BuzzSkito Mosquito & Tick Control Specialists · Published April 28, 2026 · Updated October 23, 2026
How Mosquito Dunks Work
Mosquito dunks contain dried BTI bacterial spores compressed into a slow-release tablet. When placed in water, the dunk slowly releases BTI spores into the surrounding water column. Mosquito larvae are filter feeders — they consume the spores as they feed, and the BTI proteins disrupt their digestive system. Death occurs within 24–48 hours.
Importantly, BTI is highly specific to mosquito and black fly larvae (and a few related species). The U.S. Environmental Protection Agency states that BTI toxins affect only the larvae of mosquitoes, black flies, and fungus gnats, so humans, pets, fish, frogs, dragonflies, water beetles, pollinators, and birds are not on its target list. That narrow target list is why BTI is so widely used in ponds and rain barrels; use it where the label directs.
Where to Use Mosquito Dunks (and Where Not To)
✓ Ideal locations
- Rain barrels — a quarter dunk on the Canadian label (its 0.1 to 3.0 sq. metre row); reapply at 30-day intervals if larvae are detected.
- Ornamental ponds — koi ponds and water gardens are listed use sites on the Summit label.
- Neglected swimming pools — pools that have lost chlorine and accumulated rainwater.
- Flooded low spots on your property that hold water for more than a week and cannot be drained (not a ditch that drains off the property).
- Tree holes and stump cavities that collect rainwater.
- Large bird baths you cannot change at least weekly (otherwise just change the water).
- Boats covered with tarps that pool water on top.
✗ Don’t use them here
- Active chlorinated swimming pools — chlorine kills larvae anyway.
- Puddles that dry within 3–5 days — the dunk needs water to work.
- Drinking water — don’t treat household drinking water. Health Canada’s PMRA does not permit Bti products to be applied to treated, finished drinking water.
- Water that leaves your property — the Canadian label says “DO NOT apply this product to any water that is designed to flow beyond your property limits (for example, ditches, drains, rain gutters or catch basin sumps that flow into weeping tile beds or municipal storm drain systems).”
Mosquito Dunks vs Mosquito Bits
Same active ingredient (BTI). Different format and use cases:
| Factor | Mosquito Dunks | Mosquito Bits |
|---|---|---|
| Format | Donut-shaped tablet | Granules |
| Release speed | Slow (30 days) | Fast (minutes) |
| Best for | Long-term sources (rain barrel, pond) | Quick knockdown / small water |
| Visibility | Visible in water (some mind this) | Sinks/dissolves quickly |
| Canadian registration | Reg. No. 28888, domestic class | None in Health Canada’s registry (U.S. EPA Reg. No. 6218-86); registered granules here are AquaBac 200G and My-Crobe 200G |
In Canada the choice is narrower than that table suggests: Bits have no Canadian registration, so the plan is dunks (Reg. No. 28888) in rain barrels and ponds, a registered granule such as AquaBac 200G or My-Crobe 200G for unexpected standing water its label covers, and fresh water twice a week in bird baths.
Do mosquito dunks really work?
Yes — in the water you actually treat. BTI larvicides are standard equipment for mosquito control districts across North America, and published field trials report high larval mortality within 24–48 hours of contact. One dunk covers up to 6 sq. metres of water surface on the Canadian label (about 100 sq ft on the U.S. label), reapplied at 30-day intervals if larvae are detected. What a dunk cannot do is touch an adult mosquito that is already flying.
That distinction is the entire honest answer, and it is where most of the online argument about dunks comes from. Two people can both use dunks correctly and come away with opposite verdicts, because they were solving different problems. If your mosquitoes are being produced on your property — a rain barrel, a tarp on the woodpile, a clogged gutter, a low spot that stays wet for a week — a dunk is close to a complete fix for a few dollars. If your mosquitoes are arriving from the ravine behind you, a neighbour’s green pool, a creek, or a municipal storm system, you can treat every drop of water you own and still get bitten, because none of that water was yours.
Two field details worth carrying: NC State Extension tells homeowners that dunks “last around 3–4 weeks before they need to be replaced” — slightly shorter than the label’s 30 days, so treat 30 days as a ceiling rather than a target, especially in hot weather or after heavy rain that flushes the container. And a dunk does nothing for a lawn; it is a water treatment, and there is a section on that below.
Do mosquito dunks kill anything else?
