Bacillus thuringiensis israelensis
Bti is a strain of the soil bacterium Bacillus thuringiensis that kills the larvae of mosquitoes, fungus gnats and a few related flies. It comes as Mosquito Dunks, floating rings that treat 100 sq ft of standing water for 30 days or more, as Mosquito Bits granules, and as Gnatrol, a granule stirred into water and drenched into compost. Only larvae that eat it are killed, usually within a day. It does not touch adults or pupae, so it works as a repeated treatment, not a one-off. UC IPM calls it essentially nontoxic to people, and the Dunks label allows it in ponds with fish.
At a glance
- Mode
- Stomach poison — larvae must eat it
- Water
- 1 Dunk per 100 sq ft; lasts 30+ days
- Compost
- Gnatrol 1–1.5 tsp/gal, weekly for 3+ weeks
- Speed
- Larvae killed or respond within 24 h
- Does not kill
- Adults or pupae
- REI
- 4 h on the agricultural label
How it works
Bti's spores carry proteins that do nothing until a larva eats them. In the alkaline insect gut (NPIC gives pH 9–10.5) the toxin is activated and breaks down the gut lining, and the larva stops feeding and dies. Human stomachs are acidic, which is one reason the Bacillus thuringiensis group is very low in toxicity to people and other mammals. Each strain has its own range: israelensis kills the larvae of mosquitoes, fungus gnats and related flies, while the kurstaki strain sold as Bt kills caterpillars. UC IPM says Bti kills only mosquitoes and the larvae of a few related flies.
It has to be where the larvae are feeding, and it does not last. In water, a Dunk releases Bti at the surface and it gradually settles to the bottom, where larvae feed, for 30 days or more. Bits give a quick kill within 24 hours but last only up to 14 days. In compost, UC IPM says Bti does not reproduce or persist indoors, which is why the Gnatrol label asks for a drench weekly for at least three weeks. Pupae do not feed, so any already present will still emerge as adults, and for larvae that hatch after a Dunk goes in, the label says to allow at least 48 hours.
Applying it
There are three home products. Mosquito Dunks and Mosquito Bits (Summit) are for standing water; Bits are also labelled for fungus gnats. Gnatrol is a water-dispersible granule for compost drenches. The rates below come from the EPA-accepted labels, so read your own label.
Size the Dunk to the surface
One Dunk per 100 sq ft of surface, whatever the depth. Break it: ¼ for 1–5 sq ft, ½ for 5–25 sq ft. Tie it through the centre hole so it is not flushed out.
Bits first, then a Dunk
Bits: 1 tsp per 25 sq ft or 1 tbsp per 75 sq ft, every 7–14 days. The label suggests following them with a Dunk for longer control.
Drench, weekly, three times
Gnatrol: 1–1.5 tsp per gallon, enough to wet the compost surface, once a week for at least three weeks. Bits: sprinkle on the compost and water in as usual, for three weeks.
Not drinking water, not with copper
Never add it to treated drinking water. The commercial section of the Gnatrol (VectoBac WDG) label says not to inject it with fertilisers or fungicides containing copper or chlorine, which may neutralise it; tap-water chlorine is not expected to be a problem.
Pesticide sensitivity
UC IPM says Bti is toxic only to fly larvae. The Dunks label allows it in fish habitats, and the Bits label in water with fish, aquatic life and plants. NPIC notes that a few aquatic insects and water fleas were affected in some studies. Check a product →
When to choose something else
Bti is a larvicide and nothing more. Choose something else when:
- The standing water can simply be emptied, covered or screened — that beats treating it
- You want control that lasts in compost — UC IPM says the nematode Steinernema feltiae can reproduce and give longer-term control where Bti does not persist
- The problem is the flying adults — yellow sticky cards catch gnats, and for mosquito bites follow CDC advice
- You plan to soak a Mosquito Dunk for fungus gnats — the Dunks label does not list them; Bits and Gnatrol do
- The pest is a caterpillar — that needs Bt kurstaki, a different strain
Where to buy
These are the Bti products whose labels we checked, in alphabetical order. They are not ranked. helpfulbugs takes no commission and does not sell any of them.
- Gnatrol WDG / Gnatrol Fungus Gnat Killer (EPA Reg. 73049-56)Water-dispersible granuleUS · HOME & FARM
- Summit Mosquito Bits (EPA Reg. 6218-73)GranulesUS · HOME
- Summit Mosquito Dunks (EPA Reg. 6218-47)Floating ringUS · HOME
Bacillus thuringiensis israelensis by target
Rate and timing differ by what you are releasing it against. Each page below is the program for one pest.