Fungus gnats

they damage roots, so definitely slows them down. if it goes untreated it can kill a plant eventually. The adults don't really damage the plants at all (obviously annoying as hell and you don't want that shit in your house). It's the larva that are the problem to the plants. Bti .. use it preemptively in future. You never know when the substrate you buy is coming with extra hitchhikers, and most places store that shit outside... if not, somewhere in its transportation it was still exposed and could pick up pests and molds etc. if in NA, "mosquito bits" is a top dress your irrigation will infuse the substrate with bacteria that kills larva, and "mosquito dunks" is something that can float in a reservoir (not good if using any sort of plumbing/automated watering without mesh filters). it's a bacteria i won't sepll correctly: Bacillus thurgingeniss subspecies israelis something or other... it's a common thing. it's been used on world's food crops for nearly a century i'd bet anyway, it's cheap and worht it to always avoid any fungal gnat issue even if it comes packaged with your substrate. it will take 2-3 weeks to clear up an ongoing issue or completely avoid it if you start before the problem escalates. top dress every 2-3 weeks. adjust based on what you see. seeing 1-2 gnats that never escalate is okay. I sometimes find 1-3 gnats on a sticky trap after a full grow cycle of time, which is obviously no problem.