Fungus gnats in seedling trays

[cosmotoad]

Hello, I seem to be raising a decent population of fungus gnats in my seedling trays. I've sprayed a couple times with Safer's insecticidal soap but it doesn't seem to be doing the trick, might kill some of the adults but by the next day there seems to be a bunch of new ones. Most of them are probably old enough to take them out of humidity but I'd rather keep them in there for at least a few more months, if possible. Anyone have any tips on how to deal with this? There doesn't seem to be much damage to the seedlings as of now but I don't really want to wait and see what happens.

Thank you!

(edit: i used the word 'seem' way too much in that post but i'll just leave it like that)

[greenknight]

I can tell you what will happen - the larvae will keep eating the root hairs off your seedlings, stunting their growth and maybe killing them.

Some have had success using mosquito dunks - the bacterium that kills skeeter larvae also kills the fungus gnat larvae. You can trap the adults with yellow sticky traps, as well. I found a site that has it all laid out - though I wouldn't buy from these guys, you can get the stuff much cheaper elsewhere: https://www.repotme.com/pest-control/Fu ... ntrol.html I'd be willing to bet their bug spray is just insecticidal soap, which you already have, and I doubt that putting a cinnamon stick in it makes much difference (and it's another thing that's cheaper elsewhere).

[cosmotoad]

Yeah I think maybe I'll just take the lids off for a couple weeks to dry them out, most of the seedlings are plenty big enough to be exposed now. Any idea how long the gnat eggs stay alive after drying out? I'd like to put the lids back on some day if possible, but a lot of people would probably advise against that because of contaminants and such. Anyway I'll see how this drying out goes, thank you

edit: actually after checking just now there seems to be hardly any still alive, maybe the second dose of soap killed most of them off. Also the soil has dried out a bit, which probably helped reduce the amount

[Bosenoge]

Take the lid off and pray that they dont eat everything! My treatment for a mild infestation was:completely dry the pots, repot everything in a moist mineral supstrate, get everything back in the humidity box and anyway new gnats appeared in a week or two. Now the lid is of, everything is dry and still I see one or two sometimes! I lost about a quarter of my seedlings in the process, I think that larvae hatch on to seedlings and as long there is humid atmosphere you are in danger. Only thing that I tried is soaking tobacco overnight and spraying with the liquid but it didnt help.
My only mystery is how did I get them? The seedlings were kept in a moist box which I opened once a day to rince the excess liquid. Must be some fast little buggers...

[cosmotoad]

Yeah I might have to do that.. there's a lot less of them now but if I leave the lids on they might come back in full force. There doesn't seem to be much damage done but I suppose it's all underground as of now. I'd like to put the lid back on some day but I might end up just taking it off permanently, even though the growth will be slowed down a bit

[Licespray]

Fungas gnats are the bane of my indoor palms! They get heavy infestations, nuke them with neem oil...a couple days later they’re back!

[esp_imaging]

In the UK I use a product called Gnat Off. It works very well. Maybe you can get it locally, or a product with the same active agent.
Here's a description:
"Fungus Gnat Off contains the bacterium Bacillus thuringiensis which produces a toxin that prevents the larvae from feeding. This results in their death and subsequent eradication of the problem. Fungus Gnat Off is highly effective against Fungus Gnat infestations. It is also non toxic and fast acting as well as safe to use and has no harvest interval."

[cosmotoad]

Hmm it comes up on amazon.ca but it says it's currently unavailable.. other than that I'm not sure if I'd be able to find it around here. It's mostly under control for now but if problems arise in the future I'll have to do some more research. Thanks!

[greenknight]

Try searching for Mosquito Bits, it's the same thing.

[keith]

Diatomaceous Earth mix it in your potting mix it doesn't prevent them completely but slows them way down. Doesn't harm cactus.

soil will turn white but that's OK. Use the edible kind its all over the internet for sale.

[DaveW]

A friend of mine has trouble with them in seedlings but I seldom have. Probably because I keep things on the dry side and usually top seedling containers with a layer of grit and sow seed on the top of that then wash them into the grit with a mist spray. Also perhaps there is not too much organic material in my mixes to attract them.

We evidently used to get a lot come in with commercial peat or peaty soil mixes. The name "Compost" = potting soil in UK, as well as meaning the rotting organic stuff you make compost heaps with. Presumably the UK name "potting compost" comes in the past from gardeners in the UK making up their potting soils with some old well rotted compost before peat etc was used.

The flies have more of a job laying their eggs in the soil below the grit and it holds up the newly germinated seeds and stops mould spreading if a seedling dies.

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Not tried any of these myself but See:-

viewtopic.php?t=24568

https://www.youtube.com/watch?v=ibfS0Se0Ye4

https://www.sublimesucculents.com/get-rid-fungus-gnats/