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  • Fungus Gnat Prevention and Control

    Fungus Gnat Larvae have been the bane of many fig forum members, but there are a few simple measures that can reduce or eliminate Fungus Gnat infestation, since the Gnats are attracted to wet mix and decaying plant matter. Also the Larvae are located only in the top 2 - 3 inches of the potting mix.

    1. Start cuttings in "clean or sterile" medium (without soil or compost)
    2. Better watering practice to keep top of soil dry or bottom watering.
    3. Incorporate a dry layer on top of media, 3/4" of Gnatnix or 1" - 2" of Sand.
    4. Water with BTi or mix BTi (grated mosquito dunks) into the potting medium.

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    Fungus Gnats:



    BTi , Mosquito Dunks and Bits, GnatNix:

    Mosquito Bits® When sprinkled into standing water where mosquitoes breed, biological Mosquito Bits® Kills Mosquitoes Before They’re Old Enough To Bite®! Easy to use – sprinkle or broadcast over standing water to kill mosquito larvae Quick acting, starts killing mosquito larvae within minutes Harmless to beneficial insects, pets, birds, fish, horses, livestock 8 oz. treats … Mosquito Bits® Read More »


    Please comment with any additional info and your prevention methods.
    Last edited by Admin IT; 01-01-2016, 02:38 AM.
    Pete R - Hudson Valley, NY - zone 5b

  • #2

    I don't know about the only in the top 2-3 inches.......I have seen them 8" down infesting the bark at the bottom of a tree pot. Perhaps they came in from the bottom but still they were there. Honestly I think they are just a sign of too much water, causes rot and the bugs show up to feast. I don't think they actually cause the damage. Now if you have a high population of the pests they certainly will feed on matter like roots that are not rotted. Just my observation, ymmv.
    Cutting sales will start Tuesday Nov 1 at 9:00 eastern


    • AscPete
      AscPete commented
      Editing a comment
      Yes, Its been my observation and experience also. Usually Fungus Gnats arrive after the damage has already been done since they are attracted to the rot. Their Larvae will eat the rot and fungus that they find when they hatch.

      That's why the recommendations, Clean Medium and Better Watering Practices (not over watering).

      BTW, I use #'s 1 and 2 and have tested #3 with sand and have not had any infestations of my rooted cuttings indoors.
      Last edited by AscPete; 04-19-2015, 08:50 PM. Reason: added BTW.

  • #3
    Found this interesting:
    Growing media vary in attracting fungus gnat adults. Less attractive growing media may result in an increase in plant injury because larvae feed on plant roots instead of fungi in the growing medium. Plant disease suppressive growing media may be more attractive to fungus gnats because of the greater microbial activity. Studies have shown that fungus gnat adults are attracted to particular growing media.

    I assume they are talking about "biofungicides" like B. subtilis, T22 HC, mycorrhizal fungi, etc. when they say "plant disease suppressive growing media".


    • AscPete
      AscPete commented
      Editing a comment
      "Another option, placing a layer of sand (1.3 mm or 0.005 inches) over the top of the growing medium, was proposed
      as a way to reduce fungus gnat infestations by creating an unattractive surface for ovipositing females. But not
      even a 3.1 mm (0.012 inch) layer of sand was shown to be effective in preventing adult emergence or inhibiting
      females from laying eggs."
      The quote from the attached document is somewhat humorous, they dusted the potting mix surface with a 1/ 8th of an inch layer of sand and expected it to deter the Gnats???

    • hoosierbanana
      hoosierbanana commented
      Editing a comment
      I think any potting mix can be attractive to fungus gnats indoors. Take away the preferred mix and substitute any other household item that the gnat could choose to lay eggs in its place and the sterile mix will become preferred because it has moisture.

    • AscPete
      AscPete commented
      Editing a comment
      If you also eliminate the moist surface layer, you will eliminate the Gnats.

  • #4
    I was able to control gnats in my greenhouse by using predatory nematodes in my watering can. They also attack the gnats only and leave the beneficial microbes in your soil alone.
    Zone 10b, Miami, FL


    • AscPete
      AscPete commented
      Editing a comment
      Thanks for commenting. Its one of the proven Biological control methods.
      I've never tried nematodes, but I've also never has a severe indoor infestation.

    • Rafaelissimmo
      Rafaelissimmo commented
      Editing a comment
      Pete I have never had an indoor infestation in my grow room. This is the greenhouse, which I consider an extension of the outdoors. And since there are non-inert materials there, that is how the gnats pounced upon an attractive environment for replication. I also tried some diomatecious earth sprinkled on the surface of the pots, but it can create a slimy environment in your media.

  • #5
    As a test last season I grew cuttings in 2 Liter SIPS next to a container in which I grew Fungus Gnats. All the SIPs were "bottom watered" and had 1 - 2 inches of dry potting mix at the top of the planters. Out of 20 SIPs with various mixes, including Peat and Coir none had Gnat Larvae.
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    Pete R - Hudson Valley, NY - zone 5b


    • hoosierbanana
      hoosierbanana commented
      Editing a comment
      Can't argue with that. I think fabric containers could also give a similar result with cautious watering. The moist mix at the bottom is also inaccessible, unlike normal containers with drainage holes.