Almost nothing. The U.S. Environmental Protection Agency states that BTI toxins affect only the larvae of mosquitoes, black flies, and fungus gnats. Nothing else in the water is affected — not adult mosquitoes, not dragonfly nymphs, water beetles, snails, tadpoles, frogs, fish, birds, bees, or earthworms. BTI only becomes active in the specific alkaline gut those three larval groups share.
This is the question every safety page on the internet answers backwards. Most pages, including the older parts of this one, tell you what BTI is safe for. The more useful framing is what it actually kills, because a larvicide with a three-species kill list is a fundamentally different product from a broad-spectrum insecticide, and that difference is why the EPA notes BTI is approved for pest control in organic farming operations.
| BTI kills | Where | Outside BTI’s target list (per U.S. EPA) |
|---|---|---|
| Mosquito larvae | Any standing water it is dosed into | Adult mosquitoes — already flying, never eat the spores |
| Black fly larvae | Running water; BTI is used for this at watershed scale | Mosquito pupae — they stop feeding, so nothing gets ingested |
| Fungus gnat larvae | Damp potting soil and greenhouse benches | Dragonfly and damselfly nymphs, water beetles, water striders |
| — | — | Fish, frogs, tadpoles, turtles, snails |
| — | — | Bees, butterflies, fireflies, ladybugs, earthworms |
| — | — | Birds, pets, livestock, people |
Kill list and non-target list follow the U.S. Environmental Protection Agency’s BTI for mosquito control fact sheet. One consequence worth knowing: because BTI must be eaten, it does not kill pupae — the comma-shaped “tumblers” that no longer feed. If a container is full of pupae, dunking it will not stop this batch from emerging; dump the water instead, then dunk the refill.
Are mosquito dunks harmful to humans?
No. The U.S. Environmental Protection Agency states plainly that BTI has no toxicity to people, and BTI is registered for residential use in both countries — EPA Reg. No. 6218-47 in the United States, Health Canada PCP #28888 in Canada. The Summit label carries the signal word CAUTION for moderate eye irritation only, which is a handling instruction, not a health warning about the treated water.
Sensible handling, which is all the label actually asks for: wash your hands after handling a tablet, avoid rubbing your eyes while you do it, and keep the box out of reach of children the way you would any garden product. BTI is used at municipal scale in water bodies across North America, but Health Canada’s PMRA does not permit any Bti product to be applied to treated, finished drinking water — so there is no case for dosing the water in your own kitchen. Treat your rain barrel, your pond, and your ditch; leave your drinking water alone.
What is the difference between mosquito bits and mosquito dunks?
Two different things get mixed together in this question. Mosquito Dunks are a BTI tablet you drop in standing water to kill larvae — they do nothing for a mosquito bite you already have, which is an antihistamine or hydrocortisone job. The real comparison is Dunks vs Bits: same BTI bacterium, different release speed. Dunks last about 30 days; Bits work within minutes.
The format table further up this page compares them factor by factor. What most people actually want is the decision rule, so here it is by scenario:
| What you are treating | Buy | Why |
|---|---|---|
| Rain barrel (50 gal) | Dunks (¼ dunk on the Canadian label) | Set-and-forget for 30 days; the barrel refills itself and you are not there to re-dose |
| 5-gallon bucket trap | Dunks (quarter tablet) | You want a month of coverage from a fraction of a tablet |
| Ornamental pond, koi pond, water garden | Dunks | Large surface, long-lived water, and koi ponds are a labelled use site |
| A wet low spot on your property | Dunks | Slow release survives the days between your visits; in Canada, not a ditch that drains off the property |
| Bird bath or plant saucer | Usually nothing: change the water | Health Canada advises replacing bird bath water “at least twice a week”; the labels are for water you cannot change weekly |
| Larvae you just found and want gone today | Tip the water out; if you can’t, BTI granules | In Canada, the registered granules are AquaBac 200G and My-Crobe 200G (“Allow 24 hours for maximum effect on larvae”); Bits have no Canadian registration |
| Fungus gnats in houseplant soil | Not a Canadian label use | No Canadian BTI label we checked lists fungus gnats; fix the watering and use sticky cards (the U.S. labels allow a steeped drench) |
| Animal watering trough | Dunks | Labelled use site, monthly cadence |
If you want the two formats compared in more depth, we did that separately in Mosquito Bits vs Dunks.
