How to Use Gnatrol® WDG to Control Fungus Gnats Q & A

by: Daniel Gentile


This Q & A was written for fig-growers, specifically, collectors and hobbyists growing indoors or through the winter.  But if you've landed here searching for help with a fungus gnat infestation you're in the right place.  Although the information below is written with the fig-grower in mind, all of the information is useful for any grower.  Please visit my figBid listings for all of your fungus gnat control supplies including Gnatrol WDG and Yellow Sticky Traps.  You can reach out to me from there with any questions.  Thanks for reading!

Those little gnat B********S!  Try growing anything indoors or in the greenhouse and they're sure to ruin your day.  An infestation seemingly comes out of nowhere; one day everything is perfect and the next they're swarming everywhere.  Adult gnats are not much more than a huge, buzzing nuisance to humans (they’re attracted to the carbon dioxide we exhale) and won't bite or damage plants.  But gnat larvae are the predators and will cause significant harm to your plants by feeding on finer root hairs, particularly roots of seedlings and young plants.  The rule of thumb is, if you see one gnat, take action quickly as it's already too late.  Read the Q and A below to find out more about fungus gnats, the damage they cause, and how to treat an infestation.

What are fungus gnats?

Fungus gnats are tiny, winged insects similar in appearance to mosquitoes and about the size of a fruit fly.  They are generally considered a houseplant and greenhouse pest attracted to moisture and humid conditions.  Light from windows, grow lights, televisions, computer monitors, and other smart devices will attract fungus gnats, but more often they gather near potted plants and can be found on growing media and foliage.  Fungus gnats are weak fliers and seem to aimlessly buzz around, never flying on a straight path although they hop and run very fast.  Adult fungus gnats live for about 7 days and many overlapping generations of fungus gnats will thrive at one time.

What are fungus gnat larvae?

Female fungus gnats lay up to 200 tiny eggs within the top layer of damp, organic soil.  Fungus gnat larvae hatch and will be identified as having a shiny black head and an elongated, whitish-to-clear, legless body.  Larvae will immediately begin to feed on bits of organic matter, fungi, and root hairs within the soil.  Fungus gnat larvae can cause significant root damage, stunted growth, declining health, and death to your young plants and seedlings.  Most larger plants can handle fungus gnat larvae feeding.  But damage from feeding will leave open wounds where pathogens move in.  Prolonged activity will surely ruin even the healthiest plant.  As fungus gnat larvae travel they'll leave shiny slime trails on the surface of media, grow-pots, and trunks of plants that resemble trails from snails or slugs.

What is the life cycle of a fungus gnat?

Fungus gnats occur in four stages; adult, egg, larva, and pupa.  At the right temperature, the entire life cycle lasts for about a month and many overlapping generations of fungus gnats will occur in a single infestation.  Warmer temperatures will speed up the life cycle.

  • Egg - Female fungus gnats lay around 200 eggs singly, or in clusters, within top layers of moist organic debris and prefer areas where fungus is growing.  The eggs will hatch within 4 to 6 days.  Eggs are harmless to plants.
  • Larva - Larvae hatch from fungus gnat eggs and stay in this form for 12 to 14 days.  This is the longest and the most problematic stage fungus gnat larvae feed on organic matter and roots.  Plant root systems will be damaged and growers will notice a decline in plant health during this stage.
  • Pupa - When fungus gnats enter the pupa form they become much more mobile, for 3 to 7 days the fungus gnat pupa crawls and stays within the top layers of the soil.  The pupa will continue to feed on organic matter and damage root systems during this stage.
  • Adult - Adult fungus gnats will stop feeding on a plant's root system and you will begin to notice them buzzing around.  After 7 or 8 days of adulthood, the fungus gnat will naturally die, leaving behind approximately 200 eggs to repeat the process.  Adult fungus gnats won't harm your plants and are nothing but a nuisance at this stage.  But the damage has been done and exponentially increases with every generation.

What kind of damage can fungus gnats do?

