Fungus Gnats + rep

[johnnytsmitw]

Ive had this problem for atleast a month. I put some sand on the soil and it seemed to cut waaay back on the adults, but while doing a little flush, a bunch of these maggots came up.

I sprayed them with some pyrethrin. I was wondering if I could mix some of the pyrethrin into my watering can to kill them for good, or wtf I should do...

 

[johnnytsmitw]

BUMP...

So, I will be doing a watering with pyrethrin spray in the water unless someone steps in.
I have no idea how much these things are stunting growth, if at all.

 

[YGrow]

Those are fungus gnats and they do hurt the plant. They chew on the roots and stress the plant. If you have access to a grow store, pick up some Gnatrol. Mix that up with water and soak you're plants with it. Let it sit on them for at least 24 hours before watering again, longer if possible. The gnat larvae eat the Gnatrol and die within 24-48 hours. To keep adults to a minimum, I use sticky traps, which you can also buy at the grow store or probably any garden store.

 

[RedHairs]

If I remember correctly (which I could be wrong), pyrethrin can be harmful to marijuana in certain doses. I think your best bet is to feed normally, then spray some on the surface. Let your soil dry out well before another watering. You're keeping the soil too moist for those little buggers.

 

[johnnytsmitw]

thanks guys. I did a really weak solution of the pyrethrin spray and spread it around just enough to soak in a little on the top of the soil. I am going to add some more sand to the tops as well to hopefully prevent any eggs getting laid.

 

[rollingarkansas]

If I remember correctly (which I could be wrong), pyrethrin can be harmful to marijuana in certain doses. I think your best bet is to feed normally, then spray some on the surface. Let your soil dry out well before another watering. You're keeping the soil too moist for those little buggers.

Yes I agree, you should consider letting your soil dry out just enough so the surface crusts. Just because it is dry does not mean the soil just beneath is, always use a moisture meter. Those nasty knats love moist areas and it is ideal for their larvae. Also, if there is a lot of peat moss present on the surface this attracts them. Spread a thick layer of well composted (no burnt roots) manure onto the surface. This is less attractive and hospitable to them. Plus it dries out easily and creates a barrier. The crusty soil surface should be sufficient to drive them off. If not use the pesticide. Your plants will "tell" you when they need to be watered. Just observe them carefully. I hope this helps. It sure helped me.

 

[HornedFishy]

If you want to water with something that will do it use azadirachtin. Watering with pyrethrins seems like a horrible idea, especially if you are using organic soil. There is also a product called gnatrol which my mother said worked wonders for the gnat problem with her orchids. Its a little dangerous to mix becayse its just these bacteria that kill the gnat larvae and it is NOT good to inhale. But the azadirachtin is OMRI certified.. pretty sure the gnatrol is as well.

 

[johnnytsmitw]

I am over budget right now. cant really buy anything at the moment. I let the pots dry out for three days, theyre small pots and my temps are a little high so I could already see the effects of this on the plants. I then watered just the top of the soil with the pyrethrin, which IS organic. I then put some more sand on top of the soil, and after that was dry, I aerated the top of the soil with a pocket knife to try to get some air down to the roots as I feel like the sand is cutting off oxygen.

 

[vh13]

I aerated the top of the soil with a pocket knife to try to get some air down to the roots as I feel like the sand is cutting off oxygen.

Oxygen gets introduced into the soil by watering. Aerating the top a bit more to help dry out will help. Sand works well for me, I don't like pesticides, even if organic.

I create a tight fitting mylar cover for my pots, sliced half way down the middle for the stem to grow through. Makes it harder for gnats to get in, and allows roots to fill in the top few inches of soil nicely (by blocking light).

I like to leave a small cup of water out for the adults. Bottle cap filled with a few mL of water sitting in each pot works amazingly well. They dive into the water to lay eggs and drown.

 

[growone]

this is no help at the moment, but i have had a bit of the fungus gnats myself
MG Organic soil comes with them free of charge, like the soil in other respects
anyhow, a light soil heating - 200 degrees for about a half hour - seems to do wonders
that is during soil prep, cheap, effective, and no pesticides at all

 

[HornedFishy]

@johnnytsmitw Yeah... pyrethrins are organic. So are all the little microbes and bacteria in my organic soil that do all sorts of good things for my plant. I personally wouldn't want to use it in my soil. It also has such a short half life(hence it's appeal) but this means that it is only killing the larvae in your soil. Not the gnats, not the eggs. That's why it's probably not working very well. You really need an IGR to fuck with their ability to reproduce. Considering money is a problem I would NOT be surprised if you ended up spending the same amount if not more on the pyrethrins route because you will have to use it constantly.

