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#1 corene1

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Posted 24 March 2012 - 10:04 PM

How do I kill fungus gnats without hurting my young plants. I have the gnats but have no sign of the larve. and What are mosquito dunks?

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#2 silva83tj

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Posted 24 March 2012 - 10:37 PM

mosquito dunks kill the larve of gnats...i picked some up from home depot...they look like circle cookies or something haha...i plan on using it tomorrow...had a dunk soaking in a gallon of water for over a week now...for the adult gnats i used yellow sticky fly traps and ive got over at least 200 of them bastards on there...i will post a pic...but thats how i stop by the adults and hope the dunks work for the babies.although i have not noticed any larvae....to test for larvae cut a piece of potato and leave it in your pot a couple days then turn it over and see if you seem there!

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STICKY TRAP #1
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STICKY TRAP #2
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#3 3/5King

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Posted 24 March 2012 - 10:47 PM

Holy cow thats a lot of em' I have them too but just a few adults each time I water.

"Take this quarter, go downtown, and have a rat gnaw that thing off your face! Good day to you, madam.”

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#4 Maxsack331

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Posted 24 March 2012 - 11:26 PM

haha, I'm going to go take a pic right now of the just one of the sticky traps I have down in my grow room.. they were pretty bad, but the sticky strips have helped a TON, and that with the mosquito dunks (I have these little packages which are the same thing pretty much, just not stuck together) when I water helps a lot now... and I also have a little tray with some soapy water and vinegar to kill them too.. and they get fried by the HID in the "cool tube" that I have to blow out every day

k here they are lol, there is another strip in the back of the tent that has a lot also, not as much as the one in the 2nd pic, but a good amount more than the one in the first pic.. and then all that crap in the water/vinigar/soap, are pretty much all gnats... I hate them, they really like the basil pot (the teal one in the 2nd pic) since they have a lot of cover and the soil gets dry, but not really as much as the peppers)
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Edited by Maxsack331, 24 March 2012 - 11:40 PM.


#5 Siliman

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Posted 25 March 2012 - 12:18 AM

So sad to see that level of infestation.

Forget the potato! You probably can't see the larvae-THEY ARE TINY. Drench with the mosquito dunk water, look at the runoff water with a magnifying glass. Look closely at the runoff water. Both you guys will see larvae flipping all over the place in your drained water. They will live for a couple of days, so spray them with something like Neem.

For every adult gnat on those traps, there are a couple hundred larvae IN YOUR SOIL, eating your roots.

Drench, and consider an immediate re-pot with fresh soil.

Edited by Siliman, 25 March 2012 - 12:39 AM.


#6 corene1

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Posted 25 March 2012 - 12:30 AM

Thanks everyone! I had to use a jewelers loop to identify the gnats, it is a 10 power loop so maybe I can see the larve with it too. My plants are still very small so can I soak the soil with the dunk solution and not hurt my plants? What is Deem and where can I get it. Is it a nursery item or maybe from Home Depot.

#7 Siliman

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Posted 25 March 2012 - 12:38 AM

First thing to do is just drench your plants with water and use your loop to check the runoff. If you see the tiny white flippin larvae with a black head, you got gnats. Get the dunk water activated and use it once a week. It has never harmed any of my seedlings.

My local Home Depot didn't carry Neem oil. I got a little bottle of concentrate from a hydronics store. Keep a dilute version handy with some added soap in a spray bottle. I spray the larvae in the run-off water just because they can live for a few days before dying from the dunk water.

#8 corene1

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Posted 25 March 2012 - 12:54 AM

I just read an article that said you can mix 3% hydrogen peroxide at a 4 to 1 ratio with water and water it into the soil and it will kill all the larve. It said you have to let the first 2 inches of surface soil dry first though as the larve will suspend thier actions in a drought condition. A little foaming will occur but the solution will break down into water and oxygen in a few hours. If I try it I will definitely re pot afterwards.

The website is www.freeplants.com/fungus-gnats.htm hope this works.

#9 Capsicum

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Posted 25 March 2012 - 01:12 AM

I just read an article that said you can mix 3% hydrogen peroxide at a 4 to 1 ratio with water and water it into the soil and it will kill all the larve. It said you have to let the first 2 inches of surface soil dry first though as the larve will suspend thier actions in a drought condition. A little foaming will occur but the solution will break down into water and oxygen in a few hours. If I try it I will definitely re pot afterwards.

The website is www.freeplants.com/fungus-gnats.htm hope this works.


That will work I can say. I use soap too.

"If I try it I will definitely re pot afterwards."

No need. H202 will not hurt plants, in fact is a good cleaner and keeps roots fresh and healthy. it only hurts microbes in the soil, unless your growing organic that will not be a issue.

Edited by Capsicum, 25 March 2012 - 01:16 AM.


