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I hope I'm posting this in the right place. 


I am constantly loosing the battle against fungus gnats. It's not the adults that are the problem but the larva. Everytime I think I have an answer they come back and go nuts. This time they have been destroying all my Onions from the inside of the bulb out.


We have a constant flood and drain system. Basically the water is always coming in and going out. 

We've tried Nematoads, BT Thuriside, Mosquito dunks,  Alchoal spray, washing all the rock out and starting over, placing cut potatoes on the rock and even flooding the system and sweeping all of the larva out. I have sticky traps every where and still can't get them controled. 


Please any ideas or suggestions? Our system is inside our house in an extra room, so there are certain things we can't use. And we can only afford low cost solutions. I'm willing to try anything! Our plants were looking great but the Tomato leaves are starting to be affected.


Thank you so much for any info!!

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For fungus gnats, I use Natrol, but still have to kill em off every now and then with a Pyrethrin.

i have used "Take Down" Pyrethrin Garden Spray and have had no fish death. i mix it at about 75% of its recommended rate, and spray the underside of the plants and directly into the netpots. i use a very fine head on the sprayer, with a surfactant added to help hold it there. PLEASE take this information AT YOUR OWN RISK!

i have 3400 gallons in that system and the volume of water may dilute it enough to spare the fish. 

The plants are 100% safe - they are only dangerous to small insects.  You will find some true pitcher plant species that are able to digest small rodents and baby monkeys...but they only grow in tropical jungles, lol, so you are probably good to go.  The goo they produce (sundews and butterworts) have such low levels of digesting power that it doesn't affect skin...much like our own saliva digests food slightly but does nothing to harm us.  The pitcher plants produce a water like substance (and collect water in the wild...I occasionally fill mine with distilled) that has enzymes to start breaking down the bugs that drown in them.  Again, noting that can even start to harm us.  Not that I would recommend letting your kids naw on the plants, lol, but I think that goes without saying.


You may want to keep the sticky tape going with the plants when you first start (and do a bit of research on growing them, they really are easy, especially if you are managing an AP system) to catch a crap load and then phase out the tape.  What I find is the fungus gnats enjoy the constant stagnant water in the dish of the plant and tend to nest there...then most of the adults get eaten...larvae turn to adults, some breed, some get eaten, the cycle continues.  They are definitely attracted to them though, I have some carnies in my living room and my tank is in bedroom, and I find fungus gnats in them as well.  

I asked some experience growers in my area, and they all said to use the same things that are mentioned so far, but use them all at once, and be patient. The gnats life cycle is 28 days, and nothing fish-sage will kill the eggs. So you have to let them hatch and kill them too. One aid not yet mentioned is diatomaceous earth, that is if you can devise a way to pit it on your growbed without grinding your pumps. Everyone agreed that the fungus gnat predators sound like a great solution, but I haven't found anyone at all that has actually used them. Also saw this:
Potato Cubes or SlicesFungus gnat larvae migrate to feed on the underside of potato pieces placed in media. To determine whether container media are infested, use 1-inch cubes or slices of peeled raw potato imbedded about 3/8-inch deep into media. Pick up and examine the underside of each potato and the soil immediately beneath it about once or twice a week. Compare numbers of larvae before and after any treatment to determine whether larvae are being controlled.
You mentioned it in the post, but it was news to me.
Thanks for the new hobby you've inspired, Ricky. Now I've got to get my carnivorous plant fix on. Sundews make a really cool garden addition, makes me smile to see gnats suffer. Orchard Supply has four regular carnivores to choose from; I got two sundews and a Venus flytrap. Anxious to get some pitcher plants as well. One of the sundews I planted right in the gravel, we'll see how it does with nutrient rich water as opposed to the distilled water recommended.

I have a Sundew and have used it for the Fungus Gnat problem.  The Sundew loves fungus gnats.  I bought some Sundew seeds from somebody on e-bay and they cautioned about fungus gnats.  The baby's apparently have problems when fungus gnat larvae eat the roots.  I haven't seen a problem with my one larger sundew though.

Ricky Flickenger said:

Since you are likely in a sunny spot (I'm assuming lots of light because of the tomato plants), try adding some sundews around your system.  Sundews are insect eating plants (very attractive) and act much the same way as the sticky tape does, but they are self cleaning and use the nutrients from the bugs to grow.  I use to sell these for a living and they are very easy to grow - they are native to the pine barrens of not exactly exoctic

They will not elimate the problem, but they will help control it - possibly to the point of them not effecting your onions.  