Are mosquito dunks safe for dogs, cats, birds, fish, bees, and plants?
The label, not a blanket yes, is the guide. BTI only becomes active in the alkaline gut of mosquito, black fly, and fungus gnat larvae; the U.S. Environmental Protection Agency states BTI toxins affect only those larvae and that BTI has no toxicity to people. Use it on the sites and at the rate the label lists. Below, one row per question.
| Who | Short answer | What to actually watch for |
|---|---|---|
| Dogs | Not a BTI target | Watering troughs are a labelled use site; use the label rate. The real scenario is a dog fishing the tablet out and chewing it — store the box out of reach and call your vet if a dunk was swallowed. |
| Cats | Not a BTI target | Same points as dogs, and cats rarely chew tablets. For a fountain or bowl a cat drinks from, refresh the water instead: Bits have no Canadian registration, and the registered granule labels rule out drinking receptacles. |
| Fish (goldfish, koi, mosquitofish) | Not a BTI target | Koi ponds are a labelled use site, and BTI is used routinely in stocked ponds. Fish eat larvae themselves, so a dunk earns its keep mainly in fishless ponds and the plant-choked margins fish do not patrol. |
| Birds | Not a BTI target | Birdbaths are a labelled use site. For a small bath, dumping and refilling every three days beats larvae on timing alone — they need over a week of still water. |
| Bees, butterflies, fireflies, ladybugs | Not a BTI target | EPA: BTI toxins affect only mosquito, black fly, and fungus gnat larvae. EPA also notes BTI is approved for pest control in organic farming operations. |
| Frogs, tadpoles, turtles | Not a BTI target | The larvicide acts on the mosquito wrigglers in the pond, not on amphibians. |
| Plants and vegetables | Not a herbicide | BTI is a soil bacterium. Water steeped with a piece of dunk is how gardeners treat houseplants and vegetable beds for fungus gnats. |
| Livestock (horses, cattle) | Labelled use site | Animal watering troughs are a labelled use site on the Summit label. |
| People and children | EPA: no toxicity | EPA: no toxicity to people. Label signal word is CAUTION for moderate eye irritation. Wash hands after handling; store out of reach; do not treat household drinking water. |
The Mosquito Bucket of Doom: how to build a mosquito dunk bucket
A mosquito dunk bucket is a deliberate trap, not a leak you forgot about. You fill a 5-gallon bucket two-thirds with water, let it go foul with straw or grass clippings, and dose it with a quarter of a mosquito dunk. Female mosquitoes hunt for exactly that kind of stagnant, organic-rich water, lay their eggs in the most attractive water in the yard, and the BTI kills every larva that hatches.
The method has spread widely in the United States over the last two seasons, and it is genuinely clever: instead of hunting for hidden water, you out-compete it. The build below follows the instructions published by K-State Research and Extension in Johnson County, Kansas, which credits the approach to Doug Tallamy, the University of Delaware entomologist. Mississippi State University Extension has published its own version with an urban entomologist walking through the same steps. It helps to be clear about whose behaviour the bucket is exploiting: only females bite and only females lay eggs, which is the practical upshot of the difference between male and female mosquitoes — the males in your yard are feeding on nectar and will ignore the bucket entirely, so every design decision here is about what an egg-laying female finds attractive.
The build · 20 minutes · about $4 of dunk per season
- Pick the spot first. Shade, low foot traffic, away from doors, patios and play areas. Shade slows evaporation and the females prefer sheltered water.
- Drill overflow holes about six inches below the rim. Heavy rain then escapes through the holes instead of washing larvae and treated water over the top edge.
- Fill the bucket half to two-thirds. A 2–5 gallon bucket is the size the extension services use. The headroom you leave is what makes the overflow holes work.
- Add a handful or two of straw, hay, or grass clippings — then wait. Leave it several days until the water smells genuinely foul. That smell is the attractant; a clean bucket of tap water attracts very little.
- Add one quarter of a mosquito dunk. Not a whole one. A full dunk is rated for up to 6 sq. metres of water surface on the Canadian label (about 100 sq ft on the U.S. label), which is many times what a bucket presents.
- Cover the top with hardware cloth and drop in a stick. The mesh keeps larger animals out while letting mosquitoes in; the stick is an escape ramp for anything that gets in anyway.
- Refresh the quarter dunk every 30 days through your local season, and top the water up as it evaporates. Never tip an untreated bucket onto the ground — that releases whatever survived.
