Lately, since I’ve overwintered some of my plants which include coleus, hydrangeas, and geraniums, and also some of my African Violet leaves I’m trying to propagate, tangerine plants, Christmas Cactus, spider plants, etc. I’ve noticed a lot of black gnats around my plants. I’ve tried a lot of remedies to get rid of these things including cutting little yellow rectangles with petroleum jelly to trap these little suckers.
The only remedy that works with these pests is a dish of soapy water. All you need is a saucer (I got some at the dollar store in a pack of 4 for a $1.00), water and a couple drops of dish soap (I use lemon dish soap: it attracts these bugs). I add dish soap to the saucer and fill the saucer with a cup of water. I allow the soil in my houseplants to dry out to kill the baby fungus gnat eggs. I then put the saucer near the inflicted plants and the gnats eventually seek water and go to my “trap” saucers. The soap prevents them from flying away and they eventually drown. In a couple of days, you’ll notice that there are dozens of gnats in the water! Success proven! I hope this helps anyone who are burdened with these pests.