  • #6
    I lost a bunch of beautiful, rooted cuttings back in December to those pesky gnats. Devastating to say the least. I bought a sack of that Gnat-Nix at the local hydroponic store, maybe around $10. I posted a question about it, and the few who responded were happy using Gnatrol. I had some cuttings stored in the fridge, and when I took them out to start rooting them, I put them in cups with the 3/4" Gnat-Nix on top. It is made from recycled glass, and if you look closely, it resembles puffed rice but small grains. The gnats supposedly get kind of chewed up when they try to get through it and into the soil. No chemicals in it. My cuttings are outdoors now, and NO GNATS. It definitely seems to me that it does work.
    USDA Zone 9b Wish list: Abruzzi, Pasquale, Filacciano, Tagliacozzo, Zingarella, Godfather. Any, including unknowns, from Abruzzo, Italy.


    • #7
      It seems the cuttings outside follow a completely different trend. I have never used a Gnatrol or any other gnat remedy. My best solution is to move the rooting cuttings outside and keep them in a partial shade with a slight breeze asap. No gnats whatsoever and the cutting are all happy. My biggest enemy there are the squirrels , they dig in the soil and turn the cups upside down. I think outside the gnats are not as safe as in a greenhouse or in a room, there are a lot of creatures that eat them , the wind also helps.
      USDA z 10a, SoCal. WL: Boysenberry Blush


      • #8
        I had good success preventing gnats by applying a 1/2" layer of crushed limestone on the soil. I direct pot my cuttings. Same idea as the sand.
        SE PA
        Zone 6


        • #9
          I've noticed a few gnats flying around my cuttings inside, perhaps three or four. I don't think they like the coir/perlite rooting medium. A layer of dry perlite forms on the top in the cups. I keep the cups on the dry side and only water when the moisture level is a "2" or lower. Just to be on the safe side, I've put strips and circles of fabric softener paper around the cuttings at the top of the cup which also seems to discourage gnats. When I see a gnat running around the top of a cup, I take the cup outside and it flies away. There were also spiders with webs around when the larger pots were inside which were natural predators. I've left a cup without softener paper and keep a wet medium to try and attract the gnats so I can catch them and take them outside, but it hasn't worked so far. Most of them left with the larger pots. No problem so far.
          Mara, Southern California,
          Climate Zone: 1990=9b 2012= 10a 2020=?


          • #10
            I got my share this year...a first here. I'm growing in 50% shade...open on the sides...some on heating mats

            I was running a short term experiment on cuttings from a tree I have access to with alot of excess growth. I started them in clean sand and vermiculite with about 10% coarse perlite...good clean material...and they started nicely..in January on the mats. Then I transplanted the first few to 24oz cups and a mix of potting mix leftovers and old planting mix.....maybe the problem...probably...I just ordered a pallet of sterilized mix

            all was well until this spring when it warmed up...first noticed a hatch and then a couple larvae on the sides of the cups..at the bottom of the cup...and on the sides. So I began treatment.....

            1. mounted yellow sticky pages behind all the starts to catch adults and minimize the population
            2. treated all starts and pots with a 25% solution of 3% hydrogen peroxide one part peroxide, three parts water at 6.5ph
            3. after one day, inoculated all pots with BTI...not necessarily Gnatrol but BTI, every 3 days for 9 days
            4. after innoculating the pots, sprayed with pyganic...then after one week with Monterey Take Down..both pyrenthins

            everybody's happy and looking good..not hearing any complaints...and lots of new roots and growth

            I plan on dealing with the ground cover in the shade structure as well but haven't made up my mind witch product to use...typical material.. lined, woven plastic over 4" compacted road base....I'll have a few less gnats when I'm done
            Ross B. Santa Rosa Calif zone 9b, wish list: CdD Blanc, Igo, Palmata, Sucrette, Morroco, Galicia Negra


            • #11
              Saw this on the net and thought it might be a cheap effective remedy:

              Yellow construction paper with petroleum jelly

              Smear a good amount of petroleum jelly on the surface of some appropriately cut yellow construction paper. Yellow paper is needed as this color preferentially attracts adult gnats. During watering you can gently remove the jelly paper, water and then replace.

              I'm using the 3 cup method to root my cuttings using indoor grow lights. Due to the use of a humidity chamber, it presents a optimal environment for gnats and fungus to grow.

              Note this method works only to remove adult gnats rather than eggs/larvae.

              Any thoughts?

              Malcolm - Carroll County, MD (zone 6b). Interested in cold hardy figs. Currently container growing, MBVS, St. Rita, Olympian, RdB, Beale, Sal's EL, UCD 184-15s and Desert King.


              • don_sanders
                don_sanders commented
                Editing a comment
                I tried that once and I don't know if my Vaseline was to old but it wasn't sticky enough. They would just walk across it.

                Pre made sticky traps or fly paper worked better.

              • AscPete
                AscPete commented
                Editing a comment
                I've never tried the petroleum jelly, but fly paper along with light from a cool white florescent bulb (I've use a 14 watt CFL bulb) also seems to attract the adults.

                IMO, starting off with clean potting mix with a lower pH (to reduce fungal growth) and possibly using an anti-fungal agent in the early stages can eliminate surface and soil fungal growth, http://tomclothier.hort.net/page13.html . Also eliminating adult fungus gnat in the rooting area (before starting the rooting process) will ensure that no eggs will be laid in the cuttings' potting mix.

            • #12
              One thing that I was wondering is if the fungus gnats could be propagating in a house anywhere other than pots like a disposal of something?
              Don - OH Zone 6a Wish list: Verdolino, Black Celeste


              • AscPete
                AscPete commented
                Editing a comment
                If you had a wet area that can be accessed by adult female fungus gnats, with fungus, rot and decomposing organic matter which is stagnant for at least 17 days ( time from egg to winged adult stage at ~75º F.) then the answer would be yes.

              • don_sanders
                don_sanders commented
                Editing a comment
                I'm not the best house keeper but I don't think I'm that bad, hehe.