Generally, adult fungus gnats are considered a nuisance and won't physically harm people or plants, but they can spread diseases.  Fungus gnat larvae and pupa will do the most damage by feeding on root systems, especially the finer roots of seedlings and younger plants.  As an infestation grows your plant won't be able to cover the root loss.  Seedlings and young plants will quickly enter a weakened state beyond recovery.

What are the symptoms of damage caused by fungus gnats?

Fungus gnat damage will appear similar to that of any other root-related issue.  Fungus gnats are attracted to humid conditions and moist, organic potting soil.  Declining health is often confused with overwatering so lower leaves may turn yellow and drop, and the plant's growth may slow down or stop completely.  Fungus gnats will do the worst damage to seedlings and other plants during the early stages of development and symptoms mentioned might appear on those plants first.  Other symptoms include:

  • moist rootballs
  • sudden wilting
  • loss of vigor
  • leaf yellowing
  • foliage loss
  • unexplained seedling loss
  • death of young plants

How do I know if my plants have fungus gnats?

Any of the plant symptoms listed above may be a clue but seeing insects flying around your grow area or plants is the true indicator.  Don't really know?  Try one of these methods:

  • Examine the leaves and soil line of your potted plants.  Fungus gnats will be easy to spot.  Check seedlings and younger plants first.  The greatest fungus gnat activity occurs during the early morning and evening.  Inspect your plants during those times.
  • Get some yellow sticky traps and hang them by the leaf-line of your plants.  If adults are flying around they're sure to land on the yellow stick trap and get stuck.  This is also a great indicator of the level of infestation present.  
  • Look for cream colored larva on the soil.
  • Look for slime trails on soil, leaves, pot, or trunk of your plant.
  • Press a wedge of potato into the soil and leave it for 2 days.  If larvae are present they'll be attracted to the potato, start feeding, and easily noticed.

Will fungus gnats go away on their own?

No.  Please, please, please, don't think there's only one gnat.  While you've spotted one adult gnat there is an army of larvae already chomping on the tender root systems of your plants.  Take fungus gnats seriously and begin a treatment program ASAP.

How did my plants get fungus gnats?

Fungus gnats are not your fault.  When you grow indoors or in a greenhouse you're bound to see them appear.  The tiny eggs of fungus gnats are commonly found in commercial potting soils and within the pots of other plants introduced to your indoor grow area or greenhouse.  Once fungus gnats are introduced they easily spread from plant to plant by any opening in your grow-pot.  That includes the top soil-line and drainage holes.  Plan ahead; keep growing areas clean and free of organic and other debris.  Start fungus gnat management early, before they appear.

When do fungus gnats appear?

Fungus gnats can appear anytime indoors or when the conditions are right.  Fungus gnats thrive under humid conditions with temperatures around 70˚F-80˚F with 75˚F being optimal for larvae.  Fungus gnats are attracted to damp, organic soil where larvae feed on bits of organic matter, fungi, and delicate root systems of our plants.  For those in colder climates fungus gnats will begin to make an appearance in the fall when plants are brought inside for protection.  Your fungus gnat management plan must already be in place by that time.

How do I create a fungus gnat management plan?

Creating your fungus gnat management plan is simple.  Before rooting even begins, follow these 5 steps to beat the beasts and keep your fungus gnat woes at bay:

  • Clean and Repair - It doesn't matter where you grow or how big your operation is, clean, and disinfect it now!  Grow in a closet?  Clean it!  A corner in the basement?  Clean it!  On top of the refrigerator?  Clean it!  A greenhouse?  ... you get the idea.  Starting your rooting season with a clean grow area and equipment is healthier for your plants and lessens the impact of issues as they come up.  Use soap and hot water, a general household cleaner/disinfectant, or a 10:1 bleach/water solution in a spray bottle and some paper towels.  Clean and disinfect your nursery pots and tools as well.  Change light bulbs, fix and clean shelves, test your equipment, fix plumbing and irrigation leaks, clean up standing water, fix leaking windows and doors, etc.  Store compost, organic fertilizers, and mulches away from your grow area in sealed containers.  Get it done now.
  • Treat your Potting Mix - I mix almost all of my own potting mix and add extra perlite to improve drainage for cuttings that will be rooted indoors.  But all of my indoor potting mix is now treated with Gnatrol WDG granules.  I'll add a tablespoon of Gnatrol WDG with amendments and mix them well into my potting mix.  It's enough potting soil to fill a standard 5-gallon bucket.  When using a commercial potting mix like Pro-Mix I'll do the same thing.  So if you're looking for a safe starting point use 1 tablespoon of Gnatrol WDG per 5-gallon bucket of potting mix.  It's a great front-line and lets the gnats know you're not fooling around.
  • Control Moisture - When you're growing inside water can be the enemy.  Too much water will kill your fragile fig cuttings and create an attractive environment for fungus gnats.  Stay involved daily and create a watering plan for your plants to keep them happy.  Avoid overwatering and provide adequate drainage.  Allow the surface of container soil to dry between waterings.  When your choice growing medium is somewhat light and a watered pot feels heavy, don't water!  It's probably overwatered already.  Damp soil is good enough.  I water my plants by weight.  It's a lot of work but overall success is higher and there are much fewer instances of fungus gnats.  I'll also use a clip-on fan and set it to blow lightly across the top of the cuttings.  It helps to dry out the soil line and keep fungus gnats from flying around the pots.
  • Monitor - If you're using a fan turn it off.  Visually examine the leaves, soil line of your potted plants, and grow area.  Fungus gnats will be easy to spot resting on leaves, soil, windows, walls, or you might see them in flight.  Check for excess moisture conditions.  Hang yellow sticky traps at the leaf-line of your potted plants to trap flying adult fungus gnats.  Look for larvae in the soil and slime trails on soil, leaves, nursery pot, or trunk of your plant.  Press a wedge of potato cut side down into the soil and leave for 2 days.  If larvae are present they'll be attracted to the potato, start feeding, and easily noticed. 
  • Treat - You've planned ahead and now you're ready for combat.  Most of a fungus gnat's life is spent in larva and pupa stages in the soil.  These life stages are where fungus gnats do the most damage as well so it makes the most sense to target fungus gnats here.  Don't provide your plants with additional water for treatment.  Instead, cycle Gnatrol WDG granules into your watering program at the rate of 1 teaspoon per gallon of water for a light/medium infestation or 1 tablespoon per gallon of water for a medium/heavy infestation.  If you've mixed Gnatrol WDG into your potting mix the colony will already be weakened and easy to get rid of.  Continue to hang new yellow sticky traps to trap flying adults.  Use potato wedges to trap larvae as well.  Continue to monitor.  Don't move infested plants where they can infect other plants.  Continue treatment in the growing area where the infestation was discovered.  

I see fungus gnats flying around my plants.  Should I try another natural or home-remedy first?

There are lots of natural and home-grown fungus gnat remedies out there and I've tried most of them.  Hydrogen Peroxide, vinegar or red wine traps, cinnamon, diatomaceous earth, neem oil, horticultural sand, liquid dish soap, bug zapper, peppermint oil, and so much more.  If you're growing anything more than one or two potted plants they don't have a noticeable effect.  The only natural, home-remedy that is 100% effective is women's nylon stockings.  Get the largest size you can find and squeeze your smallest grow pots in.  Tie at the stem.  You can water right through the nylons and tiny fungus gnats can't penetrate.  Don't even attempt this with rooted cuttings as you wouldn't want to disturb fragile roots.

I've heard removing the top layer of soil is very effective for controlling fungus gnats.  Is this true?

Fungus gnat adults will lay eggs in the top 4cm of soil.  While scraping and removing the top layer of soil may help, you won't get all the eggs and run the risk of damaging your precious cuttings and seedlings by destroying roots.  It's not worth it.  Gnatrol WDG is easy to use, more than effective on its own, and there's no risk of disturbing fine roots.  

What if I add a layer of sand or some other finely crushed matter to the top of the soil?  Won't that stop the fungus gnats?

