fungus gnat control and eradication

Discussion in 'Growing Marijuana Outdoors' started by thezephyr, Aug 11, 2015.

  1. #1 thezephyr, Aug 11, 2015
    Last edited by a moderator: Aug 11, 2015
    Hey everybody,

    I've got a nice outdoor grow going, but made the mistake of mixing in an older bag of soil I had laying around along with my various organic amendments. Fast forward, and now the three plants that got a bit of that soil now have rather intense fungus gnat infestations in the soil media. I have had totally successful harvests even with fungus gnat problems in the past, but this time around I really just want to get rid of them. My plants are just starting to form pre-flowers at the tips of the branches so I have some time before the fungus gnats become a problem for anything other than the root zone, but it sucks going out every morning to pluck those little bastards off my sticky buds so I'm hoping to resolve this problem and eliminate the infestation before I get too far into flowering.

    So far, I have given each of the affected plants a neem oil soil drench. theoretically, neem disrupts the feeding of the larvae, weakens the adult population, and should cause birth defects and weakness in the subsequent generations. Additionally, I'm letting my soil dry out pretty thoroughly between neem drenches. I know from experience that although this slows their reproduction, it doesn't get rid of them entirely. And although my plants are still strong as ever, consistently adding neem like this will put a strain on them.

    So now I turn to the experts here at grasscity. I'm looking for recommendations and advice from anyone who has successfully eradicated a fungus gnat population. My grow is all organic so far, but I would be willing to use inorganics if they are non-toxic to humans and pets and would dissipate/flush from my plants root system before harvest. I cannot work with diatomaceous earth because of respiratory problems.

    Here are a few options I am currently considering, and I'd love to hear any other suggestions or successful techniques from everybody:

    A neem seed meal top dressing- I'm hoping this would either discourage or make it impossible for the adults to lay more eggs. Has anyone used neem meal for this prupose?

    Predatory nematodes- This seems like a great organic option, has anyone on here used predatory nematodes succesfully against pests in the root zone?

    Gnatrol- as I understand it, this product is an insecticidal bacteria similar to BT in a granular powder which is then diluted in water for a soil drench. it says this product is safe for use on vegetables, but has anyone used this product on cannabis? should I be worried about my plant uptaking it systemically? is there any chance of toxicity from this product remaining in my plants at harvest time a few months from now? As I said before, this grow is all organic so far so I'd lean away from this option.

    Thanks everyone, hoping to get some good information together here. most of the people on these forums use pre mixed soil for their grows in some capacity, so fungus gnats are something we all deal with from time to time. hopefully this information can be a useful resource for us all. I'll be sure to post the results of whatever treatment I end up using. For those who are curious, the affected plants are two surfs up females, and a lemon skunk.

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  2. Subbing along for this. I have been using crab meal top dress which is supposed to release chitins that will kill the skeletal system of the larvae I believe. I have also been looking into the neem seed meal.
  3. #3 sb420abc, Aug 11, 2015
    Last edited: Aug 11, 2015
  4. Fungus gnats are usually caused by media that's too wet. Let your soil dry out completely before watering. The dry conditions will automatically kill the growing population. To get rid of existing population, DE is a great suggestion. GOOD LUCK!

  5. #5 mjmama25, Aug 12, 2015
    Last edited by a moderator: Aug 12, 2015
    Especially in an organic garden it can be hard to control. You don't really want to let the soil get too dry because that damages the soils micro life.

