Fungus gnats are some of the most common flies in homes, gardens, and greenhouses. These flies don’t pose a direct threat to humans but they represent a real problem for plants and flowers.
Fungus gnats hinder plant growth as they eat plants and other organic matter in the soil. Most fungus gnats can also spread the fungus to plants, eventually leading to their death.
Fungus gnats are small gray-black flies that feed on soil fungus around plants. These flies lay eggs in the soil and the emerging larvae feed on plants and plant roots. The larvae of fungus gnats also feed on soil fungus and organic materials.
One of the biggest problems of overwatering plants is that it develops fungus in the soil. You can identify this fungus as a white fluffy clump on top of the soil of potted plants or of flowers planted in the garden. Soil fungus can also be gray, green, or pale yellow.
Getting rid of fungus gnats involves understanding where they come from and how they multiply. Acting quickly when you see soil fungus can keep these flies away. However, it might already be too late at the time you see soil fungus around plants, vegetables, and flowers since fungus gnats might already be growing below the surface level of the soil.
What are fungus gnats?
Fungus gnats are small flies resembling fruit flies. They can be gray, black, or gray and black. These flies grow to a maximum size of 1/8 inch, but they are generally smaller as they only live about a week.
Fungus gnats are types of flies that grow and infest soil that’s affected by fungus. They can also infest compost and other organic materials where fungus grows.
They do not feed exclusively on fungus. These gnats also eat mulch, grass, plant roots, and even organic compost. As a result, fungus gnats can live, multiply, and feed on soil that has fungus on it as it already meets all of the criteria need to sustain fungus gnat life.
Are fungus gnats dangerous?
Fungus gnats aren’t dangerous to people. They only invade homes when looking for potted plants but are mainly found outdoors. These flies are dangerous to plants. It’s their larvae that are more dangerous than the adult fly as fungus gnat larvae feed on the hairs of plant roots.
This stops proper plant feeding and it may eventually kill the plant. The time these larvae take to kill a plant varies by their numbers. Plants affected by more fungus gnat larvae die quicker.
Where do fungus gnats come from?
Fungus gnats infest the soil of fungus-affected plants. These are all plants that have been watered too much. Watering too much or abiding by too frequent plant watering can quickly lead to the growth of fungus.
This fungus is what attracts these flies. Plants that live in dry soil are never affected by fungus gnats as the fungus doesn’t grow in dry soil.
What do fungus gnats look like?
Fungus gnats are often confused with mosquitoes given they have a long slender body, long wings, long black antennae, and long dark brown legs.
These gnats are mostly gray-to black. They have smoky-brown long wings and a black head.
What size are fungus gnats?
Fungus gnats grow to various sizes up to 1/8 inches even if they can be as small as 1/18 inches as adults. Food availability is one of the main criteria that influence the size of adult fungus gnats. The more food they have the faster they grow.
What color are fungus gnats?
Fungus gnats are multi-colored flies which makes it difficult to establish a single dominant color. They are seen as gray-black flies in scientific literature. The thorax and the head of the species are black while the abdomen is dark gray.
What do fungus gnats’ eggs look like?
Fungus gnat eggs have a round shape. They are off-white or yellow-white in coloring and are always laid just below the surface of the soil.
Fungus gnats cannot lay eggs anywhere else as their larvae need to feed on organic matter or the plants themselves.
The average fungus gnat lays up to 300 eggs in its lifetime. Most of these eggs hatch when they’re laid in a good spot with sufficient fungus to feed on.
What do fungus gnats eat?
Fungus gnats eat soil fungus. They do this until they reach adulthood. As adults, these flies eat organic matter, plants, and fungus.
How long do fungus gnats live?
Fungus gnats live a week, on average. Some of the most long-living fungus gnats may survive up to 3-4 weeks.
Eggs are laid in the ground and it takes up to 6 days for them to turn into larvae. Fungus gnats larvae live up to 4 days before emerging into adults.
This process can be considerably slower in lower temperatures as these flies love warm weather.
10 methods to get rid of fungus gnats
Getting rid of fungus gnats involves getting rid of all the elements they need to survive. These are food and moisture. Here’s what you can do to keep fungus gnats away from your plants.
1. Reduce plant watering frequency
Watering frequency is the most important aspect of the appearance of fungus gnats. Overwatering plants quickly lead to fungus problems.
Once a small amount of fungus has grown on the soil it can quickly spread to neighboring plants within a couple of days.
Very high soil humidity may also be caused by poor drainage as well as by the soil quality itself. Compost and other aired soil are not as prone to fungus as classic plant soil you find in the ground in the garden.
Reducing plant watering frequency is recommended for all species affected by fungus and fungus gnats. Even without these invasive flies, fungus on plant soil is an indication that you need to water plants less frequently.
