Fungus Gnats' Secret Hiding Places

Everyone knows or eventually discovers that fungus gnats live in damp houseplant soil. Unfortunately, they are not always polite enough to stay there.

Fungus gnat larvae develop in damp places containing fungi, algae, decaying roots or other organic material. Adults are weak flyers, but they readily wander toward moisture, light and interesting smells. This means the place covered in adult gnats is not necessarily where the infestation began.

Recycling and Drink Containers

Do not underestimate the recycling bin. In our experience, fungus gnats are attracted to:

• Old pop cans
• Beer cans
• Juice boxes
• Orange juice bottles and cartons
• Surprisingly, ordinary water bottles

Orange juice residue seems to be one of the worst offenders. However, even a water bottle can attract them. Fungus gnats love moisture, and a bottle containing a small amount of water can remain damp for a very long time.

Fruit flies are usually what you expect to go c-r-a-z-y around sugary or fermenting drink residue, but fungus gnats love these containers too. Rinse drink containers, drain them completely and keep the recycling bin clean and dry.

Wet Carpet, Mats and Fabric

Damp carpet can become an easily overlooked fungus gnat habitat, especially if it contains soil, fertilizer, plant debris, drink residue or ordinary dirt that supports fungal growth.

Inspect:

• Carpet beneath plant displays
• Entrance mats
• Fabric grow mats
• Carpet near fish tanks
• Carpet around toilets and sinks
• Areas beneath leaking windows
• Carpet underneath reservoirs and shelving
• Any area affected by a plumbing leak or spill

Carpet may feel dry on top while the underlay remains wet. If gnats continually gather over one patch, inspect underneath it rather than cleaning only the visible surface.

Fish Tanks and Aquariums

Fungus gnats are frequently noticed near fish tanks. The open water attracts moisture-seeking adults, while algae, biofilm, fish food and damp organic debris around the aquarium may provide more suitable habitat.

Check:

• Wet aquarium lids and rims
• Algae growing above the waterline
• Damp filter media
• Old fish food beneath the tank or stand
• Water spilled inside the aquarium cabinet
• Wet carpet beneath or behind the aquarium
• Damp wood, cardboard or particleboard around the stand
• Houseplants growing from or beside the aquarium
• Dead roots and plant material in aquaponic systems

The aquarium water itself may attract adults without being the place where larvae are developing. Look especially closely at permanently wet edges, filters, algae and debris surrounding the tank.

Reptile, Amphibian and Isopod Enclosures

Reptile enclosures and other humid habitats can be ideal for fungus gnats, particularly when they contain damp organic substrate.

Possible hiding places include:

• Damp sphagnum moss
• Rehydrated coco coir
• Bioactive soil
• Decaying leaf litter
• Wet wood and cork bark
• Uneaten food
• Fungal growth beneath hides
• Drainage layers containing old water
• Water spilled beneath the enclosure
• Isopod and springtail cultures

A clean, healthy bioactive enclosure deliberately contains moisture and decomposing organic material. Unfortunately, fungus gnat larvae appreciate many of the same conditions.

Avoid applying unapproved pesticides to an occupied enclosure. Treatment choices must be safe for the animal and any beneficial organisms living in the substrate.

The Bottom of Plant Pots

The soil surface may appear dry while the bottom of the pot remains wet. This is particularly common with:

• Large pots
• Decorative pots without drainage
• Self-watering planters
• Plants standing in deep saucers
• Pots inside waterproof sleeves
• Compacted or decomposed growing media

Fungus gnat larvae feed on fungi, algae, decaying material and fine roots. Correcting poor drainage and allowing the upper portion of the growing medium to dry can make the pot less hospitable. 

Under Pots, Saucers and Trays

Remove the saucer and inspect underneath it. A nearly invisible mixture of water, fertilizer residue, dead roots and spilled potting mix can remain wet indefinitely.

Check:

• Beneath plant saucers
• Under propagation trays
• Inside tray grooves
• Beneath flood tables
• Under rubber or capillary mats
• Between greenhouse benches and walls
• Under pots sitting directly on shelves

A tiny amount of wet debris may be enough to sustain larvae.

Forgotten Bags of Growing Media

An opened bag of growing medium can become infested before the product ever reaches a plant.

Inspect bags of:

• Potting soil
• Peat moss
• Coco coir
• Compost
• Worm castings
• Seed-starting mix
• Damp sphagnum moss
• Used growing media
• Bark mixes containing decomposed fines

Keep unused media closed, covered, dry and off damp floors. Organic growing media can become infested when it gets wet during storage.

Propagation Domes and Seedling Trays

Propagation areas offer nearly perfect fungus gnat conditions: warmth, constant moisture, tender roots and limited airflow.

Look underneath:

• Seedling inserts
• Rooting cubes
• Peat pellets
• Rockwool
• Humidity domes
• Clone collars
• Capillary mats
• Bottom-watering trays

The exposed surface may look clean while water, algae and dead roots collect underneath the cells.

