Bt granules are small, corn cob-based pellets coated with Bacillus thuringiensis, a naturally occurring soil bacterium that kills insect larvae. They’re most commonly used to control mosquito larvae in standing water and fungus gnat larvae in potting soil. The granules work by releasing bacterial toxins that are lethal to specific insects but harmless to people, pets, and plants.
How Bt Granules Work
The active ingredient is a protein toxin produced by Bacillus thuringiensis bacteria. When a larva eats the toxin, it destroys the lining of its gut, killing the insect within hours to days. The toxin only activates in the highly alkaline digestive systems of certain insect larvae, which is why it has no effect on mammals, birds, fish, or adult insects. The granules themselves are just a delivery system: they float on water or sit in soil, slowly releasing the bacterial toxin where larvae feed.
Different subspecies of the bacterium target different insects. The most widely sold granules use BTI (subspecies israelensis), which specifically kills mosquito and fungus gnat larvae. Other subspecies target caterpillars or beetle larvae, but those are more commonly sold as liquid sprays or powders rather than granules.
What Bt Granules Control
BTI granules have two primary uses. The first is mosquito control: when spread on standing water where mosquitoes breed, the granules release the larvicide at the water’s surface. Mosquito larvae feed at or near the surface and ingest the toxin as it disperses. This makes the granules effective in birdbaths, rain barrels, drainage ditches, tree holes, and any other spot where water sits long enough for mosquitoes to lay eggs.
The second major use is killing fungus gnat larvae in houseplant soil. Fungus gnats are those tiny flies you see hovering around potted plants. They lay eggs in moist potting mix, and the larvae feed on plant roots, potentially stunting or damaging your plants. To treat gnats, you soak the granules in water for about 30 minutes to make a “tea,” then use that water on your plants. The dissolved toxin reaches the larvae in the soil where they live and feed.
How to Apply Bt Granules
For Mosquitoes in Standing Water
Sprinkle the granules directly onto the surface of any standing water. Most product labels recommend about one tablespoon per 25 square feet of water surface, though you should follow the rate on your specific product. The granules begin releasing the toxin quickly, often within minutes. For ongoing mosquito control, reapply every 7 to 14 days, since the active toxin breaks down relatively fast in water, with a half-life ranging from less than one day to about 46 days depending on conditions like sunlight and temperature.
For Fungus Gnats in Potting Soil
Add about four tablespoons of granules to one gallon of water and let it sit for at least 30 minutes. Strain out the granules (or leave them; they won’t hurt anything) and water your plants with the solution. Repeat with each watering until you stop seeing adult gnats, which usually takes two to three weeks since you’re killing larvae but need to wait for the existing adults to die off naturally. Some people also sprinkle dry granules directly on the soil surface before watering, letting each watering session release a fresh dose of toxin.
How Long They Last
The toxin itself is relatively short-lived. In the environment, it breaks down with a half-life of less than one day to 46 days, depending on exposure to sunlight, soil pH, and microbial activity. UV light is the biggest factor in breaking down Bt on surfaces and in shallow water, which is why reapplication is necessary for outdoor use.
The bacterial spores, on the other hand, are far more persistent. In the top two inches of soil, spores degrade over the first two weeks but then stabilize and can remain at a consistent level for at least six weeks. Below the soil surface, spores can persist for months or even years. Soil acidity matters: in acidic soil (around pH 4.3, typical under pine trees), spores break down faster than in neutral soil (around pH 6.5). Other soil microbes also help break down the spores over time. For practical purposes, the killing power fades well before the spores disappear, so you’ll need to reapply regularly regardless of spore persistence.
Safety for People, Pets, and Wildlife
Bt is one of the safest pest control options available. The toxin activates only in the alkaline gut of specific insect larvae. Human and animal stomachs are acidic, so the protein passes through without effect. You can safely use Bt-treated water in birdbaths where birds drink and bathe, in rain barrels used for garden irrigation, and in ponds where fish live. It’s approved for use in organic farming and gardening, and the EPA classifies it as a low-risk biopesticide.
The selectivity of BTI granules means they won’t harm bees, butterflies, earthworms, or beneficial insects in your garden. Only larvae that ingest the toxin in water or moist soil are affected, and even then, only mosquito and fungus gnat larvae are susceptible to the israelensis subspecies. This makes Bt granules a good fit if you want targeted pest control without collateral damage to pollinators or soil health.
Tips for Getting the Best Results
Bt granules only kill larvae, not adult insects. If you’re dealing with an existing swarm of adult mosquitoes or a cloud of fungus gnats, the granules won’t provide instant relief. They break the breeding cycle, so expect results over one to three weeks as the current generation of adults dies off and no new larvae survive to replace them.
Store unused granules in a cool, dry place. Heat and moisture can degrade the toxin before you even apply it. Most products have a shelf life of one to two years when stored properly. For standing water outdoors, reapply after heavy rain, since fresh water dilutes the active toxin. And for fungus gnats, addressing overwatering is just as important as treatment: letting the top inch of soil dry between waterings makes the environment less hospitable to egg-laying adults in the first place.