#10 Siliman

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Posted 25 March 2012 - 01:14 AM

That's a good article! Prior to your H2O2 idea, it recommends "BTi" as a drench (which is the active ingredient in mosquito dunks) as also effective.

The only problem with waiting for the soil to dry out, the larvae are munching away. That is why the yellow sticky traps are good as a monitor--if no action on the traps, then no real worries.

By the way, the 'sticky traps' the others posted are fly strips. While they are yellow, and sticky and catch gnats, there are also smaller flat ones that can be placed horizontally to indicate your level of infestation. The sticky traps won't wipe out big infestations, but they will take out the occasional straggler and let you know if something bad is brewing down below.

Edited by Siliman, 25 March 2012 - 01:17 AM.


#11 irishmaverick

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Posted 25 March 2012 - 01:42 AM

Are mosquito dunks safe to use on seedlings? I know that mosquito dunks don't normally harm the plants in anyway, but I'm a little nervous to treat my seedlings with these dunks. I do have a gnat problem though so I need to take action somehow.

#12 corene1

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Posted 25 March 2012 - 02:21 AM

OK , I watched them die HAHAHA! Now have to hope my plants don't. My pots are set into a soil heating box so they are surrounded by warm soil up to the top of the pots,it looks like a big planting tray, they are 5x5x3.5 deep ,when I pulled them out you could see the little larve on the out side of the pot maybe 4or5 to the sq.inch and very few on the inside. I have some pots that are only soil and no plants so I checked all the soil in those pots after treating it and saw lifeless larve. Just couldn't go to sleep tonight knowing something was hurting my plants!

That's a good article! Prior to your H2O2 idea, it recommends "BTi" as a drench (which is the active ingredient in mosquito dunks) as also effective.

The only problem with waiting for the soil to dry out, the larvae are munching away. That is why the yellow sticky traps are good as a monitor--if no action on the traps, then no real worries.

By the way, the 'sticky traps' the others posted are fly strips. While they are yellow, and sticky and catch gnats, there are also smaller flat ones that can be placed horizontally to indicate your level of infestation. The sticky traps won't wipe out big infestations, but they will take out the occasional straggler and let you know if something bad is brewing down below.


They are the same things we used to hang in the cow barns. Couldn't figure out what a sticky trap was till I saw the pictures.

First thing to do is just drench your plants with water and use your loop to check the runoff. If you see the tiny white flippin larvae with a black head, you got gnats. Get the dunk water activated and use it once a week. It has never harmed any of my seedlings.

My local Home Depot didn't carry Neem oil. I got a little bottle of concentrate from a hydronics store. Keep a dilute version handy with some added soap in a spray bottle. I spray the larvae in the run-off water just because they can live for a few days before dying from the dunk water.



I am definitely going to start a control program for them. Thanks!

#13 Siliman

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Posted 25 March 2012 - 12:19 PM

Are mosquito dunks safe to use on seedlings? I know that mosquito dunks don't normally harm the plants in anyway, but I'm a little nervous to treat my seedlings with these dunks. I do have a gnat problem though so I need to take action somehow.


Several university Ag departments recommend BTi (aka mosquito dunks). It breaks down fast, which is good, but must be re-applied. Like I said before, it has never harmed my plants.
Here's more information: ucdavis PESTNOTES

OK , I watched them die HAHAHA! Now have to hope my plants don't. My pots are set into a soil heating box so they are surrounded by warm soil up to the top of the pots,it looks like a big planting tray, they are 5x5x3.5 deep ,when I pulled them out you could see the little larve on the out side of the pot maybe 4or5 to the sq.inch and very few on the inside. I have some pots that are only soil and no plants so I checked all the soil in those pots after treating it and saw lifeless larve. Just couldn't go to sleep tonight knowing something was hurting my plants!


They are the same things we used to hang in the cow barns. Couldn't figure out what a sticky trap was till I saw the pictures.


Couple of comments: keeping your soil warm after plants have sprouted might not be a great idea. Plants can get leggy--and the f-gnats love the heat.
The sticky traps used for monitoring are flat, not the long curly fly strips. After you study the habits of the f-gnats, you find it is better to have traps flat near the base of the plants, as they are lazy flyers and like to crawl around rims of containers. I'll post some pix in a second

last year's sticky trap:
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This year--much better after applying several pre-emptive countermeasures:

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Edited by Siliman, 25 March 2012 - 12:39 PM.


