What's Bugging You? Fungus Gnats


Fungus Gnats – Photo Courtesy of Whitney Cranshaw, Colorado State University, Bugwood.org

On my top 10 list of the most irritating things in the world is fungus gnats.  If you have tropical plants in your home, at one time or another, you have had fungus gnats and know what I’m talking about.  They are especially bothersome in winter and early spring. 

Fungus gnats look like fruit flies (but are not related) and are tiny, dark, short-lived gnats.  They love moist tropical plant soil.  The winged adults are more a nuisance than anything.  They are very weak fliers and make short hops from plant to plant to lay their eggs.  The yellowish-white eggs are laid as clusters or strings in the top inch to half inch of the soil.  It is the larvae that can damage the roots of tropical plants.  The larvae are extremely tiny.  They feed on root hairs, dead leaves lying on the soil and organic matter in the soil.  A heavy infestation of larvae can damage a plant’s root system, letting in disease and encouraging rot.  Pupation occurs in the soil and the winged adult emerges to begin the life cycle all over again.  Adults live for 7-10 days with each female laying up to 200 eggs during that time.  Indoors they can reproduce year- round.  They do not feed on the plant leaves or bite people. 

A heavy infestation of larvae can cause a plant’s leaves to yellow and stunt its growth.  They are also capable of spreading the plant pathogen that causes damping off and death of seedlings.


Fungus Gnat Larva – Photo Courtesy of Whitney Cranshaw, Colorado State University, Bugwood.org

That’s all fine and good you say but what I really want to know is how do I get rid of these things!  Here is a list of steps you can take to rid yourself of these pesky gnats.

When you discover you have a fungus gnat problem, make sure you treat all of your plants.  These little gnats can get around quickly.    

Let the top inch or two of the soil dry out between waterings.  This is especially important in winter – a time when people tend to overwater their plants.  This discourages the female fungus gnat from laying her eggs.

When you do water, water from the bottom.  The top of the soil will remain dry while the bottom absorbs the water in the drip tray. 

Replace the top half inch of soil in your pot(s) with horticultural grade sand (not play sand) or diatomaceous earth.   Diatomaceous earth is especially effective as it shreds the body of the insect as it walks on the soil’s surface. 

Place yellow sticky traps close to the affected plants.  Fungus gnats are attracted to yellow and the adults become stuck to the trap and die.  Replace the traps every few days.

Peel a potato, cut it into pieces resembling French fries and insert a couple into the soil.  Fungus gnat larvae love potato.  Leave it in the soil for three to four hours.  When you remove the potato stick the larvae will be attached.  Throw this in the garbage and insert new sticks of potato. 

If the fungus gnats have just invaded a couple of your pots, repot those plants using new tropical soil.  Remove the plants from their pots, clean as much of the soil off the roots as you can into a garbage bag and repot the plants into fresh soil.  Immediately throw out the old soil. 


Mosquito Dunks

Mosquito Dunks are made of a natural bacteria and are safe for humans and pets.  The natural bacteria is called BTI ( bacillus thuringiensis var. israelensis).  It kills the larvae by infecting them which damages them internally. 

Fill a one- gallon jug with water and drop in one mosquito dunk.  Let soak overnight.  In the morning, remove the mosquito dunk from the water – it can be used again.  Water your plants.  The bacteria leaches into the water and will now infect and kill the larvae.  Repeat this process every time you water your plants for a three to four weeks.  I would combine the mosquito dunk water with the use of the yellow sticky strips to kill them above and below the soil.  

Carnivorous plants, such as sundews, venus fly traps and pitcher plants can be a help as well.   Do not rely on them exclusively. 

Nematodes are also effective.  Do nematode soaks every 10-14 days until the problem is resolved.   

FYI – Fungus gnats are attracted to the carbon dioxide we exhale.   

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