    Nuke Em is said to be a very very effective soil drench. One of my friends was just saying that they haven't found anything better for controlling gnats.
  6. I am having good luck with an aloe/silica/crab meal drench and foliar spray. In 2 days most flying are gone so we will see if it kills the larvae.
    Mosquito Dunks can be found near the citronella candles at HD..
    Mosquito Dunks biological mosquito control contains Bti, a natural mosquito larvicide, which kills mosquito larvae, but is harmless to birds, fish, wildlife, and pets. Simply place a Dunk in any standing water to control mosquito larvae for up to 30 days. Each Dunk treats up to 100 square feet of surface water regardless of depth. Perfect for Koi Ponds, Hydroponic systems, animal watering troughs, rain barrels, birdbaths, stock tanks, planter saucers, or any other standing water.\t\t\t
    • Kills mosquitoes before they are old enough to bite
    • The professional mosquito control product available for the homeowner. Used and recommended by mosquito abatement professionals and Health Departments for over 25 years. The quintessential highly effective low environmental impact mosquito control product!
    • Harmless to honey bees, beneficial insects, pets, and wildlife! Kills mosquito larvae and nothing else!
    • Labeled For Organic Gardening by the US EPA.
    • Unused, dry Dunks retain their potency indefinitely and can be halved and quartered for small areas of water
    • Contains Bti a species specific bacterium and food grade inactive ingredients-----------------------------------------------------------------------------
    Break up a dunk and sprinkle a pinch or more depending on pot size for 30 days of Fungal Gnat control.. Hang a dunk in your watering pail as well.. It's a sister product to the BT-k most of us outside growers use to keep the caterpillars-bud worms in check.. BNW
  8. BT @ 1 -2 Tbls per gallon of water kills them then add the DE to the top once it drys a little, the de is only effective when it's dry
  9. If using hard pots get a good run off or if small pots place in some kinda tub to get the bottoms good
  10. #10 thezephyr, Aug 14, 2015
    Last edited by a moderator: Aug 14, 2015
    Thanks to everybody for the great responses.
    Here's a quick update on my fungus gnat situation as well as the recipe for the soil drench I've been using:
    Along with dryer soil, the neem soil drench seems to be quite effective. For the first drench I used 2 tablespoons of high quality neem oil and 1.5 teaspoons coco wet emulsifier mixed in a 2.5 gal watering jug, and reduced to 1.25 tablespoons neem for all subsequent drenches. so per gallon that would be roughly 3/4 of a tablespoon neem oil and about a half teaspoon of the emulsifier on the initial drench. the oil blends best if you first mix it with 1/3 hot water and then two thirds cool water stirring constantly. I let my soil dry pretty thoroughly (not entirely because its too hot) and administered the drench twice to each affected plant until it started to run off. I'm using fabric air pots which run off pretty quickly. as a warning, for some plants neem administered systemically can cause stress and leaf yellowing or burn. This wasn't a problem at all for me, but I've seen it shock plants before especially recently established clones. if you're worried about your plant getting shocked by the neem, water or feed with nutrients as normal, and wait 20-30 minutes while the plant uptakes the water and feed before administering the drench. this will limit the amount of neem soil drench the plant is able to uptake. When pouring the soil drench, pour evenly over the entire surface of the soil medium for an even distribution of the oil, and pour some over the base of the main stalk to discourage adult fungus gnats from congregating on the stems of your plants. I administered the drench every other day, alternating with a light amount of pure water as needed to keep the plants from drying out and wilting in the heat.

    the adult fungus gnat population is in a sharp decline, I don't see any of them on one plant and on the others I now see only 1-3 hovering close to the soil instead of many of them flying all through the shaded areas in the plant. however, I'm not convinced that this has taken care of the eggs or the next stage of larvae. I'm considering getting some crab meal tomorrow morning for a top dress, and if I see a resurgence I'll most likely go with some variant of the BT. I've also been spraying with a mix of 1/4 optic foliar atak foliar feed/anti-fungal and 3/4 neem oil dilution mixed in a basic hand sprayer. the plants loved the combination, and it made the leaves incredibly green and shiny on the few plants that were looking pallid from heat. My grow's no longer 100% organic, but this is a medical grow so I'm being very careful with what I use. I'm not using anything that will leave residuals in/on my plants at harvest, nothing toxic, nothing with a considerable heavy metal content, and I'm making sure not to upset my soil biology.

    let's keep the thread going, there's a lot of great information here. I'm intrigued by the predatory nematodes. has anyone on here used them, or any other kinds of beneficial insects or beneficial bacteria? I like the idea of the bt, but I'm a little hesitant to use the mosquito dunks since I'm not really sure if they're intended for edible grade gardening.
  11. Keep up the neem oil for another week or so. You already have interrupted their life cycle. I think eggs take about three days to hatch. Some say that it might take a couple of weeks for all the bad bugs to go away.

