Question
 Hi, thank you for taking time to answer my question.  I just read the treatment for black flies. But I still have a question; After removing the top layer of soil, where the larvae lives, and after I place sand or coarse coir or any of the other coarse soils that you mentioned, could these type of soils be harmful for the plants?  Would I have to remove the sand, once the black flies are gone? I am also planning to repot in spring some of theses plants so would it be ok to mix new soil with the layer of coarse earth.  I live in Edinburgh Scotland, and it is very humid, once I get rid of this pest, could I get it again?
In case you need to know what type of plants I have: 2 Banana Plants, 1 Christmas Cactus, Ivy, 1 gardenia, 1 palm tree and 1 Jasmin ivy

Answer
Hi Mariana, I do not know where you got your information but found some additional information for you that may help...
Those flies are fungus gnats. The larvae are probably in the soil that you used when you repotted. If you have any left, I suggest that you discard it. If you recently repotted, then you might consider removing just the soil that you added and replacing it with a peat-based, soilless potting mix, such as Pro Mix.

Otherwise he is what you need to know about treating fungus gnats.

Adult fungus gnats fly around and are an annoyance, but they are not harmful to people. Each gnat lives for about 5 days. The trick is to get rid of the next generation - the gnat larvae that live in the top layer of the soil. The larvae feed on decaying organic matter. Decaying pine bark in potting mixes and decaying plants roots feed the larvae.

Try to keep the soil as dry as possible. Remove all loose soil from the surface and put a light layer of coarse coir (coconut husk) or sand or diatomaceous earth on the soil surface. These substances have sharp edges that carve up the larvae. (Recent studies indicate that fine-textured peat moss also deters gnat larvae.)

Another safe technique is to place ?inch slices of raw potato on the surface of the soil to attract the larvae. After a day or so, discard the slices along with the larvae inside. Repeat this until there are no more larvae in the potato.

For more serious infestations try Knock-Out Gnats to treat fungus gnats available from Gardens Alive for about $20. See

http://www.gardensalive.com/item_display.asp?ProductNumber=1962

Another bio-control method is Gnat Not, a parasite that destroys gnat larvae and other soil pests. It comes on a sponge in plastic (5 weeks shelf life) that is rinsed into water and applied to the soil. For information, go to http://www.goodbug-shop.com/gnatnot.htm

Detection trick: Add a little water to the soil and then look very closely for tiny fungus gnat larvae swimming in the water as it pools on the surface. You need good light and good eyes to see them. If you don't, then your plant is probably gnat free.

Prevention is often the best remedy. Use sterile potting mixes that are free of bark chips. The potting mix should have ample drainage material, such as perlite so that it drains well and allows the soil to dry out frequently. Fungus gnats can nearly always be traced back to overwatering and/or poor soil quality.

Please let me know if any of this is unclear or if you have any additional questions.

Regards,
Will Creed, Interior Landscaper
Horticultural Help, NYC

You can E-mail me directly at: [email protected]