Question
I have little black flies coming from my house plants. I kill them , and the next day or two they are back. Someone told me that the eggs are in the soil. Can you please tell me what I can do to get rid of the eggs that are in the soil? Is there something that I can put in the soil to kill the eggs that won't kill my plants ,but get rid of the flies?

Answer
Fungus Gnats?  If these are THOSE little black flies, do little damage to a plant, but they are unsightly little pests.

The Gardenline (http://gardenline.usask.ca/pests/gnat.html) posted by the University of Sasketchawan has a whole webpage on these little critters: "The larva (worm) is the only form to feed, and it feed mainly on fungus and organic matter in the soil."

Too many larvae and there is a chance they will start going for the roots of the plant.  Minimal damage.

To catch them, make yellow sticky flags to trap them.  They love the color yellow, and get stuck to the paper.  For directions, see the Yardener website (http://www.yardener.com/YellowStickyTraps.html).

You can also use a dash of B.t. (Bacillus thuringiensis) on the soil.  That's the bacteria that causes Fungus Gnat disease, to destroy them without using insect spray (even worse to use because you're inside).  The B.t. makes them sick and they die.  It is harmless to mammals including humans.

If these are thrips, which dine on pollen and new plant shoots and damage flowers, you have to use other ways to get rid of them.  On his Thrips IPM page (http://ipmofalaska.homestead.com/files/thripsipm.html) gardener Rocco Moschetti describes a novel, effective way involving heat treatment: "Researchers found that an air temperature of 104?F with a relative humidity of 10% was sufficient to kill Western Flower Thrips (an environment that was fatal to plants within 4 days)."  That's high, dry heat he's talking about.

Thanks for writing.  Keep me posted.