My sister asks what she can do about the tiny flies on her indoor cactus plant. First of all, the tiny flies are called fungus gnats. These pesky insects pose little or no threat to plants, but they get annoying to us people. The best treatment/prevention is to avoid overwatering. So, little sis, just let your cactus dry out for a while in between each watering. For most houseplants, you should let the surface of the soil dry out before watering anyway. With a cactus you could let it get really dry in between waterings. If fungus gnats get too bad, some people recommend putting an inch-thick layer of sand on top of the soil. There are also sprays - like Gnatrol - to kill the little buggers. I really should get some sand on all my houseplants. Last winter I grew about 100 delphiniums from seed in several windows around my house. I needed to keep the soil moist for the little sprouts, which led to an unfortunate increase of fungus gnats. The delphiniums are happily planted outside now, but the gnats live on in my numerous pothos, philodendrons and other houseplants. I really hate it when one flies up my nose when I'm falling asleep . . . and I'm embarrassed to think of what visitors must think if I don't explain the situation. I'll have to put sand on my next shopping list!