Step six is the one people improvise and get wrong. Window screen is too fine — it keeps the female out along with the raccoon, which turns the whole build into a bucket of stagnant water doing nothing. What you want is quarter-inch galvanised hardware cloth, cut a little oversize and pressed into the rim so the mesh stays taut: large enough for a mosquito to pass, small enough that nothing with a spine can. One small roll covers every bucket you will ever build. Check hardware cloth on Amazon.ca →
How many buckets? The people running this method typically put out one bucket per quarter-acre, sited at the shaded edges of the property rather than in the middle of the lawn. A 6-pack of dunks broken into quarters gives you 24 bucket-months, which is more than enough for two or three buckets across a full northern season.
Where a bucket helps and where it does not. A dunk bucket is an oviposition sink: it competes for the eggs of females that are already in your yard looking for somewhere to lay, and it works on water you control. It has no effect on adults arriving over the fence from a neighbour’s yard, a ravine, or a creek — the same limitation every larvicide shares, including the barrel dunk you already have. That off-property pressure is what a barrier spray on vegetation is for. The two work on different halves of the same problem, and there is no reason to run only one of them.
How to use mosquito dunks: dosing rates, quartering a tablet, and how often to replace
On the Canadian label the rate is one dunk for up to 6 sq. metres of water surface, half a dunk for 3.0 to 6.0 and a quarter for 0.1 to 3.0 (a rain barrel), reapplied at 30-day intervals if larvae are detected; the U.S. label says one dunk per 100 sq ft. Almost every container people actually treat is far smaller than either, which is why the extension services and experienced users break tablets into quarters, and why the Canadian dose table near the top of this page starts at a quarter dunk. Depth barely matters; the Canadian label doses by surface “regardless of depth”. The table below follows U.S. extension practice; in Canada, use the Canadian label’s amounts.
| Water source | How much dunk | Replace | Note |
|---|---|---|---|
| Rain barrel, ~50 gallons | 1 dunk (¼ dunk on the Canadian label) | Every 30 days | The Canadian label table puts 0.1 to 3.0 sq. metres of water at ¼ dunk |
| 5-gallon bucket | ¼ dunk | Every 30 days | K-State Research and Extension rate for the bucket method |
| Plant saucer, small bird bath | A small broken piece of dunk (Bits have no Canadian registration) | Weekly, or just refresh the water | Refilling every 3 days beats the larvae without any product |
| Pond or water garden, up to 100 sq ft | 1 dunk | Every 30 days | Koi ponds and water gardens are labelled use sites |
| Pond larger than 100 sq ft | 1 dunk per 100 sq ft of surface | Every 30 days | Spread them out rather than clustering in one corner |
| Drainage ditch or wet low spot | 1 dunk per 100 sq ft | Every 30 days, sooner after heavy rain | Flushing rain is the main reason coverage runs short; in Canada, never a ditch that drains off your property |
| Unused or neglected pool | 1 dunk per 100 sq ft | Every 30 days | A stopgap — drain or restore chlorination when you can |
| Animal watering trough | 1 dunk | Every 30 days | Listed use site on the Summit label; top up rather than draining |
| Watering can for houseplants, 1 gallon | ¼ dunk steeped 24–48 hours | Each watering for 3 weeks | The fungus gnat method — see the section below |
Break the dunk into quarters
The BTI is distributed evenly through the tablet, so a quarter tablet treats a quarter of the water. Score it with a utility knife or snap it by hand — they are brittle and break cleanly along the ring. This is the single biggest cost saving available on this product: a 6-pack broken into quarters becomes 24 small-container treatments, which is a whole season of several buckets, saucers, and tree holes from one box. The Canadian label agrees that dunks “can be used whole or in pieces”. Store the leftover pieces dry.
30 days is a ceiling, not a promise
NC State Extension tells homeowners dunks “last around 3–4 weeks before they need to be replaced” — slightly shorter than the 30 days on the label. Heat, heavy rain, and water turnover all shorten it. The practical rule: check at three weeks, and if you see wrigglers, replace early rather than waiting out the calendar month.
Do mosquito dunks still work after they sink?
Yes. A dunk keeps releasing BTI whether it floats or sits on the bottom, and Summit states this on the label. Mosquito larvae are filter feeders that graze through the whole water column rather than only the surface, so they take up the spores wherever the tablet ends up. Sinking is normal as the tablet absorbs water and starts breaking down.
