Bti (Bacillus thuringiensis subspecies israelensis) is a naturally occurring soil bacterium whose spores produce toxins that specifically target the larvae of mosquitoes, black flies, and fungus gnats. It is the active ingredient in the familiar donut-shaped "mosquito dunks," and it is the right tool for standing water you cannot simply dump: yard drains, catch basins, rain barrels, ornamental basins, stubborn low spots. The EPA maintains a plain-language explainer on Bti for mosquito control, which notes the agency has registered multiple Bti strains across dozens of products for residential, commercial, and agricultural use.
Why larviciding is such good leverage
Larvae are the trapped stage of the mosquito. They live in the water, cannot leave it, and are concentrated in a small area. The EPA's page on controlling mosquitoes at the larval stage describes the strategy: larvicides target larvae in the breeding habitat before they can mature into adults and disperse, which reduces the adult population in nearby areas. Killing one larva today is cheaper than hunting one flying, biting adult next week, and per the EPA life cycle timeline, "next week" is literal: the cycle can complete in as little as 4 days in warm weather.
How Bti products work in practice
University of Kentucky extension entomologists cover homeowner Bti use in ENTFACT-005, with two practical details worth knowing:
- Dunks are slow-release. One donut-shaped dunk is typically recommended per 100 square feet of water surface, and it breaks down slowly when wet, releasing Bti over about a 30 day period. Drop one in the catch basin at the start of the month and it is still working at the end.
- Granules act faster but shorter. Granular Bti formulations begin releasing toxin more quickly than dunks, at the cost of residual life, which makes them the better fit for small containers like flower pots and tree holes.
Placement beats quantity. The goal is every body of water that persists most of a week and cannot be dumped, screened, or drained. Run the Yard Breeding-Spot Checklist first; most yards end up with two to five legitimate larvicide targets, and the rest of the list is solved with a bucket flip or a screen.
The safety picture, without spin
Bti's appeal is its specificity: per the EPA, its toxins specifically target and only affect the larvae of the mosquito, black fly, and fungus gnat. The EPA's Bti page addresses the common questions directly, including human health and effects on wildlife such as honey bees, and the products carry labels like any pesticide. The honest homeowner rules are the same as always: buy an EPA-registered product, read the label, dose per the label, and do not expect a larvicide to do an adulticide's job. Bti in the drain does nothing about the adults already resting under your deck; that layer is covered in what is in a barrier treatment.
Where Bti fits in a full program
Our service uses the same logic we recommend to DIYers. Every visit includes a source sweep: dump and fix what can be dumped and fixed, larvicide the water that must remain, then hold the treated perimeter where adults rest, on a 10 to 14 day rhythm. Bti is the "treat" verb in that sweep. If you want us to find your larvicide targets for you, that is part of the free yard assessment.
FAQ
Is Bti safe for birdbaths and pet water?
Use products per their labels for listed sites. For a birdbath, the simpler answer is usually to skip treatment entirely and dump and refill it weekly, which interrupts the cycle mechanically. Save Bti for water you cannot refresh.
How fast does a dunk start working?
Dunks release gradually and persist for about 30 days; granular formulations start faster with a shorter residual, per University of Kentucky extension. Either way larvae stop developing into biting adults, though adults already flying are unaffected.
Can mosquitoes become resistant to Bti?
The EPA discusses resistance on its Bti page; the multi-toxin mode of action makes resistance development less likely than with single-compound insecticides, which is one reason Bti has remained a mainstay of larval control programs.