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Avatar for MinaPelerina
Jun 1, 2020 8:22 AM CST
Thread OP

Hi dear plant lovers,

I am seeing little flies on the dirt of my beloved calatheas and on my banana (they are on the same shelf).

I got Calatheas few months ago and I was watering them too much, they got root rot, and 2 months ago I cleaned the roots, cut the unhealthy ones and repotted them.

I thought the problem was solved, but now the flies appeared again! And there are more and more again. I am really careful with the watering, I am always waiting for leaves to drop a bit before watering, and checking that the soil is not too wet. They are actually thriving and looking so good, and I want them to stay like this. I have been checking the roots but they look healthy.
I went to one of the house-stores and the lady suggested I put the fine sand on the top of the soil and with this preventing the flies to go out and lay more eggs.
My question is: has anyone tried this? Do you think it helps??
If not, what should I do? Repot them again and use some sort of anti-fungi device?
Because I am afraid to stress them out unnecessary if I just check the roots again in new sand.

I would appreciate any advice!
:) Thank You! Hurray!
Avatar for Helicopterplantmum
Jun 1, 2020 8:32 AM CST
Region: Europe
I'm not a professional in any way but I have heard that putting a layer of clay pebbles or other non-organic material can prevent flies from laying eggs. Also, yellow tape traps and letting the soil dry out a bit between watering should help too from what I've read.
Hope this helps and I'm not wrong! Crossing Fingers!
Jun 1, 2020 9:02 AM CST
Virginia, USA (Zone 6b)
Avoid fine sand if possible. It won't fix your problem but instead will get mixed in with the potting soil, compact around the roots and cause more drainage problems. Have you tried watering with Bti (Bacillus thuringiensis subspecies israelensis) to kill the larvae? If you're in the US you can get this in the form of mosquito dunks at most nurseries and big box stores. The tape traps mentioned by Helicopterplantmum will catch the adults.
Avatar for MsDoe
Jun 1, 2020 11:42 AM CST
Southwest U.S. (Zone 7a)
I haven't used sand, and I don't know if that would work. I think what you have is probably fungus gnats, not flies and not fungi. An anti-fungal won't help with this. Repeated re-potting is likely to just further stress the plant, I wouldn't recommend it.

I have used the Bt-i, it helps a lot but won't totally eradicate them. I have "Mosquito Bits" instead of dunks. I just sprinkle them on the soil and water normally. Bt-i also works against mosquitoes, I have it on hand to treat an open drain outside, and also use it for the fungus gnats on indoor plants. This and yellow sticky traps are the least-toxic treatment.

If you decide to really nuke them with a most-toxic treatment--Bonide Systemic Houseplant Insect Control Granules is one product that works well. It contains imidacloprid, which kills the gnats but is also highly toxic to bees, butterflies and other beneficial insects. Please read instructions carefully, and don't use it outside or on any flowering plants.
Avatar for MinaPelerina
Jun 2, 2020 12:38 AM CST
Thread OP

MsDoe thank you so much for your advice, I will follow it, the last part I think. All of my plants are inside plants so that will fit well.
Will report how it goes Smiling
Jun 8, 2020 8:08 AM CST
Name: Laurie b
Western Washington (Zone 7b)
Houseplants Orchids Region: Pacific Northwest Region: Mexico Sedums Tropicals
Back in the 70's, it was a practice to use sand. But only the builders sand that had very fine glass in it. The tiny glass in it would chop up the gnats as they emerged from the soil to make thier first flight out of the soil. It did work, but other sands did not. Also, if it got on your furniture and went unnoticed it could scratch the heck out of it. I use the mosquito bits and am happy with that. However I admit that if one of those gnats flies up my nose in the dark, I am pulling out the Bonide in a heartbeat. This insecticide will kill bees so it's a terrible idea to use it outside.
Jun 8, 2020 9:20 AM CST
Name: Will Creed
Prof. plant consultant & educator
The safest and often most effective way to treat fungus gnats is to let the soil dry out as deep into the pot as possible and also to remove any loose soil from the top. The gnat larvae live in the upper layer of the soil and they require constant dampness to survive. It does take time and careful watering but it can be quite effective.
Will Creed
Horticultural Help, NYC
Contact me directly at [email protected]
I now have a book available on indoor plant care
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