No.  It's nearly impossible to completely cover the soil line with a thin layer of sand and there are still drainage holes.  A thick layer of sand has a minimal effect and may even compound the problem by keeping too much moisture in the pot.  A thick layer of any commercial product of this nature will break the bank for sure.  Not worth it.

Why should I use Gnatrol WDG instead of stronger chemical controls?

Of course, you'd never spray any type of pesticidal chemical for fungus gnats inside the home.  For hobbyists and smaller nurseries, chemical control for fungus gnats is rarely warranted.  They're not necessary and Gnatrol WDG is natural and way more effective over the long term.  Please resist the urge to use commercial foggers and low-toxicity pyrethrins.  

What is the active ingredient in Gnatrol WDG and how does it work?

The active ingredient in Gnatrol WDG is Bacillus thuringiensis, subspecies israelensis, strain.  More commonly known as Bti.  Bti is a highly selective biological larvicide that contains unique toxicity characteristics very specific for fungus gnats.  Bti spores that are eaten by mosquito larvae release toxins into the mosquito's gut, causing the larvae to stop eating and die.  Bti is only effective against actively feeding larvae, and does not affect fungus gnat pupae, adults, or eggs.

What is Bti?  Is Bti harmful to humans, pets, my plants, or the environment?

First discovered in Israel in 1976, Bti is a biological or a naturally occurring bacterium found in soils.  Bti contains spores that produce toxins that specifically target and only affect the larvae of the mosquito, blackfly, and fungus gnat.  Since humans, animals, birds, and fish have acidic and not alkaline digestive tracts, Bti toxins have no effect and has been approved for pest control in organic farming operations.  It has been well tested by many studies on acute toxicity and pathogenicity (ability to cause disease) for Bacillus thuringiensis including studies specifically on Bti.  Bti can be applied safely to fungus gnat habitat without a detrimental impact on food crops, water supplies, or honey bees and can be used up to the day of harvest on all edible plants.  

Does Gnatrol WDG contain any other ingredients beside Bti?

Yes, Gnatrol WDG does contain other proprietary organic ingredients.  The other ingredients are preservatives and like Bti, they are not harmful to humans or animals.

I see Gnatrol WDG is listed as organic, but the directions say to wear protective coverings and to not inhale dust from the product.  

Gnatrol WDG is safe for all organic gardening.  The safety precautions are the same for Gnatrol WDG as any product you'd find useful to control pests, no matter how safe the product may be.  Always read and follow directions, use common sense, and wear the appropriate PPE.  Dust from anywhere can be a lung irritant especially if you have asthma or any other lung conditions triggered by naturally occurring or concentrated dust particles.   But the high level of safety of Bacillus thuringiensis makes it safe to work with in situations where exposure to pesticides during mixing and application are likely.

Is Gnatrol WDG safe to use around my terrarium, vivarium, reptile enclosure, or aerogarden?

Yes.  Gnatrol WDG is safe to use around any kind of terrarium, vivarium, reptile enclosure, or aerogarden.

I just watered my plants and cuttings but want to start using Gnatrol WDG right away.  Should I water them again with Gnatrol WDG?

NO!  Don't risk overwatering your precious cuttings and young plants.  It's hard to resist the urge but please have patience.  You'll have to wait until the next watering and work Gnatrol WDG into the cycle.  If your established plants are very wet you can try sprinkling a bit of Gnatrol WDG into the pot.  Only use a tiny bit; about 1/8 of a teaspoon per 6" (diameter) pot.  Don't try this with newly rooted cuttings or young plants.  Broadcast as best you can.  This is a good time to set up some yellow sticky traps as well.  Please have patience.  Gnatrol WDG works!  

Will Gantrol® kill annoying adult fungus gnats?

No, Gnatrol WDG kills fungus gnat larvae and not the adult fungus gnats.  The adult stage of a fungus gnat is short; maybe 7 or 8 days.  And Gnatrol WDG begins to work right away.  So, depending on how bad the infestation has grown you should expect to see minimal results within 24 hours, noticeable results in a week, and all colonies eliminated in 10-14 days.

How should I apply Gnatrol WDG?  What are the application rates?