The only real drawback is bookkeeping: once a dunk is out of sight you stop noticing it, and “I think there’s still one in there” becomes a month of untreated water. Put the replacement on a calendar reminder instead of relying on spotting the tablet, and if you want a visual check, drop the next one in a mesh bag tied to the barrel handle.
There is a version of this problem you can design out entirely. A rain barrel that a female cannot get into needs no tablet at all, and the fix is a fine mesh screen stretched under the lid and over the downspout inlet — the two openings people forget are openings. Screen first, dose second, and the dunk becomes a backstop for the barrel you cannot fully seal rather than the only thing standing between you and a month of wrigglers. Check rain barrel screens on Amazon.ca →
Do mosquito dunks expire?
Yes: the Canadian label says to “use them within one year from date of manufacture to ensure microbial purity and potency.” It also says to store them “in the original closed container in a cool (25°C or less), dry place”, and the box has a date-of-manufacture line, so leftovers carry over to the next season only while they are inside that year. Keep them sealed in a bag or a lidded bin; moisture is what ruins a dunk fastest.
A tablet that has been wetted, dried out, and re-wetted — the one that sat in a puddle on the shed floor, or the half-used pack left open in a humid garage — has already released part of its BTI and will not give you a full 30 days. That is the one to replace rather than trust. If you buy the 20-pack for the price break, the storage discipline is the whole trick: seal it, keep it off the floor, and keep it out of the sun.
Mosquito dunks for fungus gnats and houseplants
Mosquito dunks work on fungus gnats because fungus gnat larvae are one of only three things BTI kills. The U.S. Environmental Protection Agency lists them alongside mosquito and black fly larvae. You do not bury a tablet in the pot — you steep a quarter of a dunk in water for 24 to 48 hours, then water your plants with that BTI-infused water at every watering for three weeks.
In Canada, read the label first. The Canadian Mosquito Dunks label (Reg. No. 28888) is “for use only when mosquito larvae are present in” standing or containerized water, and fungus gnats are not on it. The same label says: “It is an offence under the Pest Control Products Act to use this product in a way that is inconsistent with the directions on the label.” The steeped-dunk method below follows U.S. practice; in Canada, the watering fix and the sticky cards are the part of the plan that needs no pesticide label.
Two things confuse people about this. First, the BTI kills larvae in the soil, not the adults flying around the window, so for the first week nothing appears to change while the adults you already have live out their lives. Second, a fungus gnat goes from egg to adult in roughly two to three weeks, so a single treated watering does almost nothing — you have to keep the treated water going long enough to catch every generation still in the pot.
| Week | What to do | What you should see |
|---|---|---|
| Setup | Break off ¼ of a dunk, drop it in a 1-gallon jug of water, steep 24–48 hours. Put yellow sticky cards on the soil surface for the adults. | Cloudy-ish water. Sticky cards start filling immediately. |
| Week 1 | Water every plant with the BTI water instead of plain water. Let the top inch of soil dry between waterings. | Adults still flying — these hatched before you started. Not a failure. |
| Week 2 | Keep watering with BTI water. Re-steep a fresh quarter dunk when the jug runs low. | Noticeably fewer new adults on the sticky cards. |
| Week 3 | Final round of BTI waterings. Replace sticky cards. | Close to zero new adults. The cycle is broken. |
| After | Stop the BTI. Bottom-water where you can and stop overwatering — soggy soil is the actual cause. | No recurrence unless a new infested plant comes in. |
Under the U.S. label, Bits are easier here than dunks. Mosquito Bits are the same BTI in granule form, and you sprinkle them straight onto the top inch of potting soil and water them in — no steeping, no jug, no tablet sitting in a decorative pot. In Canada that shortcut is not available: Bits have no Canadian registration, and the Mosquito Dunks label covers mosquito larvae in water only (see the note above). Either way, the durable fix is watering habits: fungus gnats breed in permanently damp soil, so letting the top inch dry between waterings does more than any product.
The one item in that week-by-week table that is neither a dunk nor a bag of Bits is the yellow sticky card, and it is doing real work in this plan. BTI reaches only the larvae in the soil, so the cards are what remove the adults already in the air — and, more usefully, they are the measurement: a card that stops filling up in week three is how you know the cycle actually broke instead of merely quieting down. Check yellow sticky cards on Amazon.ca →
Which mosquitoes do dunks actually stop in the United States?
