They don't bite or spread disease, but that doesn't make fungus gnats popular house guests. They are small insects attracted to the moist soil of indoor plants and are said to be among the most common plant pests worldwide. Although an overload of fungus gnat larvae can damage the roots of plants, this usually only occurs in greenhouses.
However, dealing with an infestation of fungus gnats can be harder than you anticipate since there is no simple product available to rid the house of them. A few control methods, like using sticky yellow paper, can trap adults readily but have no effect on eggs or larvae. Generally, you will need to use consistent management and safe prevention techniques (commonly termed integrated pest management) to live fungus-gnat-free over the longer term.
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What Do Fungus Gnats Look Like?
- Species name: Orfelia and Bradysia species.
- Physical characteristics: The adults are
tiny, flying insects about 1/16 to 1/8 inch long. They look a little like
small mosquitoes with gray bodies and long, slender legs. The wings are
usually clear. The insects have
segmented antennae that are larger than their heads.
- Wings or wingless: Adults have clear wings but are not strong fliers. You are more likely to see them on or near potted plants indoors and compost or wet mulch piles outdoors.
- Species varieties: The common Bradysia species has a wing vein shaped like a "Y."
- Life cycle and appearance: The insects are born from eggs laid in moist soil. They develop into larvae with shiny black heads and elongated bodies that stay in the soil until they pupate.
How to Get Rid of Fungus Gnats
Fungus gnats seem to appear in force overnight — one morning, there they are, flitting around the soil of your plants. If only there were a way to turn them off as quickly and easily. Easy pest solutions are available for some houseplant pests but not for fungus gnats. Killing off the adults is not difficult, but since the insects spend most of their life under the soil, you need to deal with the eggs, larvae, and pupae as well.
Since the generations of fungus gnats can be overlapping, new adult gnats appear cyclically. By regularly using a combination of steps toward prevention and eradication of the fungus gnat population, success is entirely within reach.
Use Mosquito Dunks
Mosquitoes may be the outdoor equivalent of indoor fungus gnats in terms of their ability to annoy humans, although mosquitoes also bite, causing itching and sometimes spreading disease. Anyone with outdoor water features knows the importance of preventing mosquito larvae from populating ponds and fountains. A product called "mosquito dunks" can be helpful in the fight against fungus gnats as well.
Mosquito dunks are dry pellets that contain bacteria called Bacillus thuringiensis subspecies israelensis. These bacteria destroy the larvae of many types of flying insects, including mosquitoes, fruit flies, and fungus gnats. Simply put a mosquito dunk pellet into your full watering can the night before you water your houseplants. Take it out in the morning and use the water for any houseplants attacked by the gnats. The bacteria that leeched into the water will now kill the larvae in the soil. Alternatively, you can buy the bacteria in powder form in a garden store.
Yellow Sticky Traps
In terms of eradicating adult fungus gnats, yellow sticky traps might be the most effective weapon available. They are made of a yellow note card covered in a sticky adhesive, and they can catch vast numbers of adult gnats. Look for yellow sticky traps in a garden store and then cut them into small squares. Put the small squares on the soil surface or attach them to small spikes that hold them just above the soil.
Adult gnats, attracted to the color yellow, will land on a card and get trapped there. Admittedly, the cards full of dying gnats are not attractive, but they do help resolve the problem. Flypaper ribbons can work too, but these can also get stuck to plant leaves, human hair, and furniture and provide more power than needed for the weak gnats.
Cider Vinegar Traps
If you prefer a DIY approach to pest management, you may prefer making your own fungus gnat trap of apple cider vinegar. Just pour a small amount of vinegar into a small, shallow container and add an equal amount of water so that there is at least 1/4 inch of liquid. To this, add a few drops of liquid dish soap and mix it up. Put the trap on the soil of an affected plant and check it every few days.
Raw Potato Traps
For something even simpler than the cider vinegar traps, consider putting together raw potato traps. These traps catch larvae that are living under the soil surface before they emerge as adults.
Just slice up a potato and put the cut side on the soil surface. Leave the potato chunks in place for a few days and then remove them. Assuming larvae are present in the soil, they will be attracted to the potato. Remove and dispose of the potato pieces and replace them. This is also a good way to monitor your plants for fungus gnats.
