Fungus gnats (Bradysia species) are small, mosquito like insects
...that commonly infest the growing medium in which houseplants and outdoor potted plants grow. Often, when outdoor plants are brought indoors for the colder months, fungus gnats harbored in the potting mix are brought indoors, where they can quickly infest the soil of other houseplants.
Adult fungus gnats are tiny flying pests that can swarm like an annoying black cloud whenever an infested houseplant or outdoor potted plant is moved. Anyone who has grown houseplants has probably dealt with fungus gnats.
During their short seven to ten day life span, adult female fungus gnats can lay up to 300 eggs each in potting soil. Eggs typically hatch into larvae within six days and the larvae feeds on the organic matter in the soil and the tender plant roots. The larvae feed for 12 – 14 days before changing into the pupal stage. In this stage all feeding ceases and after 4-6 days the pupa change into adults to complete the life cycle. Adult fungus gnats typically only consume water and flower nectar but during these last 8 to 10 days of life the breeding stage occurs and the females lay their eggs, starting the whole life cycle again. Fungus gnats can be difficult to control because they thrive in moist potting soil and it's hard to separate them from the mixture. Fungus gnats lay their eggs in potting soil and once the eggs hatch, and the larvae live in the top 2-3 inches. In the larval stage, the fungus gnat maggots may cause plant damage by eating tender plant roots. After about two or three weeks in the potting soil, the fungus gnat maggots become the tiny black adult gnats that fly erratically when a potted plant is moved.
So What Do They Look Like?
Adults are tiny, grayish black, flies loosely resembling mosquito (1/8" long) with long legs and transparent wings. Often mistaken for fruit flies, they are not good at flying and will usually flee from potted plants during watering. Larvae or maggots (1/4") have shiny black heads and an elongated, somewhat whitish body. Females lay their tiny eggs in that attractive moist organic material or soil.
Do They Live Long?
Fungus gnats exist through four stages of life. They go from egg (4-6 days) to larva (12-14 days) to pupa (4-6 days) to adult (8-10 days). At 75ºF, the eggs hatch. The complete life cycle from egg to adult lasts around 4 weeks depending on temperature. A generation can be produced in about 17 days. The warmer it is, the faster their cycle and the more generations that will be produced. Due to their short life cycle, they have many overlapping generations throughout the year.
How to Control Them?
Fungus gnats are attracted to rich organic material such as most potting soils. Their larvae thrive in this soil, especially when it is excessively moist. An important step in reducing fungus gnats in houseplants is to let the potting soil dry between watering. As the fungus gnat eggs and larvae need moisture to survive, allowing the soil to dry will kill many of the eggs and larvae. Letting the potting soil dry out between watering helps control fungus gnats.
Sometimes though, more aggressive measures are needed to control the pests. Typically many homeowners have relied on chemical insecticides to kill both adults and larvae. While effective, there are consumer concerns with treating indoor plants with chemical insecticides, especially around children and pets.
Fortunately, there are now natural, non-chemical solutions that control fungus gnat infestations:
- Yellow Sticky Traps – often used for monitoring insect populations, yellow sticky traps placed around the plants close to the soil can catch many flying fungus gnats and helps prevent more eggs from being laid.
- Horticultural Oil – products like Summit Year-Round® Spray Oil can be used to kill the adults that are congregating on the plants. Horticultural Oil kills soft bodied insects like fungus gnats by blocking their spiracles (breathing tubes), but is harmless to beneficial insects, people, and pets when used as directed. It kills on contact with no residual so treatment must be continued every few days.
- The most effective non-chemical solution is to use Summit’s Mosquito Bits®, a product that contains Bti (bacillus thurengiensis israelensis), a naturally occurring soil bacterium that is a very specific toxin to mosquito larvae, black fly larvae, and fungus gnat larvae. When made into a “tea” as directed and used to water the plants, the Bti goes into the soil and is consumed by the hungry fungus gnat larvae which then die. Continued use over a three week period generally breaks the fungus gnat life cycle and eliminates the problem.
What Kind of Damage Do Fungus Gnats Create?
If you have fungus gnats, you may notice sudden wilting, poor growth, and yellowing in your plants. If you have a severe infestation, you could lose a large portion of your plants.
These plants take fungus gnat infestations especially hard:
- Geraniums
- African violets
- Carnations
- Poinsettias
Unlike other gnats, adult fungus gnats don’t bite people. They're just a nuisance. The larvae, however, is another story. Since the larvae eats roots and other organic matter in the potted soil, nothing good will come from their unwanted presence in your home. Follow our treatment suggestions to keep your plants free of fungus gnat damage.
How to Control Them?
Fungus gnats are attracted to rich organic material such as most potting soils. Their larvae thrive in this soil, especially when it is excessively moist. An important step in reducing fungus gnats in houseplants is to let the potting soil dry between watering. As the fungus gnat eggs and larvae need moisture to survive, allowing the soil to dry will kill many of the eggs and larvae. Letting the potting soil dry out between watering helps control fungus gnats.
Sometimes though, more aggressive measures are needed to control the pests. Typically many homeowners have relied on chemical insecticides to kill both adults and larvae. While effective, there are consumer concerns with treating indoor plants with chemical insecticides, especially around children and pets.
Fortunately, there are now natural, non-chemical solutions that control fungus gnat infestations:
- Yellow Sticky Traps – often used for monitoring insect populations, yellow sticky traps placed around the plants close to the soil can catch many flying fungus gnats and helps prevent more eggs from being laid.
- Horticultural Oil – products like Summit Year-Round® Spray Oil can be used to kill the adults that are congregating on the plants. Horticultural Oil kills soft bodied insects like fungus gnats by blocking their spiracles (breathing tubes), but is harmless to beneficial insects, people, and pets when used as directed. It kills on contact with no residual so treatment must be continued every few days.
- The most effective non-chemical solution is to use Summit’s Mosquito Bits®, a product that contains Bti (bacillus thurengiensis israelensis), a naturally occurring soil bacterium that is a very specific toxin to mosquito larvae, black fly larvae, and fungus gnat larvae. When made into a “tea” as directed and used to water the plants, the Bti goes into the soil and is consumed by the hungry fungus gnat larvae which then die. Continued use over a three week period generally breaks the fungus gnat life cycle and eliminates the problem.
What Kind of Damage Do Fungus Gnats Create?
If you have fungus gnats, you may notice sudden wilting, poor growth, and yellowing in your plants. If you have a severe infestation, you could lose a large portion of your plants.
These plants take fungus gnat infestations especially hard:
- Geraniums
- African violets
- Carnations
- Poinsettias
Unlike other gnats, adult fungus gnats don’t bite people. They're just a nuisance. The larvae, however, is another story. Since the larvae eats roots and other organic matter in the potted soil, nothing good will come from their unwanted presence in your home. Follow our treatment suggestions to keep your plants free of fungus gnat damage.