The species a dunk is best aimed at is Culex — and in the United States that is the one that matters most. The U.S. Centers for Disease Control and Prevention identifies Culex as the primary West Nile virus vector, and West Nile is the leading cause of mosquito-borne disease in the contiguous United States. Culex breeds in exactly the foul, organic-rich standing water a dunk treats.
That is a stronger case for a $13 box of tablets than most people realise. In the United States, the CDC reports more than 1,300 people develop severe neuroinvasive West Nile illness each year and more than 130 die. The larvae behind those infections are not developing in a pristine woodland pond — they are developing in rain barrels, buckets, clogged gutters, catch basins, and neglected pools. Larvicide is the intervention that meets them where they actually are. In Canada, Culex pipiens is likewise the species behind most West Nile surveillance detections, and Public Health Ontario and the Public Health Agency of Canada both put eliminating standing water first — the Canadian framing on the rest of this page is nuisance biting and black fly, which is accurate for most GTA yards, with West Nile as the reason the standing-water advice is not optional.
| Species | Where in the United States | Where it breeds | Why a dunk matters |
|---|---|---|---|
| Culex pipiens (northern house mosquito) | Northern states | Foul, organic-rich water: rain barrels, buckets, clogged gutters, catch basins | CDC: primary West Nile vector. The single best target for a dunk. |
| Culex quinquefasciatus (southern house mosquito) | Southern states | Same stagnant, nutrient-rich containers and storm drains | CDC: primary West Nile vector in the South. |
| Culex tarsalis | Western states | Irrigation water, ditches, ponds, flooded fields | CDC: primary West Nile vector in the West. |
| Aedes albopictus (Asian tiger mosquito) | Southeast, Mid-Atlantic, lower Midwest | Small containers — tires, saucers, toys, tree holes, bottle caps | The classic U.S. container breeder. Water volumes are tiny, so use a broken quarter dunk or Bits. |
| Aedes aegypti | Florida, Hawaii, Texas, Puerto Rico, U.S. Virgin Islands, American Samoa | Containers in and immediately around the home | CDC: spreads dengue, chikungunya and Zika. Container control is the front line. |
| Anopheles quadrimaculatus / An. freeborni | Most of the continental United States | Sunlit permanent water, marsh edges, ponds | Historic malaria vectors. Larvae lie flat under the surface film rather than hanging head-down. |
Species ranges and disease associations follow the U.S. Centers for Disease Control and Prevention. If you are not sure what is in your water, the wriggler identification guide is here: how to identify mosquito larvae.
How many mosquito dunks does your season actually need?
A 6-pack is a season in Canada. It is not a season in Florida. Everywhere else on this page assumes a May-to-September window, which is correct for the GTA and for the northern United States. Further south the breeding season runs far longer, and the arithmetic changes with it — a single rain barrel on the Gulf Coast can consume roughly a dozen dunks a year. Find your row before you decide between the 6-pack and the 20-pack.
| Region | Treat from | Dunks for one 50-gal barrel | Buy |
|---|---|---|---|
| Gulf Coast, Florida, Hawaii, Puerto Rico | Close to year-round | ~12 | 20-pack — a 6-pack does not cover a season here |
| Southeast & Southwest (GA, SC, TX, AZ, southern CA) | ~March to November | ~9 | 20-pack, or two 6-packs |
| Mid-Atlantic & Midwest | ~April to October | ~7 | One 6-pack plus a couple spare |
| Northern tier U.S. (New England, Upper Midwest, Pacific NW, Mountain) | ~May to September | ~5 | 6-pack covers it |
| Canada — GTA and southern Ontario | May to September | ~5 quarter dunks (the Canadian label puts a barrel at ¼ dunk) | One 6-pack covers several barrels for the season |
Two adjustments to that table. Multiply by the number of separate water sources you are treating — a barrel, a pond and a ditch is three dunks a month, not one. And in a hot, wet summer, count on replacing early: NC State Extension’s 3–4 week interval is the realistic one, so the totals above are a floor rather than a ceiling. Quartering tablets for buckets and small containers is what keeps the bill from moving.
Do mosquito dunks work for a lawn?
