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keith wrote: ↑Fri Apr 07, 2023 1:57 pm Hi mmcavall, I also used to have terrible problems with fungus gnats but now just a few . I use diatomaceaous earth powder in my soil mix, have been for 20 years I think it keeps them away.
Could also be that I moved away from a area that had swarms of fungus gnats from neighbors raised planters . They would come through cracks in a fence. Very hard to grow seedlings with fungus gnats around.

jerrytheplater wrote: ↑Fri Apr 07, 2023 3:15 pm Regarding Fungus gnats: I read the label on using Mosquito Dunks or Mosquito Bits for Fungus Gnats. Then I went onto the manufacturer's website and see their product will work for them. I tried it and it really did kill them. I didn't have the bits, just the dunks, so I broke one of the dunks up into bits, soaked it to make a tea, strained out the bits and watered with the tea. No more Fungus Gnats.

https://summitchemical.com/fungus-gnats-what-are-they/ Home Depot and other stores sell them.

MrXeric wrote: ↑Sat Apr 08, 2023 10:50 pm I sometimes get fungus gnats in my seedling propagation box over the winter when it's humid. Since I water the seedlings more frequently they don't have the chance to really dry out and kill the bugs, so I treat with imidacloprid. It's used as a sytemic but I believe it can work as a contact pesticide, especially on the larvae. One treatment is enough.

keith wrote: ↑Mon Apr 10, 2023 1:53 am I add it to the soil mix as I make it. Food grade is what you want about 5% by volume. If you use it wear a mask until its wet its a very fine powder. It shows up at the drain holes and on top of the soil, a white residue as a tiny crystalline structure. I've used it for 20 years at least but used to use a high fired pool filter type don't use that it contains silica bad for the lungs. Best for seedlings they get the worst of fungus gnat problems.

LateBloomer wrote: ↑Mon Apr 10, 2023 6:41 pm I don't treat my fungus gnats... this is a bad idea if you have very few seedlings of a specific species but over 30+ seedlings most will survive without issue. I only have them when I start my seeds since the top layer is topsoil but once repotted into 100% mineral mix they just disappear.

From what I've experienced if the seedlings make it to 6 months the gnats will not kill them at worse they will only slow down growth a little bit. The cacti will produce different alkaloids to combat the gnats and make themselves less attractive to them. But some species seem to lack this ability like mammillaria boolii didn't 'combat' them naturally.