    Use neem cake also. It helps with bugs in the soil. Also, plants uptake the oil into there system. It deters bugs.

    Lady bugs and praying mantas also work. However, they tend to love nature and really don't think about just staying in on place.

    I used neem oil and just started using it every other day. I spray it on. The first two days were a heavy application. The next couple will be a lite one.

    The gnats and mold got crazy over the last week or so.

  12. The OG BioWar foliar + root pack tea has always worked wonders for me on fungus gnats. Great stuff, plants love it
  13. #13 thezephyr, Aug 16, 2015
    Last edited by a moderator: Aug 16, 2015
    Quick update on the fungus gnat situation:
    I have the gnats confined to just a couple plants, but the population seems pretty entrenched on those. I thought I'd eliminated them for a minute there, but the weather turned cool all of a sudden and the lower temps/slightly higher moisture seems to have prompted yet another hatch. Neem drench has proven to be very effective at bumping off the adults, they only hang around for about a day after a hatch and I'm seeing less and less stuck to my yellow sticky traps. After a week of searching for it I was completely unable get the neem seed meal locally, so I'm getting some in the mail tuesday. I'm amending my top soil with crab shell meal and some sand instead of DE, then top dress with the neem seed meal when it arrives. if this isnt effective within 1.5-2 weeks, I'm going to try finding an organic bacterial larvicide at a local nursery.
    • Disagree Disagree x 1
  14. Hey BrassNwood, what BT-k product are you using? I'm in a new location and dealing with different pests and environmental factors than I've dealt with before.. I'm starting to see a lot of moth/caterpillar action in this new garden, and I'm thinking I'd better get on top of it.
  15. #15 BrassNwood, Aug 17, 2015
    Last edited by a moderator: Aug 17, 2015
    I'm using Bonide Thuricide from the local Armstrong's garden center.. About a half dozen companies make some version of it and it's most popular use is on tomatoes.. It'll usually have a Caterpillar on the face and BT in bold print along with Worm and Caterpiller Killer..
    BT is destroyed by sunlight and even when not directly exposed it's very short lived so it's a weely application for me.. Twice a week if the zapper is full overnight.. Evening spraying is best if your in a dry climate but I spray at sunup most times with good results.. Minimum mix ratio has proven effective..
    I bought a new cheap 10 dollar gallon pump sprayer for each compound I spray and the 40 dollars was well worth the knowledge nothing else has been used.. Label clearly.. I'll look for the pics of the week after I sprayed with an old sprayer with some herbicide residue in it.. Looks like I used a flame thrower on the plants..
    I run this 15 watt bug zapper as well but more for the design.. Once I cut away most of the plastic guard it made it easy to use a 3 inch paint brush to sweep it clean each morning and the low power doesn't affect the flowering plants just 5 feet in front of it..

    Ace hardware ordered me a new zapper when my old one died after 3 years of continuous use and shipped to the local store free when everybody else said seasonal item try next spring..
    Powder Mildew is my other big threat and I use Green Cure for that also once a week.. Hydro store for that or in a pinch household baking soda will work at a fraction the price but I don't know the long term effects of the sodium bicarbonate vs the potassium bicarb that the cure is made of.. 1 table spoon per gallon is the ratio..

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  16. Excellent post man, I really appreciate that you included specific usage instructions for your techniques with each product. I got a real good shopping list for my next trip to the plant nursery. beautiful garden too, of course the cannabis looks great but that's also a really nice old jade plant you have there. have a good one brassnwood!
  17. fungus gnat and neem meal update:

    I haven't seen any fungus gnats at all since applying down to earth organics neem seed meal as a top dressing. The neem meal appears to have stopped my fungus gnat problem. its only been a few days so I can't rule out the possibility that its between hatches, but the weather went through a hot period and then an overcast period, and there were no hatches when the temperature dropped like there have been previously. so it's looking like it works pretty well for pest control. however, I don't like what the amendment has done to the consistency and water retention characteristics of my topsoil.