Hydroponic and Aquaponic Equipment

Fungus gnats do not require a traditional pot of soil. They can survive around moist roots, algae and decomposing material in hydroponic and aquaponic systems.

Inspect:

• Algae-covered rockwool
• Wet clay pebbles containing root debris
• Net pots
• Exposed root crowns
• Irrigation channels
• Filters containing plant debris
• Drip emitters
• Spaces underneath reservoirs
• Dead roots caught in pumps or plumbing
• Leaking fittings and hoses

Algae, prolonged wet surfaces and decaying organic matter are important pest habitats.

Drains and Sink Overflows

A clean, functioning drain is not normally the first place fungus gnats breed. A drain containing wet potting mix, algae, roots or decomposing plant matter is another story.

Inspect:

• Floor drains near plant displays
• Greenhouse drainage channels
• Sink strainers
• Sink overflow openings
• Garbage-disposal splash guards
• Floor cracks beside drains
• Condensate drains

Physically scrub away organic buildup. Pouring a product down the centre of the drain may leave the film coating its sides untouched.

Fuzzy, moth-like insects are probably drain flies. Small hump-backed flies that run quickly across surfaces may be phorid flies. Fungus gnats love dirty drains too. 

Toilets and Plumbing Leaks

A clean, regularly used toilet is unlikely to produce fungus gnats. Investigate more closely when:

• The toilet is rarely used
• Organic residue remains beneath the rim
• Water leaks around the base
• The wax seal has failed
• The floor or subfloor remains damp
• Nearby carpet or mats stay wet
• A neighbouring floor drain contains organic buildup

Flies repeatedly appearing around the base of a toilet may indicate a moisture problem that deserves attention.

Compost and Organic Fertilizers

Compost and worm bins naturally contain moisture, fungi and decomposing material. Fungus gnats may also appear around:

• Compost tea equipment
• Fermenting plant extracts
• Damp alfalfa or kelp meal
• Used tea leaves or coffee grounds
• Spilled organic fertilizer
• Dirty measuring cups
• Garbage containing vegetable scraps

A dry bag of fertilizer is not automatically a gnat problem. Once organic material becomes wet and begins decomposing, however, it can become suitable habitat.

Dead Leaves and Decaying Roots

Look beneath dense foliage and inside crowded containers for:

• Fallen flowers and leaves
• Dead seedlings
• Failed cuttings
• Rotten bulbs or corms
• Mushrooms
• Diseased roots
• Plant debris beneath benches
• Roots trapped in propagation equipment

Fungus gnat larvae primarily consume fungi and decomposing material, but they can also feed on tender roots. Algae, fungi and plant roots are among their principal foods.

Algae in Unexpected Places

Algae is not always bright green. It may appear as a brown, black or nearly invisible slippery film.

Check:

• Inside watering cans
• Around humidifiers
• Beneath capillary mats
• Inside clear tubing
• On wet concrete
• In condensate trays
• Under leaking irrigation fittings
• Inside buckets
• Around permanently wet windowsills

Material Brought Indoors

Fungus gnats naturally live outdoors in moist leaf litter and decaying vegetation. They can enter with:

• New plants
• Outdoor plants brought inside
• Garden soil
• Collected moss
• Compost
• Used pots
• Leaf litter
• Previously used growing media

Fungus gnats commonly breed outdoors in leaf litter and other decomposing organic matter.

Windows, Lights and Computer Screens

Finding adults somewhere does not prove they bred there. Adult fungus gnats often gather around:

• Windows
• Grow lights
• Computer screens
• Illuminated signs
• Light-coloured walls
• People’s faces

They may have emerged from a wet hiding place several metres away. Adult fungus gnats are weak flyers but are attracted to light traps.

How to Find the Source

Place yellow sticky cards in several different locations:

• Horizontally above suspect pots
• Beside recycling
• Near wet carpet
• Beside fish tanks
• Near reptile enclosures
• Under propagation benches
• Beside drains
• Near stored growing media
• Around hydroponic equipment

Label each card and compare the catches after several days. A raw potato slice placed on damp growing media can also attract larvae, making their translucent bodies and black heads easier to spot.

Attraction Is Not Always the Same as Breeding

This is the important distinction.

An orange juice container, water bottle or open fish tank may attract adult gnats without producing them. Meanwhile, wet carpet underlay, a forgotten bag of soil or debris underneath a plant tray may be quietly supporting the larvae.

Clean the places where adults gather, but continue looking for persistent moisture, fungi, algae and decomposing organic material.

The Bottom Line

When fungus gnats keep returning, inspect every place containing some combination of:

• Water
• Persistent dampness
• Fungi or algae
• Decaying organic material
• Tender roots
• Poor airflow

Fungus gnats love water far more than many people realize. Check the plant pots—but also check the pop cans, beer cans, juice boxes, water bottles, wet carpet, fish tanks, reptile enclosures, drains and everything hiding underneath.