Each year in the super year spring/late winter I get a pretty dramatic fungal gnat problem in some of my plants and it is entirely off the charts maddening. I haven’t found a way to avoid this completely but I have picked up a couple tricks that help keep the issue at bay and more short-lived.
Years ago I tried the ol’ vinegar and soapy water thing. The vinegar can draw them out of the soil but catching a couple a week is not a successful endeavor. For me the best combination has been insecticidal soap and sticky paper. This brand of sticky paper is not the most attractive or subtle, but they work the best of the four or so different brands I’ve tried. I doubt either of these are as quick and effective as chemical-based bug killers, but I’ve come to accept the tradeoff. If I don’t catch the issue early enough and things get unbearable I’ll reluctantly bring out this botanical-based insecticide and it works instantly.
Before I even begin seeing these flies buzzing around certain plants I know which ones are infected. I’ve noticed that they stop growing and can develop brown spots on the leaves. Through trial and error I’ve learned that the greatest contributor to a plant being an ideal fungal gnat host is if it is low-light and the soil retains moisture. The problem has only ever been with plants that are in very low-light rooms, meaning that my ZZ Plants are consistent targets. The best thing I’ve been doing to combat these ultimate enemies is bring these susceptible plants into bright rooms during the day; it can be a pain to remember but worth it and the ZZ Plant in particular can tolerate some sun luckily.
Another place where these little stinkers can thrive in the house is a sink drain, eventually finding their way to plant soil. If I catch one flying out of a drain I pour very hot vinegar water down it after clearing it with a plumbing snake and that will usually take care of it.