Mosquito Bits are one of the most effective ways to kill fungus gnat larvae in houseplant soil. The product contains a naturally occurring soil bacterium that targets larvae in the growing medium, killing them within 24 to 48 hours. It’s safe for your plants, pets, and household, and a single container can treat dozens of plants.
How Mosquito Bits Kill Fungus Gnats
The active ingredient in Mosquito Bits is a bacterium called BTI (Bacillus thuringiensis israelensis). It produces protein crystals that are toxic only to the larvae of specific insects: mosquitoes, black flies, and fungus gnats. When fungus gnat larvae feed in the soil and ingest BTI, the proteins destroy their gut lining. They stop feeding and die within 24 to 48 hours.
BTI is highly selective. It does not harm earthworms, beneficial soil microbes, or adult insects. It has been registered with the EPA since 1961, and research has shown that ingesting BTI proteins does not cause cancer, genetic mutations, or birth defects in mammals. It’s safe around children, dogs, and cats.
The Tea Method: Best Way to Treat Houseplants
The most popular approach for houseplants is brewing “Mosquito Bit tea,” which means soaking the granules in water and using that water on your plants. This distributes the BTI evenly through the soil where larvae are feeding.
Here’s how to make it:
- Ratio: 4 tablespoons of Mosquito Bits per gallon of water. Some growers use as little as 1 tablespoon per gallon with good results, so there’s flexibility here.
- Soak time: Let the bits sit in the water for at least 30 minutes. For stronger results, soak for 2 to 4 hours.
- Strain and water: Remove the bits (a cheesecloth bag or fine strainer works well) and water your plants as you normally would.
You can reuse the same bits for a second or third batch before they lose potency. Warm water helps release the BTI faster than cold.
Sprinkling Bits Directly on Soil
The alternative is to scatter dry Mosquito Bits on top of the soil and water them in. This works, but it has a drawback: the corn cob granules that carry the BTI sit on the surface and stay moist, which can encourage mold growth in humid indoor conditions. The tea method avoids this entirely since you’re only adding the treated water. If you do top-dress, use a light layer and remove any granules that start to grow fuzzy.
How Long It Takes to Clear an Infestation
BTI only kills larvae, not eggs or adult gnats. That’s fine, because the adults only live 7 to 10 days and can’t bite or damage your plants. The real harm comes from larvae feeding on roots. But it does mean you need to keep treating through the full life cycle to catch every generation.
At typical room temperatures (65°F to 75°F), fungus gnats complete their entire life cycle in three to four weeks. Females can lay up to 200 eggs in the soil during their short lives, and larvae reach full size in two to three weeks before pupating. This means a single treatment won’t be enough. You’ll need to water with BTI tea every time you water for at least three to four weeks, possibly six weeks for heavy infestations. Each watering kills the current batch of larvae, and eventually no new adults emerge to lay eggs.
Most people notice a dramatic drop in flying adults within the first week or two. The stragglers you see after that hatched from eggs already in the soil before treatment began.
Mosquito Bits vs. Mosquito Dunks
Both products come from the same company and contain BTI, but they’re designed differently. Mosquito Bits are small granules that release BTI quickly, providing a fast kill within 24 hours. Mosquito Dunks are larger, donut-shaped tablets designed to float in standing water and release BTI slowly over 30 days or more.
For houseplants, Bits are the better choice. They dissolve faster, making them ideal for brewing tea. Dunks work too (you can break off a quarter of a dunk and soak it overnight), but they take longer to release their BTI because the concentration is lower per volume. Some growers use Bits for the initial knockdown and then switch to Dunks for ongoing prevention in plant saucers or watering cans.
Tips for Faster Results
BTI works best as part of a combined approach. Letting your soil dry out between waterings is one of the simplest ways to reduce fungus gnat populations, since larvae need moisture to survive and females prefer damp soil for laying eggs. If your plant can tolerate it, allow the top inch or two of soil to dry completely before the next watering.
Yellow sticky traps placed near your plants will catch adult gnats and reduce the number of eggs being laid while the BTI handles larvae below the surface. Together, these three steps (BTI tea, drier soil, sticky traps) can eliminate even stubborn infestations within a month.
One thing BTI won’t do is prevent reinfestation from new soil. Fungus gnat eggs often hitchhike in bagged potting mix. If you repot frequently, consider watering new plants with BTI tea as a preventive measure right from the start.