#14 Maxsack331

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Posted 25 March 2012 - 01:39 PM

yeah that one trap that I had that was absolutely cover has been in there for.. a month probably, but I know it is still a lot, I had forgotten to use the dunks the last couple times watering, I was more concerned about the watering and the way the plants were first, that I forgot about the gnats, BUT I will be back on watch and planning on wiping them out for the most part, there are not too to many now, the traps are just from when there were tons in there right when I started using the dunks.. I am tempted to use the H2O2, but I have organic stuff in there, particularly mykos and azos, I don't know how it would affect the algae, but the azos it would not be good for.. and they will be going outside in a month so once they are in the ground nature should take care of it, I actually think I am going to sprinkle a bunch on the soil once I get everything potted up just to do a little more clearing out lol, even though it will probably be a waste

#15 zappa883

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Posted 25 March 2012 - 03:41 PM

Man that is a hefty yield of bugs! Never seen that many !

#16 Siliman

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Posted 25 March 2012 - 04:03 PM

yeah that one trap that I had that was absolutely cover has been in there for.. a month probably, but I know it is still a lot, I had forgotten to use the dunks the last couple times watering...


Same thing happened to me last year. I missed a couple of weekly cycles of dunk water and the gnats came back pretty fast. I learned to keep a dunk pellet in a bucket of water and apply with ferts on most watering days.

You're right about getting the plants outside--gnats are doomed out there, IMO. But if the plants stay in containers, I would swap out their dirt, which most people would do anyway when they pot up.

Man, I'm so glad I don't have them this year. They are my arch enemy. :flamethrower:

#17 frosty

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Posted 25 March 2012 - 04:34 PM

One bad experience will teach you the benefit of prevention.
My tips:
1. If indoors use potting medium. Avoid organic potting soils. Yeah, I mean MG organic choice potting soil. I don't want to start an argument but a sterile medium is going to prevent gnats better. It is my recommendation and you can recommend MG OC for preventing gnats.
2. Let the pots dry out a little between watering
3. Air flow. They are lazy flyers and thus can be blown away.

Since I did those three things I have never had a gnat.

BT, Strips, scraping off the top of the soil, fans will all help. I have also noticed taking them out in the sun seemed to help.

#18 Maxsack331

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Posted 25 March 2012 - 06:33 PM

I was talking to Orangehero when I went to pick up a few plants from him,(big thanks btw) and he said last year he had issues with the gnats eating his seeds, and I asked him what he meant by that, and he said they get inside the seed and eat it.. and when you dig them up, you find just the empty shell.. so now, even though I don't really have many, maybe only 1-2 gnats here and there in the shelf where I keep the seedlings, I have been finding my chocolate bhuts seeds when I was wondering why they wouldn't germinate with just the shell.. so I think I am going to go on a killing spree with the gnats to get all of the ones that I can

I have a lot of air flow, and I let the soil almost competely dry out, and I put a little layer of pine mulch on the top of the pots which I think is a little dry layer, even though they can climb down, but all the soil gets pretty dry before I water again.

but anyways, when you guys say change the soil out when you pot up.. .you mean knock all of the soil from the roots and put it in all new soil? The only problem I can think with that, is that I use soil/compost that I have from outside.. so not sure how much better it would be, BUT I have been starting to add the little packets of the BTI stuff, the grains to the soil when I mix the soil, and the ones I keep in 5 gallon pots will be outside for the summer. but hopefully those gnats get mostly killed during that time.

#19 Siliman

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Posted 25 March 2012 - 07:41 PM

@ Frosty--good tips there. Easiest way to get f-gnats is bring them in yourself. Last year, I used pine fines as a major ingredient in my medium, and it surely was the source of my gnats. This year, I did use some potting mix for my overwintered plants, but it was high quality stuff. I was lucky not to get any gnats, but I also used a liberal dose of Diatomaceous Earth wherever gnats might tread. Next year, it will be ProMix BX only.

@ Max, if I was sure there was heavy infestation of gnat larvae, I would knock off most of the old soil when potting up. Maybe spray the roots area with a Neem solution. I know this will set the plant back, but it would be better than re-inserting the gnat larvae in the new container. Also, I'm not sure about using the dunk pellets in the soil itself as you described. It seems like it would work, but it is designed (I think) to be used as a liquid drench.

Edited by Siliman, 25 March 2012 - 07:49 PM.


#20 corene1

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Posted 25 March 2012 - 08:37 PM

Well I think I have my problem fixed. Re potted all of my plants and got rid of anything that could be remotely infected with larve. As of this evening they look good, no wilting or discoloration. Got a tip from the lady at the nursery , she said I could put a little bit of clean sand on the top of the soil so it will stay dry and that it will be a deterent for the gnats. I took it one step farther and sprayed the sand with Neem before I sprinkled it on top of the soil. I felt bad for my little plants, the roots were really chewed up. Probably why they are small for the amount of time they have been sprouted. They sprouted in mid Febuary and are only about 1 1/2 inches tall and barely 2 sets of real leaves hopefully this will give them what they need. I have certainly seen some really nice plants on this website that is for sure. Again , Thank you for all the advice.




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