i normally have a few compost flies around my plants, but they've never been a huge problem. but this year, my original Spider plant, which i was worried about as it had hardly any roots, and is showing signs of new growth, appears to be infested with them. they seem to love the Spider plants, particularly this one, but my other plants, Begonias, Hoya bellas and Money plants, are hardly affected at all. i know they're supposed to be harmless, i don't know about larger numbers.