Ironically, even though the larvae can chomp plant roots, they don’t do much damage in the scheme of things either.
Fungus gnats are minor houseplant pests that can harm plants but won’t harm humans. They’re about the same size as small flies and they can be really irritating.
Adult fungus gnats live for a week, at most, but the lifecycle of these insects is continuous. No sooner has the adult laid eggs in your houseplant soil, than eggs laid earlier are already hatching.
While there are ways to control fungus gnats with chemicals and biological treatments, the key is good management when you recognize that the most common cause of these irritating insects is overwatering, you’re on your way to getting rid of them.