Please be aware that most of the suggested application rates are lower than recommended by the manufacturer.  The stated rates have been determined to be effective for fig growers and based on actual usage.  Your own setup may be different.  Please use common sense and vary accordingly.  Gnatrol WDG contains Bti.  Bti does not reproduce or persist indoors, so infestations in potting media will require repeated applications to provide control.  Mix Gnatrol with water and apply as a soil drench, or spray onto media using a hand-pump spray bottle or other spray equipment.

Always work Gnatrol WDG into your scheduled watering program and not in addition to it.  Apply Gnatrol WDG with adequate water by soil drench to sufficiently wet the soil surface above.  DO NOT APPLY GNATROL WDG IN ADDITION TO YOUR REGULARLY SCHEDULED WATERING.  Mix Gnatrol WDG into your watering can and:

  • For actively growing, well-established, indoor overwintered, and adult fig trees only, make 2 weekly applications in the suggested range rate for heavy infestations.  
  • For new and young plants every 7 days for any level of infestation.
  • For newly rooted fig cuttings only as needed.  If you have to make the choice of overwatering a rooted cutting with Gnatrol WDG or not treating it, don't treat it.  Once Gnatrol WDG is worked into your watering schedule the overall health of your entire setup will improve and you'll be able to treat the few that may have been overwatered into the Gnatrol WDG regimen.  Please have patience.

After the initial infestation has subsided, regular follow-up applications using the suggested light infestation rate will establish a long-term maintenance program.  Gnatrol WDG should not be injected in combinations with fertilizers or fungicides containing copper or chlorine, as this may neutralize the active ingredients. (Chlorine levels in potable water supplies should not present a problem with Gnatrol WDG performance).  DO NOT apply soil drenches to plants under stress or follow applications with excessive amounts of water.  Use Yellow Sticky Traps to gauge your level of infestation and apply Gnatrol WDG as follows:

For smaller setups, home use, and rooting indoors, mix Gnatrol WDG as follows:

  • For a light infestation use 1/2 tsp per gallon of water.
  • For a medium infestation use 1 tsp per gallon of water.
  • For a heavy infestation use 3 tsp per gallon of water.
  • For maintenance use 1/4 tsp per gallon of water.

For larger setups, greenhouses, and commercial applications, mix Gnatrol WDG as follows:

  • Light infestation, 3.2 to 6.4 oz/100 gallons applied as a soil drench
  • Heavy infestation, 13 to 26 oz/100 gallons applied as a soil drench

Can I use Gnatrol WDG in my drip system?

Yes, Gnatrol WDG can be injected into a drip system.  Please consult the manufacturer of your injection device for appropriate mixture and injection rates.  

Can I mix Gnatrol WDG ahead of time and save for later applications?

Once Gnatrol WDG is mixed with water it will be effective for up to 72 hours so it's a good idea to only mix what you need for an application.

Where can I buy Gnatrol WDG?

Thank goodness you're in the right place.  You can buy Gnatrol WDG in small quantities right here on  Click here to see the 1 oz, 2 oz, 8 oz packages and pricing.

How should I store my package of Gnatrol WDG?

Make sure to seal your package of Gnatrol WDG tightly and store it in a cool [59-86°F (15-30°C)], dry place.  Gnatrol WDG has a 2-year shelf life.

Where are some other places I can use Gnatrol WDG?

Gnatrol WDG can be used in almost any kind of indoor grow setup and greenhouses.  Use Gnatrol WDG to control fungus gnats anywhere you use a potting mix and around your vegetable starts including tomatoes, leafy and cole crops, cucumbers, peppers, and eggplants.  Gnatrol WDG works great with fig tree cuttings, fruit tree seedlings, young ornamentals, and even fragile tropicals.  Gnatrol WDG also works very well controlling gnats around composting toilets.  After emptying your compost, simply mix 2 tablespoons of dry Gnatrol WDG granules with about 1-1/2 gallons of coconut coir and add back to the toilet.  As compost is created from waste it mixes with the coconut coir which picks up moisture and presto, no more gnats!