Identification and Life Cycle
The Fungus Gnat, Bradysia spp., is often found in homes and offices near houseplants often running or resting on soil or leaves. They are tiny non-biting flies, that resemble mosquitoes and can be a nuisance in large numbers.
The adults are approximately 1/8 – inch long and are black or dark brown with long thin legs and an antennae. Their wings have a distinctive “Y” shaped vein. The larvae, which can be found 1 to 3 inches below the top of the soil, are worm-like in appearance with a translucent body and a black head.
They have four stages of metamorphosis; egg, larva, pupa, and adult. The adult female can lay 100-150 eggs and can hatch in as little as four days. The larva develop quickly and are fully grown in 2-3 weeks feeding on plant roots and leaves that are touching the soil’s surface. They then pupate in or on the material of which the eggs were deposited. The adults will emerge in about 7 days and will feed on fungai, algae and decaying plant matter. They have a life span of 7 to 10 days.
Habits and Damage
Fungus gnats are attracted to fungal growth. The adults lay their eggs in organic matter and moisture rich environments such as compost piles, leaf litter and potted plants. The adults do not carry human diseases, but cause damage by transmitting pathogens to healthy plants which can cause root rot and other plant fungal diseases.
The larvae feed directly on roots, stems, and seedlings and can severely damage plants. Larvae also leave behind slime trails, which can also attract slugs and snails.
Activity is usually higher during the late fall and winter months. This may be due to houseplants that are maintained outdoors during the summer, and if colonized by the gnats, will increase in numbers indoors during cold months because of warmer temperatures and humidity. They are weak fliers, flying short distances and can be seen accumulating around window frames.
Common Signs Of Fungus Gnat Activity
- Indoor visual sightings. The size of a gnat is tiny. They are noticeable in large numbers flying out of the soil when you water your indoor potted plants.
- Outdoor visual sightings. Gnats breed in decaying organic matter and can be seen flying out of leaf mold, organic mulch, grass clippings and compost.
- Slime trails. These look like tiny, narrow “slime” trails on soil surfaces that resemble slug or snail trails.
- Larvae. These worm-like insects are buried under the soil and chew on the roots of plants. A severely infested plant may drop foliage or turn a pale color.
Prevention and Management
The best strategy for fungus gnats is prevention. An Integrated Pest Management (IPM) program can help effectively control gnats by offering an appropriate treatment for your level of infestation.
Outdoor fungus gnat problems may result from over-wet soil conditions, or diseased roots. The larvae feed on decaying matter and healthy or diseased roots in soil. The gnats can live in soil fungi, algae under benches, and on damp mossy benches.
Indoor infestations can be the result of over-watering your houseplants, or from developed eggs already in the soil. Damage to indoor plants ranges from restricted growth, discoloring, wilting and even plant death.
The following steps can be taken to help prevent or reduce a potential problem with fungus gnats.
- Do not overwater your plant seedlings in the spring. This can cause fungal growth and attract fungus gnats.
- Avoid overwatering your houseplants. Allow the soil to dry between watering, especially the top 1 to 2 inches.
- Carefully inspect the soil of plants before bringing them into your home. Re-pot if necessary.
- Monitor your plants by using yellow sticky traps around the base of the plant. The yellow color attracts gnats and will capture the adults on the adhesive.
- Biologically monitor and trap larvae by cutting a chunk of potato and placing it on top of the soil near the plant. The larvae will come to the surface and eat the potato, and it can then be thrown away.
- Clean up free-standing water and repair any plumbing or irrigation systems leaks that can cause fungal growth on property.
- Minimize organic debris around your home such as compost piles, decaying leaf & grass clippings and avoid excessive fertilizing with manure, blood meal or similar organic materials.