They can not be planted IN the system, the roots wont be able to handle all the nutrients/minerals in the system and will die.  The easiest thing to do is to plant them in  some containers (no holes, these are swamp plants, LOVE water logging) and set the dishes on top of your flood and drain system.  The plants don't get that big - so a bunch of small containers spread across your system would work well.  They just need lots of light, same as your tomatoes, and are good with 55 - 85 degrees.  The gnats will be drawn to there smell/pheremones and the constant water of the dish.  You'll start to see them accumulate onthe leaves after a couple days.  These plants don't like a lot of nutrients in their substrate (they seem to thrive in anerobic systems, opposite of our lovely tanks) so you can most likely use what you use in your system to plant these guys. If you collect rainwater or have distilled water around, that's what to use for these plants, no tap water.  

Not to complicated and once set up, just keep filled to the crown with water.  Good luck


"BT Thuriside", as you mentioned trying, is the kurstaki variety, and will not work with fungus gnats.  Bacillus thuringiensis isrealensis (BTi) has worked very well for me in the past, but it must be the isrealinsis variety of BT.  

I've found better results with multiple (e.g., daily) applications using enough solution to soak into the first few inches.  I've also used "mosquito dunks", which I break up into small pieces* and mix into the top inch of the grow media (if your problem is that bad, try both!).  The beneficial (predatory) bacteria will eventually end up in your water column, but as long as your system is somewhat mature it shouldn't effect it ... I've actually never found it to be detrimental either way.

You can also refer to Molly's presentation on insect control at

 *To break it up, I put the donut shaped dunks in a plastic bag and pulverize it with a hammer.

I've used mosquito dunks in AP system and Duck systems and as far as I can tell, they haven't hurt anything.

By now Cher's gnats have found my growroom in Colorado.  I have MILLIONS of larvae in the gravel.  Problem is, my time wouldn't work for me and I have just left it on constant flood/drain.  I'm eventually going to change from gravel to a cleaner media and make a few other adjustments to the system so maybe it is best now to move the fish out temporarily, fumigate and start over with fresh media and plants.  What do you think about that approach?
Growzay said:

You should address the problem which is most likely the constant flow watering cycle. If I were you I would try getting a timer and setting the timer to cycle on ( water) once an hour for fifteen minutes. It would be off for forty-five minutes giving the roots time to become unsaturated. Also, you should check your watewr level and make sure it is at least an inch below the surface of the grow media. That alone should eliminate you problems substantially. To kill the larva you should try spraying the roots lightly w/hydrogen peroxide- 1 part- 3 part water.  Keep fresh sticky yellow traps available to knock down the breeding population and to help identify any other types of pests that may enter from the outside. As a last resort you should consider using a pyrethrum fogger in the room while your not home.

We had a fungus gnat outbreak in our Boulder greenhouse. We had pretty good results with Hypoaspis Miles (H. Miles). They're a small predatory mite that feed on thrip, springtails and fungus gnat larvae.

There is also a product out there by Growstone called Gnat Nix. I have not used it myself, but it may be something worth checking out also.



Mosquito dunks have the bacteria that will also work against fungus gnats.  Trick is you probably need to pulverize the dunks or get the granules so you can mix them with water and then spread or spray over your media.  Or perhaps if you can sprinkle the bits of a dunk or the granules over your media bed and then flood the bed deep enough for a while to wash the bits down into the water and let them dissolve some perhaps that will help against the gnats.

  I think it is unlikely that switching media to something "cleaner" will help.  The Fungus gnats are after the moisture and roots, the lack or presence of "soil" makes little difference to them.  Sticky traps may help catch the adults but the particular bacillus thurengensis that is in Mosquito dunks can also infect the fungus gnat larva and is probably easier to get on short notice than the beneficial bugs (but I think I would also order the beneficial bugs too to help take care of the ones that don't die of the BT.

I think you can find that strain of BT in a product called "Gnatrol" by Valient

Yes, Gnatrol is the same BT strain as mosquito dunks but in a liquid form for use in greenhouse/nursery production where you would apply it with sprayers or even directly through the fertilizer injection systems and I know a place where I can get it.  Unfortunately the 5 gallon bucket size is the smallest they offer and it costs over $300 so I haven't been willing to spring for it.

Oh when I search "Gnatrol" by Valient apparently they have it in a water dispersal granular form now.  Still, since they don't say where to buy it to what size it comes in, I have to guess it is still basically the same stuff I found before that I couldn't justify spending that much money for when a package of mosquito dunks doesn't cost that much and I already had them on hand.

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