How to Use Mosquito Bits for Fungus Gnats

Mosquito Bits are corn-cob granules coated with BTI (Bacillus thuringiensis subspecies israelensis), a soil bacterium that kills the larvae of mosquitoes, black flies and fungus gnats. For houseplants, the most common way to use them is to steep the granules in water, then use that water every time you water your plants for three to four weeks. The BTI is eaten by gnat larvae feeding in the top couple of inches of soil and kills them within a day or two, which breaks the breeding cycle. It does not kill the adult gnats flying around, so pair it with yellow sticky traps and let the top inch of soil dry between waterings. Done consistently, this combination clears most fungus gnat infestations in about a month. Here is exactly how to dose it, how often to apply it, and the mistakes that make it seem not to work.

Part of our Pest Control guide →

In this guide

1. How BTI Works on Fungus Gnats
2. Method 1: The Soaking Method (Most Common)
3. Method 2: Sprinkling Granules on the Soil
4. Mosquito Bits vs Mosquito Dunks
5. How Long Does It Take?
6. Combine BTI With These Steps
7. Troubleshooting: Why Isn’t It Working?
8. Using BTI for Seedlings and Propagation
9. Preventing a Comeback
10. Other Treatment Options
11. Safety Notes
12. Frequently Asked Questions

How BTI Works on Fungus Gnats

Fungus gnats lay eggs in moist potting soil. The eggs hatch into tiny, clear-bodied larvae with shiny black heads that feed on fungi, decaying organic matter and, in heavy infestations, fine plant roots. After about two weeks they pupate and emerge as adults, which mate and lay more eggs. The whole cycle takes roughly three to four weeks at room temperature, and each female can lay a couple hundred eggs.

When larvae eat BTI, crystal proteins produced by the bacterium are activated in their alkaline gut and destroy the gut lining. The larvae stop feeding within hours and die within a day or two. Because those proteins only activate in the guts of certain fly larvae, BTI is considered very low risk to people, pets, birds, fish, earthworms and beneficial insects such as bees. That selectivity is why it is popular for indoor use. Mosquito Bits are an EPA-registered pesticide, so read and follow the label, which is the legal and practical guide to use.

Before you treat, make sure you are dealing with fungus gnats. Small, weak-flying dark insects that hover around pots and walk on the soil surface are usually fungus gnats; insects that gather around fruit bowls and drains are more likely fruit flies. Our guide to fungus gnats vs fruit flies explains the differences.

Method 1: The Soaking Method (Most Common)

Soaking extracts the BTI into water you can pour through the whole root zone, which reaches larvae deeper in the pot than a surface sprinkle.

1. Check your product label for the houseplant or fungus gnat rate. A commonly used home method is about 4 tablespoons of Mosquito Bits per gallon of water; follow your label if it differs.
2. Add the granules to a gallon jug or watering can of room-temperature water.
3. Let it steep for about 30 minutes, stirring or swirling a couple of times.
4. Strain out the granules if you prefer (a kitchen sieve or coffee filter works), or leave them in; the spent granules can be sprinkled on the soil surface.
5. Water your plants with the BTI water as you normally would, soaking the root ball until a little drains from the bottom.
6. Use the batch the same day. Discard any leftover after 24 to 48 hours and make fresh for the next watering.

Water every infested plant and any plants nearby, because adults move between pots. Use BTI water at every watering for three to four weeks, then continue for another week or two if you still see adults on sticky traps.

Method 2: Sprinkling Granules on the Soil

You can also sprinkle a light layer of granules on the soil surface and water them in. Each time you water, a little BTI washes into the top layer of soil where larvae feed. This is easy and works well as a maintenance treatment, but it is less thorough than soaking because BTI release is uneven, and in some pots the damp corn-cob granules can grow a little harmless mold. Use the amount specified on your label for container plants, and scratch the granules lightly into the top half inch.

Some growers combine both: they sprinkle granules on the surface and water with BTI soak water for the first few weeks of a serious infestation.

Mosquito Bits vs Mosquito Dunks

Both products use BTI. Dunks are solid donut-shaped cakes designed to float in standing water such as rain barrels, ponds and birdbaths, slowly releasing BTI for about 30 days. Bits are granules that release BTI more quickly and are easier to measure for watering cans. For houseplants, Bits are more convenient. If you only have Dunks, you can break off a piece and soak it in your watering can, but dosing is less precise. Check the label; some Dunks labels address fungus gnats and others do not.

