I first realized my current grow was infested with fungus gnats about three weeks ago. I used spinosad soap first, I had it on hand from the thrips infestation I suffered last summer. I mixed the spinosad soap at 5 tablespoons per gallon for the first treatment, soaked the pots at lights out with treated water, then watered with half strength nutes until runoff at lights on. I did a second treatment 5 days later. The soap did not stop the larvae from hatching, I was still getting between 10-20 new gnats per day on the yellow traps.
I researched the sand, DE and B.T. (mosquito bits) and decided the sand and/or DE was not ideal, so I decided to try mosquito netting bags with a drawstring. I tied the bags around the pots to keep a) flying gnats from laying eggs and b) to isolate the gnats hatching from each pot, hopefully killing them before they lay eggs again.
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As you can see, I am using air pots - my first time switching from fabric pots. I am pretty sure the little buggers have been crawling into the holes on the sides of each pot and laying eggs, so not using just the top layer of soil. The air holes are like a Hotel California for the gnats. I wrapped the pots with tape to try to isolate them even more.
I am now on my 2nd day after my 2nd application of
Monterey LG6332 Bacillus Thuringiensis (B.t.) Worm & Caterpillar Killer Insecticide/Pesticide Treatment Concentrate, 16 oz from Amazon only $14.64 for a pint, all ready to mix (no bits to make tea).
I am happy to report that after finding at least 30 new flying gnats 2 days ago, this morning I found only 2 new flying gnats caught outside the nets (in the tent), and no new gnats in the nets (hatched from the pots). It appears the B.T. treatment is definitely the way to go.
Needless to say, the damage done to the plants has been considerable. I must admit, I had no idea how much damage they can cause in so little time. Without the yellow sticky traps, I'm not sure I would've realized the extent of the problem, because I never say more than one or two flying around when I looked in the tent.
I now realize that if one or two is flying around, there are tens or even hundreds crawling all over the leaves, hiding where you cannot see them. I have confirmed this by accident - twice in the past week, I have pruned lower branches and tossed the pruned material in the trash can (no lid) in my bathroom. Over the next couple days, I caught 4-5 gnats flying around the bathroom each time. There is really no other source than the leaves I left in the trash, and they do not appear otherwise. I am convinced they were on the leaves.
I'm not ashamed to show these pictures, but I warn you, they are not pretty! These are some of the worst looking plants I have ever grown, I still can't believe the damage done. I burned them a bit trying to deal with the deficiencies before realizing the problem was the gnat larvae feasting on my root system. The flowers are still growing, they smell like lemon pledge and they are quite sticky so I am pretty optimistic about the end result, especially considering three weeks ago I thought both plants might end up a total loss.
These pictures were two weeks ago, 3 out of the tent:
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These were taken this morning, sorry they are in the tent - I watered yesterday and they are too heavy for me to lift:
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I am week 6.5 in flower, week 7 starts on Saturday.
This is all the result of using Fox Farms Ocean Forest potting soil without letting it dry out first. I have always opened a bag and let it sit and dry out for months before using it. I forgot to leave it out this time, used a newly opened bag, and paid the price. Never again.
If you can't dry out the soil, treat the soil with boiling water and let it sit for a day before using it. I am now keeping my soil dry and protected.
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Good luck gnat hunting.