    before, the topsoil was a thin layer of loose soil above the root ball, or roots evenly up to the surface. the plants with fungus gnats are getting a little bit rootbound, but I was hoping to put off transplant until the fungus gnat problem was completely eliminated. now that its starting to break down into the soil mix, the neem meal and top soil have merged into a dense natural clay type mixture which retains substantially more water than the roots below, and therefore dries out well after the root zone. this a) makes it more difficult to discern when root zone is starting to get too dry, b)raises concern that the increased moisture will, despite neem's insecticidal properties, encourage more fungus gnat breeding, and c) makes it impossible for me to water my plants when they overheat without waterlogging the soil. as I said, they need a transplant anyway so I'll be putting them in larger smart pots. It think the transplant should solve my muddy topsoil problems because I'll redistribute it more evenly to keep the texture and water retention more consistent with the soil below it. I'll probably end up using eb stone black gold organic soil amended with perlite, pine/fir bark mulch, and some of the neem seed meal to keep any existing fungus gnat eggs or larvae from propagating into the new soil mix. One transplant will be difficult because the plant is supported by (and completely intertwined with) a tomato cage which is in turn held together by the edges of the fabric pot. im thinking instead of removing the pot entirely, I'll end up cutting some large windows into the sides of the fabric pot and slashing the bottom to give the roots a way expand into the soil of the larger container while still retaining some of the structure of the pot to keep the cage structurally sound.

    In regards to the neem meal, its an extremely effective pest deterrent. I dont see anything hovering or crawling around my soil, and since the addition fewer insects have been inclined to land on or eat the leaves. but I suspect it may be used to greatest advantage blended evenly into the soil mix at the same time as the other organic amendments before planting. I'll definitely be adding this to my soil blend from now on. Sometimes I include a base layer of perlite at the bottom of my larger pots, I'll definitely blend neem seed meal into the perlite to keep the lower roots pest free. If you do use it as a top dressing it might work better to dust a small amount of it evenly over the topsoil after watering as needed for pest control, instead of adding 1-1.5 cups to the topsoil as the product instructions on the box recommend.
  18. That is a great update, I just received my shipment of neem and karanja cake so I will be starting those next watering. The crab meal still seems to be keeping them at lower numbers do I am sure they will all but vanish once I add the new amendments.
  19. I took another look at the garden before dusk and saw that one of my plants (surf's up) was suddenly looking upset and quite root bound (drooped, palid). It seems that because the top soil was suddenly holding so much water, less of the water made it all the way down in to the perlite reservoir at the bottom of the pot. So even though the top soil was totally sodden, the lowest parts of the root ball were totally dried out. It was indeed extremely root bound. I didn't have a large enough smart pot or soil mix ready, so I sliced the fabric pot away and transplanted her into a very deep hole slightly wider than her 7 gal pot. The hole was filled with just the dense native clay soil mixed with perlite. Interestingly, the strongest plant of that strain currently is planted in straight hard pack native clay. It started off as a weak seedling, so I put it into the native soil and watered it occasional with leftover nutrients from the other plants while waiting for the darwinian process to occur. it took off like crazy, apparently pushing roots through some rock hard clay in the process. Given its success, I think the native soil with perlite will do well for this adult transplant. It seems like above any specific nutrient requirement, these plants really just need a huuuge root system with good microbials to thrive. I ended up removing most of the the topsoil because the effect of the neem is already systemic within the plant and to remove the possibility of lingering fungus gnat eggs. I planted it with soil much higher up the stem to encourage further root growth into its new perlite/clay top soil. Even if there are still some live eggs down there, I dont think they'd ever be able to make it to the surface. The root ball itself is in a rich organic soil and I'll be feeding it the same nutrients as my other plants with adjustments as needed. I still have one surfs up which seems like the same pheno that i'll be transplanting into a larger pot with my usual soil mix, so it'll be interesting to see how the flavor compares. I've never done a transplant right before dark before, but it's definitely looking better since it hit the new soil.
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  20. the other 2 plants which had the fungus gnats are still showing no signs of resurgence, and will be transplanted tomorrow into larger fabric pots.

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