Question
Our peace lily has an infestation of small black flies. We previously had a peace lily die. Its roots seemed to be rotten, but appeared to be being eaten by some small white worms. We are concerned that this plant is heading the same way. What can we do to control the pest?

Answer
Stuart,

You are keeping your peace lily too wet and the roots begin to rot. This attracts fungus gnats which are what you are seeing and the white worms in the prevoius plant were the larva of the fungus gnats. Fungus gnats are a close relative to fruit flys which appear out of nowhere when you have a too old banana on your kitchen counter.  They take advantage of it and the fungus gnats take advantage of the rotting roots, they are not causing them. To solve the problem you need to cut the amount of water you give the plant in half. You have to empty the drain tray under the plant an hour after each watering. It cannot stand to sit with a drain tray full of water under it. It is not a swamp plant. It absolutely must be in a pot with a drain hole and drain tray. As I said cut the amount of water you give the plant in half. Also do not water the plant until it is dry on the surface of the soil for at least 4 days. The plant should be noticeably lighter when it is dry than when it is wet. Allowing it to get very dry b etween waterings will keep it much healthier and get rid of the fungus gnats. You can kill any larva in the soil by watering next time it is dry with a 50/50 mix of water and rubbing alcohol. Good luck.

Darlene