Mosquito Bits for Fungus Gnats: How to Use Them (Bti)
Written by Mustafa Ayan Fact-checked against university extension/IPM sources Last reviewed: October 14, 2026
Direct answer
Mosquito Bits contain Bti (Bacillus thuringiensis israelensis), a bacterium that specifically targets fly larvae such as fungus gnat larvae. Use a product labeled for fungus gnats, follow the current label's rate and reapplication schedule, and combine it with drier soil and sticky traps, since it targets larvae rather than adults.
Do Mosquito Bits work on fungus gnats?
Yes — Mosquito Bits are one of the best-supported ways to kill fungus gnat larvae. The product is simply Bti (Bacillus thuringiensis israelensis) in a granule form. When larvae eat it, the toxin damages their gut and they stop feeding and die. Adults are not affected, so pair it with sticky cards.
Mosquito Bits is labeled for mosquito larvae in standing water; using it on houseplant soil is a common home application of the same active ingredient. Gnatrol is the Bti product labeled specifically for fungus gnats, so if you want the fully labeled route, use that and follow its directions.
What Bti is
Bacillus thuringiensis subspecies israelensis is a naturally occurring bacteria. It produces a toxin that affects the gut of certain fly larvae, including fungus gnats and mosquitoes. It does not work on adult gnats, and products labeled only for caterpillars (a different Bt strain) will not help.
How to use it on houseplants
Bti products are generally applied either sprinkled on the soil surface and watered in, or pre-mixed with water and applied as a drench. Follow the current label for the exact amount, preparation method, application site, and retreatment interval — directions vary by product and jurisdiction, and there is no single generic recipe that's correct across brands.
Bti does not persist forever indoors, which is why labels typically call for reapplication on a schedule — follow what the label says, and use whether you're still seeing adults on traps as a guide to whether to continue past that. Treat each application as that pot's scheduled watering, not an addition to it, so the dry-top habit stays intact between rounds.
How long until Mosquito Bits work?
Larvae that eat Bti usually stop feeding quickly and die within a day or two, but you won't see fewer flying adults right away, because pupae and eggs already in the soil still emerge. Expect trap counts to fall over roughly one to two weeks, and keep monitoring beyond that. Reapply on the label's schedule if adults keep appearing.
Safety
Bti is a narrow-spectrum bacterium — it's used in organic growing operations because it targets specific fly larvae rather than a broad range of organisms. That's a different claim from "safe for everything, no precautions needed." For handling, storage, and any precautions around children, pets, or edible plants, follow the current product label rather than a general statement here, since specifics can vary by product and formulation.
Where Bti fits in the overall plan
Dry soil stops new eggs. Traps catch adults. Bti kills the larvae already in the pot. Together they cover the life cycle. Beneficial nematodes (Steinernema feltiae) are another strong larval option if Bti is hard to find where you live — but they're a living product, not a shelf-stable one. They need to arrive cold, be applied within days, and kept in consistently moist soil to survive, which makes them a better fit for someone who can apply them right away than a backup sitting in a cabinet.