How Long Does It Take?

Timeline	What you should see
Days 1 to 3	Larvae in treated soil begin dying. Adults still flying.
Week 1	Fewer new adults emerging; sticky traps still catch existing adults.
Weeks 2 to 3	Noticeable drop in adults as no new generation emerges.
Weeks 3 to 4	Few or no gnats on traps if every watering included BTI.
Week 5 and beyond	Switch to occasional maintenance treatments or stop if traps stay clear.

Adults live about a week and keep laying eggs, which is why you must keep treating long enough to catch every egg that hatches. Stopping after one or two waterings is the most common reason BTI seems not to work.

Combine BTI With These Steps

Sticky traps for adults

Yellow sticky cards placed at soil level or on a stake just above the soil catch adults before they lay more eggs. They also act as your monitor: count gnats on the cards each week to see whether the treatment is working. See our picks for the best sticky traps for plants.

Let the soil dry

Gnats need constantly moist soil. Let the top 1 to 2 inches dry between waterings for plants that tolerate it, such as pothos, snake plants, ZZ, succulents and most aroids. Bottom watering keeps the surface drier. Empty saucers after watering and make sure pots drain.

Top-dress with sand or grit

A half-inch layer of coarse sand, fine gravel or a commercial top dressing makes the surface dry and less attractive for egg laying. It works best combined with BTI.

Check your potting mix

Bags of potting mix stored open, especially mixes heavy in peat, compost or bark, can arrive carrying gnat eggs. Keep bags sealed, and consider treating new mix with BTI water before potting. Mixes with more perlite or pumice dry faster and support fewer gnats. For more causes, see why houseplants get fungus gnats.

Troubleshooting: Why Isn’t It Working?

• Not treating long enough: keep going for at least three to four weeks.
• Using old soak water: BTI breaks down quickly. Make a fresh batch for each watering.
• Skipping plants: treat every plant in the room, including ones without visible gnats.
• Overwatering: constantly wet soil keeps reinfestation going faster than BTI can catch it.
• Old product: BTI loses potency with age, heat and humidity. Store Bits sealed in a cool, dry place and replace product that is several years old.
• Wrong pest: if insects persist on fruit or drains instead of soil, you may be fighting fruit flies or drain flies.
• Rotting organic matter: dead leaves on the soil surface or a decaying root ball can sustain a population. Remove debris and repot rotted plants.

Using BTI for Seedlings and Propagation

Seed-starting trays and humidity domes are ideal gnat nurseries: warm, constantly moist, and full of peat-based mix. Larvae in seedling trays do more harm than in mature pots because they can chew tender roots and stems, contributing to damping off. Bottom-water trays with BTI soak water from the day you sow, and keep a sticky card propped at the edge of each tray. Vent humidity domes as soon as most seeds germinate so the surface dries a little between waterings. The same approach works for cuttings rooting in moist mix and for overwintered plants brought indoors in fall, which often carry gnats in from the patio.

Preventing a Comeback

Once traps stay clear for two weeks, stop routine BTI treatments but keep a sticky card in a few pots as an early warning. Quarantine new plants for two to three weeks and water them with BTI during that time. Repot into fresh mix every couple of years, and water by need rather than by schedule, especially in winter when plants use less water and soil stays wet longer.

Other Treatment Options

If BTI alone is not enough, you have other options. Beneficial nematodes (Steinernema feltiae) and predatory soil mites (Stratiolaelaps scimitus, sold as Hypoaspis) hunt larvae in the soil and are popular in greenhouses. Diluted hydrogen peroxide drenches kill larvae on contact but can also harm beneficial microbes and fine roots if overdone. Repotting heavily infested plants into fresh, fast-draining mix removes most larvae at once. Our complete guide on how to get rid of fungus gnats compares these methods, and the pest control guide covers other common houseplant and garden pests.

Safety Notes

BTI is considered low toxicity to people and pets, and many BTI products are allowed in organic growing. Still, store it out of reach of children and pets, wash hands after handling, and do not let pets eat the granules, since the corn-cob carrier can upset digestion. Avoid inhaling the dust when measuring.