To combat gnats on houseplants, start by understanding their cause (overwatering, poor drainage). Identify the type of gnats (phorid flies, fungus gnats). Signs of infestation include flies, larvae in soil, and yellowing leaves. Control measures include traps (apple cider vinegar, dish soap), natural remedies (diatomaceous earth, neem oil), and chemical treatments (insecticides, nematicides). Prevent infestations by watering wisely, improving drainage, removing debris, avoiding over-fertilizing, and isolating houseplants from moisture sources.