2. Store plant soil in airtight containers
Stored plant soil might already have fungus in it without even realizing it. Most people store plant soil in the plastic bags it comes in. These are often out in the yard and rain might easily get inside.
Rainwater can cause fungus growth in plant soil to the extent it can’t be eliminated when you decide to use it for potted plants.
All plant soil should be stored in airtight containers. Alternatively, you may store it in zip-lock plastic bags which don’t allow any excess moisture and water to make its way to the soil
3. Cover soil in sand
One of the most elegant solutions against fungus gnats involves coring the soil in the sand. This prevents fungus gnats from laying eggs.
These invasive flies need to lay eggs in soil affected by fungus. They cannot do this when you put even a thin layer of sand on top of the soil and in your pots or the garden over the plant soil.
A thin layer of sand is sufficient to stop fungus gnat eggs from growing in the soil. 1 inch of sand can stop these flies from laying eggs, which essentially stops them from multiplying.
Too much sand on the soil limits the amount of water that gets to the root of the plant and it may be as detrimental as having no sand to protect the plants at all.
4. Spray the soil with neem oil
Neem oil is a type of essential oil known to deter flies such as fungus gnats or bugs such as aphids. This essential oil is mixed with water and then sprayed over plants to keep all flies out.
Neem oil-sprayed plants that already suffer from fungus are not at risk of being invaded by fungus gnats. Even if fungus gnats lay eggs in the ground next to these plants, the emerging larvae will not like the taste of the plant that’s been soaked in neem oil and water.
5. Fix leaky irrigation systems
Leaky irrigation systems are known for puddling water problems. A leaky system is responsible for overwatering and the spread of fungi on the soil.
These types of automated systems tend to fail under constant exposure to UV lights with cracked pipes. Water pipes need to be replaced to fix the leak and to prevent too much water from getting to the plants and soil.
6. Don’t let water puddle in saucers beneath plant pots
Puddling water is a sign that the soil sits in high humidity favoring fungus growth. This type of problem is typical in the saucers of plant pots which hold a lot of water.
If you see your saucers full of water all the time you know you are overwatering the plants. At this point, you can either water the plants less or you can consider re-potting the plants in better drainage soil or a larger pot.
7. Use sticky traps
Sticky traps against plant flies work. They attract flies by their bright colors. Once flies land on these traps they cannot escape.
You can add sticky fly traps around the plants in your garden and even in the pots of indoor plants. Fungus gnats are very common in greenhouses which means these traps should always be present in greenhouses as well.
Sticky traps work if they have a bright color. They are often inspired by carnivorous plants which are also known for their vivid coloring and sticky texture.
As carnivorous plants, sticky traps attract by color and trapped by a glue-like substance. You need to add multiple sticky traps around the garden if you plan on catching all fungus gnats.
One of the limitations of this method is that sticky traps can get full quickly. All types of flies and bugs get stuck on the surface of the trap eventually covering it completely. These traps need to be swapped frequently.
8. Cover drainage holes on the lawn and around the garden
Fungus gnats are very good at picking out the right location to lay eggs in. Since they can lay as many as 300 eggs per week, fungus gnats need to find multiple places to lay the eggs in.
This is why they often migrate to drainage holes, areas of high humidity. You need to cover the drainage holes in your backyard to prevent this.
Drainage holes are common around the driveway and on the patio. Make sure to cover all of them as fungus gnats can fly short distances to find a better place to lay eggs in.
9. Limit or avoid using fertilizer
Plant fertilizer is sometimes responsible for fungus growth. People tend to use too much fertilizer which enables organic material buildup, high humidity, and the growth of fungus.
One common planting mistake is to use undiluted fertilizer. This is known to cause immediate fungus growth in moist soil. You should refrain from using too much soil fertilizer.
10. Use vinegar traps
Vinegar traps are a common solution against adult fungus gnats. These are simple traps with vinegar, water, and diluted soap which kill these flies.
You need to add vinegar and soap to a tray with water. This tray is then placed next to plants. Humidity attracts these flies and the vinegar inside kills them.
The solution has its limits as these flies don’t lay eggs in water. They lay eggs in the ground which means fungus gnat eggs survive even if all adults are killed.
Vinegar traps should be used with other preventive methods such as reducing watering frequency.
Fungus gnats are small mosquito-like flies that feed on and reproduce in plant fungus on the ground. This fungus is the result of high soil moisture.
Overwatering is one of the main reasons fungus grows on plant soil. This can be accidental as in the case of water leaks or on purpose as in the case of overwatering plants.
Fungus gnats are known for their attraction to plant fungus which they also spread to other plants. You can eliminate them by adding sand on top of the fungus-affected ground as they cannot lay eggs in the sand.
Fungus gnats can be completely prevented by reducing plant watering frequency. Dry soil or partially moist soil doesn’t contribute to the growth of fungus.