Mosquito dunks vs bits

Last updated: October 2026

Mosquito dunks and mosquito bits both contain Bti, Bacillus thuringiensis subspecies israelensis, in two different forms. Dunks are floating, slow-release rings labeled to release Bti for 30 days or more, while bits are granules labeled for a quick kill within 24 hours and control for up to 14 days^6,7. Both work only on mosquito larvae that eat them⁴.

What is the difference between mosquito dunks and mosquito bits?

Both products are larvicides: they act on the immature stage of the mosquito (Culicidae) in the standing water where it develops, before adults can emerge and disperse². Larvicides for mosquito control come as liquids and as tablet, pellet, granular and briquet formulations². Dunks are the briquet form and bits are the granular form, and the difference between them is how fast the Bti reaches the larvae and how long it keeps coming^6,7.

A dunk is a floating ring, and its label says it releases effective levels of Bti for 30 days or more under typical conditions, with active material released at the surface and gradually settling to the bottom⁶. Bits are spread over the water surface, release the larvicide into the water, and it settles where larvae eat it⁷. The bits label claims a quick kill within 24 hours and control for up to 14 days in water of low to moderate organic content, and it suggests following bits with dunks or briquets for longer residual control⁷.

In North America, larvicide season follows mosquito activity: New York State health guidance places dunks from April to September⁵, and in summer a mosquito can go from egg to adult in a week or less⁴. In the Southern Hemisphere the seasons are reversed, so the same treatment window runs through the warm months, roughly October to April.

How does Bti kill mosquito larvae?

Bti is a naturally occurring soil bacterium whose spores produce toxins that target only the larvae of mosquitoes, black flies and fungus gnats¹. Larvae have to eat it: the proteins act as a stomach poison that damages the cells of the mosquito midgut⁴. Insects exposed to these bacterial larvicides have trouble digesting food and die of starvation².

That feeding requirement sets the limits. Bti does not work on pupae, because pupae do not feed⁴, and it does nothing to adult mosquitoes. All mosquitoes pass through egg, larva, pupa and adult, and the first three stages live in water³. The cycle typically takes about two weeks but can range from 4 days to a month³, and in summer it can be completed in a week or less⁴. Treated water therefore needs to keep active Bti present as new larvae hatch.

EPA has registered five strains of Bti in 48 pesticide products for residential, commercial and agricultural use¹. EPA also reports no documented resistance to Bti as a larvicide, including in mosquito populations treated with it for decades¹.

When should you use a dunk and when should you use bits?

Use a dunk where water sits for weeks: rain barrels, ornamental ponds, unused swimming pools, animal watering troughs, tree holes and roof gutters are all on its label⁶. Dunks can be anchored with a string through the center hole or staked in place so they are not flushed out, and they can be broken into portions for small containers⁶. They can also be placed ahead of time in dry spots known to flood, where they float up and start releasing Bti when water arrives; alternate wetting and drying does not reduce their effectiveness⁶.

Use bits when you want larvae killed fast, or for many small or scattered sites⁷. The bits label lists sites such as water gardens, flower pots and saucers, bird baths, rain barrels, roof gutters, ditches and old tires⁷. The bits label itself suggests a combined approach: bits first for the quick kill, then dunks or briquets for longer control⁷.

• Rain barrel or pond that holds water all season: a dunk, anchored, and replaced on schedule⁶.
• Heavy larval activity you want stopped now: bits, followed by a dunk for residual control⁷.
• Water with a lot of organic matter: bits at the label's more frequent interval⁷.
• Houseplant soil with fungus gnats: bits, which carry a fungus gnat use on their label⁷.

How do you use mosquito dunks in a rain barrel, pond or birdbath?

Start by removing water you do not need. Empty and turn over buckets, unused flower pots and wheelbarrows, clean clogged gutters, and put lids on outdoor trash cans; larvicides belong only on small areas of water on your own property that cannot be drained⁵. Change the water in pet dishes, troughs and bird baths every week⁴. In New York State health guidance, the places left for dunks are pool covers, ornamental ponds, fountains and rain barrels⁵.

For rain barrels, the first defense is a sealed barrel or window screen over every opening, with clean filters⁴. Dunks are the fallback where screens or sealed barrels are impractical⁴. In a 2024 study of extension-recommended rain barrel treatments, every treatment cut mosquito larvae by more than 99% compared with untreated open buckets, but the authors reported practical problems, including dunks breaking apart⁸. Check a dunk in a barrel regularly and replace it when it has crumbled or reached the end of its labeled period^6,8.

The label sizes the dose by the water's surface area, regardless of depth, and allows larger amounts in heavily polluted water⁶. Follow the product label for the amount and interval. Neither form is for treated, finished drinking water⁶.

Are dunks and bits a risk to fish, pets or bees?

Bti's toxins affect only the larvae of mosquitoes, black flies and fungus gnats, and studies indicate minimal toxicity to honey bees¹. The dunks label states that the product can be used in fish habitats⁶. The bits label allows application to water with aquatic life, fish and plants, and to areas used by people, pets, horses, livestock, birds and wildlife⁷. UC IPM describes Bt products for ponds as killing mosquito larvae without affecting people, other animals or plants⁴.

The precautions that do apply come from the label. The dunk carries a CAUTION signal word because it causes moderate eye irritation, so avoid eye contact and wash after handling⁶. Drinking water is product-specific: some Bti products may be used in certain drinking water such as cisterns and others may not, so read the label for any water people will drink^1,6.

How long do dunks last, and when should you start?

A dunk lasts about 30 days, so reapplication is likely across a season⁵. Some larvae that hatch after a dunk goes in may partly develop before they die, and the label allows a minimum of 48 hours for their control⁶. Bits need retreatment every 7 to 14 days: every 7 days where larvae are abundant or the water carries organic matter, and every 14 days where there are few larvae and little organic matter⁷.

Start when mosquitoes become active and water stands for more than a few days⁵. New York State health guidance recommends dunks from April to September, when mosquitoes are active and hatch more often in warm weather⁵. Mosquito eggs can survive dry conditions for a few months and hatch when flooded³, which is why pre-flood placement of dunks is on the label⁶.