@growne There is no reason in you buying organic soil if you are going to sterilize it. Pretty sure organic soils are by definition non-sterile because they have all sorts of microbes creating the nutrients. That's why organic fertilizers have such low NPK ratings, their nutrients are in non-available forms until the microbes change them. If you are using organic fertilizers you are just cheating yourself, and if you are not then you are simply paying way too much for soil. If you are looking for a nice organic soil I recommend fox farm ocean forest. I have never gotten any nasties in it, although it prolly does cost a little more.

 

[growone]

There is no reason in you buying organic soil if you are going to sterilize it. Pretty sure organic soils are by definition non-sterile because they have all sorts of microbes creating the nutrients. That's why organic fertilizers have such low NPK ratings, their nutrients are in non-available forms until the microbes change them. If you are using organic fertilizers you are just cheating yourself, and if you are not then you are simply paying way too much for soil. If you are looking for a nice organic soil I recommend fox farm ocean forest. I have never gotten any nasties in it, although it prolly does cost a little more.

200 degrees really should not sterilize, most bacteria are quite tougher than that
i feed with organic teas which have plenty of microbes in their own right
fungus gnats are not so tough, at least i seem to have good luck so far

EDIT: did a little more reading, just for my own sake
200 degrees maybe a little too warm, i see 180 recommended for pasteurization(not sterilization) of soil
this seems to be the optimum point for preserving your beneficials
which is the temps i will use next time

 

[johnnytsmitw]

Oxygen gets introduced into the soil by watering. Aerating the top a bit more to help dry out will help. Sand works well for me, I don't like pesticides, even if organic.

I create a tight fitting mylar cover for my pots, sliced half way down the middle for the stem to grow through. Makes it harder for gnats to get in, and allows roots to fill in the top few inches of soil nicely (by blocking light).

I like to leave a small cup of water out for the adults. Bottle cap filled with a few mL of water sitting in each pot works amazingly well. They dive into the water to lay eggs and drown.

thanks dude. good idea. and cost effective

 

[johnnytsmitw]

@johnnytsmitw Yeah... pyrethrins are organic. So are all the little microbes and bacteria in my organic soil that do all sorts of good things for my plant. I personally wouldn't want to use it in my soil. It also has such a short half life(hence it's appeal) but this means that it is only killing the larvae in your soil. Not the gnats, not the eggs. That's why it's probably not working very well. You really need an IGR to fuck with their ability to reproduce. Considering money is a problem I would NOT be surprised if you ended up spending the same amount if not more on the pyrethrins route because you will have to use it constantly.

@growne There is no reason in you buying organic soil if you are going to sterilize it. Pretty sure organic soils are by definition non-sterile because they have all sorts of microbes creating the nutrients. That's why organic fertilizers have such low NPK ratings, their nutrients are in non-available forms until the microbes change them. If you are using organic fertilizers you are just cheating yourself, and if you are not then you are simply paying way too much for soil. If you are looking for a nice organic soil I recommend fox farm ocean forest. I have never gotten any nasties in it, although it prolly does cost a little more.

I hear what youre saying. I just already have the pyrethrin and only have about a month to go before im done. its a tiny grow so before the next one Ill look into the chemicals youre talking about. we also have basil and lavender so im gonna have to do something about those bc the sands not cutting it for them.

 

[ethics213]

I use neem oil as a soil drench and put up some yellow sticky cards.

 

[MR420MIKE]

Oxygen gets introduced into the soil by watering. Aerating the top a bit more to help dry out will help. Sand works well for me, I don't like pesticides, even if organic.

I create a tight fitting mylar cover for my pots, sliced half way down the middle for the stem to grow through. Makes it harder for gnats to get in, and allows roots to fill in the top few inches of soil nicely (by blocking light).

I like to leave a small cup of water out for the adults. Bottle cap filled with a few mL of water sitting in each pot works amazingly well. They dive into the water to lay eggs and drown.

Awesome idea thanks for that!

 

[i.NeeD.A.LiGhTeR]

I just read that watering plants with 1 part hydrogen peroxide mixed with 4 parts water will kill any larvae in the soil and it should be done once a week till they are gone, but is it ok to do to marijuana?

 

[GrizzlyAdams]

Sup dude. Get in your bunker cuz I'm about to drop a bomb called knowledge.

"Bacillus thuringiensis (Bt) - is the best known of several bacteria that are fatal to caterpillars, larvae and maggots"
"Neem - Most effective against caterpillars and other immature insects including larvae of white-flies, fungus gnats, mealybugs, and leafminers"
"Need oil - Effective against spider mites, fungus gnats, and aphids. It is also a fungistat against powdery mildew and rust."

Kill those little bastards man, neem oil is cheap, so is a Bt pesticide. All of these are organic and if used 2 weeks pre-harvest will not impact your bud.

 

[newb1egrower]

two words - predatory nematodes

 

[MR420MIKE]

two words - predatory nematodes

I just bought nematodes. Gonna try them out. Also got 1500 lady bugs for the other critters.
Next step is the (BT)..

Thanks guys for the input.