Unless you have a large estate with dozens of houseplants attacked by fungus gnats, you probably won't need or want to use chemical control. If you need something more powerful, pyrethrins or a pyrethroid insecticide (labeled for houseplant use) can provide temporary, fast-acting control.
Don't try to knock the adult gnats out of the air; spray the surface of the soil where adults typically rest. Pyrethrins have low toxicity to people and pets. Some products use a petroleum-derived synergist (piperonyl butoxide, or PBO) to increase the insecticide's effectiveness, but these are not recommended since they are more toxic to beneficial insects.
When to Hire a Pro
Generally, you won't need a professional pest company to get rid of fungus gnats. Usually, an infestation can be cleared up by the homeowner, and additional problems can be prevented with an integrated pest management approach. Pest control services can be expensive and are usually best reserved for pests that damage the property or make the home uninhabitable, such as termites or rodents.
However, if the fungus gnat population just doesn't seem to be getting smaller over weeks of effort, you may want to consider professional help. Ask neighbors and friends for a recommendation and get a clear understanding of the work to be done before you sign a contract.
How Quickly Can You Get Rid of Fungus Gnats?
Spraying adult insects with pyrethrins is probably the fastest way to get rid of adult fungus gnats but perhaps not the entire fungus gnat infestation. That's because each female fungus gnat adult, though living for only a week, can lay some 100 to 300 eggs in that time. The single most effective solution is to allow the soil to dry out between waterings, but a variety of traps used with preventive measures will probably deal with the problem most efficiently.
Where Fungus Gnats Come From
Although fungus gnats are an annoyance indoors, they are not exclusively an indoor insect. They also live outdoors, often near compost piles or areas of wet mulch or ground. Look for them outdoors in warm weather when the ground is moist. They won't be around during cold winters. All it takes is for one female fungus gnat to get inside your home through an open door or window, and the fungus gnat process begins.
Fungus gnats can also enter your home inside bags of potting soil or compost or even in the soil of new potted plants you purchase. Be sure to buy bagged soil from reliable sources. It's equally possible for them to get into your houseplants' soil if you take them out in the summer to get a little sun.
If your houseplants are attracting fungus gnats, it is likely that you are overwatering them. These pests are attracted by warm, moist, dark locations, and potted plant soil that is kept soggy or wet fits the bill perfectly. Similarly, if soil offers poor drainage or if you have plant containers without proper drainage holes, the soil will be wet enough to serve as a breeding ground for these pests.
These insects live as adults for only a week, but they multiply very quickly. A female can lay hundreds of eggs in her short life and many overlapping generations are possible. Once eggs are laid in the soil, they can hatch in as little as three days if the temperatures are warm. They then become maggotlike larvae that are about 1/4 inch long when hatched. These larvae can pupate in as little as two weeks, becoming adults that mate and continue to propagate indoors.
How to Prevent Fungus Gnats
Prevention is almost always easier than eradication of insect pests, and this is true of fungus gnats. These pests are attracted to moist potting soil, and that's where a female fungus gnat will lay her eggs. When you overwater plants, the top of the soil remains wet and attractive to the gnats. Allowing the soil surface to dry out between waterings may be the most effective way to stop the fungus gnat cycle. In fact, overwatering is very bad for houseplants in and of itself and is one of the main reasons houseplants die.
When you dry out the soil on the top of the pot, the eggs, larvae, and pupae will die. Don't kill the plant by eliminating all irrigation; just go as long as possible between waterings. Always empty saucers after watering to prevent standing water. In fact, maintaining your house to prevent water leaks and standing water anywhere is an important part of keeping any indoor insect pests at bay.
Do Fungus Gnats Cause Damage or Spread Disease?
Adult fungus gnats are annoying to have in the house but are not a danger to plants or people. In greenhouse and nursery settings, their larvae might damage roots in young plants and stunt growth in seedlings if there is a significant infestation.
Occasionally, houseplants suffer from root damage as well if the population of fungus gnat larvae is very high. This will usually only occur in moist, organically rich soil. Root damage causes wilting, but this is not a reliable indicator, as other conditions can also cause houseplant wilting, including improper irrigation, soil that doesn't drain well, and root decay fungi.
These pests are attracted to any houseplant with moist soil, especially moist soil that is rich in organic material. It is said that African violets, poinsettias, and geraniums are especially vulnerable to fungus gnat larvae feeding on roots.