Little black flies (aphids?) wrecking seedlings

[kathyd]

Hi

I have both an indoor heated propagator and a large trough in the porch with a heated cable in, and at the moment they're both full of seeds or seedlings, some veg and some flowers. However, we seem to have a load of very small black flies which live on them, and when disturbed seem to dive into the compost and down the edges between the compost and the pots to hide... They appear to be sucking the plants dry like greenfly, and I have no idea how to deal with them! I've tried a really weak solution of bio washing up liquid, and although it doesn't appear to be damaging the seedlings it also doesn't seem to have any effect on the flies. If I 'chase' them with the spray they just hide in the compost again! What can I do? Anything? They're driving me nuts!! I've already lost most of my peppers, I don't want to lose anything else - please help!

[WilliamD]

They're fungus gnats. Try putting a layer of fine sand on the soil. That stops them getting in and out. And water from below so as not to disturb the sand.

[WilliamD]

Actually, having just read this article about them, I wonder whether top dressing with sand might still allow the larvae to attack the roots of your seedlings. Maybe one of the treatments they mention would be better.

[kathyd]

Thanks William, it look interesting

[Jungle Jane]

I had fungus gnats last year,they're a nuisance but didn't do any damage. I just squashed them daily. This year I'm watering from below as they lay eggs in the top bit of moist compost,let the compost dry out a bit? I just found this tip that might help?
*Yellow sticky traps, as mentioned above work well, but shallow containers with either Apple Cider Vinegar or White Wine also do well. *As the vinegar or wine ferment, the adults are drawn to the sweet smell and fall into the liquid and drown. *An added bonus is that vinegar is a natural deodorizer and will rid the air of gnats and smells alike. *As for the wine, gnats aren't fussy, so a cheap bottle is recommended unless you want to enjoy a glass yourself
The British Gardener: Fungus Gnats

[kathyd]

Thx Jane . Well maybe it's not the gnats then, but 'something' is biting my tiny seedlings off at ground level and/or eating their roots. And 'something' is also sucking the leaves of the slightly bigger ones until they shrivel and fall off! I can't let the top of the compost dry out too much or the tiny seedlings will die. I actually used to water from the top sparingly and have never had any problem before (apart from seedling dying from lack of water lol ), but this year I've followed the advice to water from below and the compost is lots moister and full of gnats. I'm obviously doing something wrong, * sigh *.

[mothhawk]

The larvae will tunnel into the stems at ground level, as well as eating the roots, so it probably is them doing the damage. You can put slices of raw potato on the soil to cover it, and the larvae will be attracted to them. Leave for at least 4 hours, then you can lift them up and see how bad the problem is.
A drop of washing up liquid in the vinegar will ensure any gnats going for a drink don't get out again.

Fungus gnats do prefer peat mixes, as they like the semi-decomposed plant material. I only use soil based mixes now such as JI, and am not bothered by the gnats any more.
Oh and they do carry damping off disease, which is a fungus. Clue's in the name

Good site for info:
Controlling Fungus Gnats Organically - Small Footprint Family

[Jungle Jane]

Yes my plants were quite big so a seedling would suffer more with root damage,I did notice my leaves yellowing (tomato) I assumed it was nutrient deficiency,probably was deficient because of the gnats. I would avoid watering for 2 or 3 days if the compost is wet? If you're worried at day 2 sit them in water for 5 mins,maybe less. It's good if the top of the compost is dry,they wont beable to lay eggs.

[kathyd]

Another useful link from Mothawk, thx