No — not the way the question is usually meant. A dunk is a water treatment, not a turf treatment. Scattering tablets across a lawn does nothing at all, because there is no standing water for the BTI to dissolve into and no filter-feeding larvae in dry grass. This is the most common misunderstanding about the product, and it produces a lot of “dunks did nothing for me” reviews.
Where dunks pay off around a lawn is the water that lawn drains into: the low spot that stays wet for a week after rain, the catch basin at the kerb, the ditch along the property line, the clogged gutter overhead, the tarp on the woodpile. Treat those and you cut the mosquitoes your own property is producing. The adults already flying over the grass came from somewhere else and need a different tool — that is what a barrier spray on vegetation does, and here is what a full season of that costs once lot size is factored in. Before you treat anything, it is worth walking the yard for the water you have not noticed yet: hidden mosquito breeding spots.
Of the water sources in that list, the clogged gutter is the one worth fixing rather than dosing. It is above head height, so nobody inspects it; it holds exactly the leaf-rich soup Culex prefers; and a dunk in a gutter washes out with the next real rain. A gutter guard or leaf screen removes the breeding site permanently instead of putting it on a thirty-day replacement schedule, which is the better trade on any run you need a ladder to reach. Check gutter guards on Amazon.ca →
The Bigger Mosquito Strategy
Mosquito dunks solve only HALF of the mosquito problem — they kill larvae in the water you treat. They don’t affect adult mosquitoes flying in from your neighbour’s yard, the ravine behind your house, or the creek down the street. That off-property pressure is the whole story on ravine-backed lots, which is why mosquito control in Toronto is almost never solved by treating your own standing water alone. A dunk also does nothing about the bites you already have — it kills the larvae that would have become next month’s mosquitoes, not the welt on your arm tonight, and that is a separate question answered in why mosquito bites itch and how long they last.
For fuller mosquito control in your GTA yard, combine three strategies:
- BTI dunks (Reg. No. 28888) in the standing water on your property that the label covers — it handles larvae from your own water
- Habitat modification — drain anything you can, mow grass short, clear leaf litter
- Professional barrier spray — BuzzSkito’s seasonal program targets the adult mosquitoes flying onto your property from off-property sources, with a residual applied according to label directions and renewed on a schedule
We run that third layer out of Mississauga across 19+ GTA cities — if you are treating a rain barrel here at home, the companion page for mosquito control in Mississauga covers what the barrier spray side of the season looks like.
Related Reading
- Natural Mosquito Repellent — What Actually Works
- Mosquito Repellent Complete Guide 2026
- Hidden Mosquito Breeding Spots in Your Backyard
- BuzzSkito Professional Mosquito Control Service
Frequently Asked Questions
What are mosquito dunks?+
Mosquito dunks are small donut-shaped tablets containing Bacillus thuringiensis israelensis (BTI), a naturally occurring soil bacterium. When dropped into standing water, the BTI is consumed by mosquito larvae and disrupts their digestive system, killing them; the Canadian label says to allow a minimum of 48 hours for control. On the Canadian label each dunk treats up to 6 square metres of water surface (a quarter dunk for up to 3), reapplied at 30-day intervals if larvae are detected; the U.S. label says 100 square feet. In Canada the registered product is Summit Mosquito Dunks, Reg. No. 28888, sold through hardware chains and garden centres.
Are mosquito dunks safe for pets, fish, and humans?+
Follow the label rather than a blanket promise. BTI specifically targets mosquito and black fly larvae: the U.S. Environmental Protection Agency states that BTI toxins affect only the larvae of mosquitoes, black flies, and fungus gnats, and that BTI has no toxicity to people. The Summit label lists use sites including rain barrels, koi ponds and water gardens, birdbaths, roof gutters, unused swimming pools, and animal watering troughs. Store the box out of reach of children and pets — dogs occasionally chew on dunks — and call your veterinarian if a tablet is swallowed.
Where should I put mosquito dunks?+
Use mosquito dunks in standing water that cannot be drained, including: rain barrels (a quarter dunk on the Canadian label), ornamental ponds and water features, neglected swimming pools, large bird baths you cannot change weekly, tree holes, flooded low spots, and any container holding water for more than a week. In Canada, not in water designed to flow off your property: the label names ditches, drains, rain gutters and catch basin sumps. Do NOT use them in puddles that dry out within a few days — they need water present to work.
How long do mosquito dunks last?+
The Canadian label says dunks release their BTI “for up to 30 days” and to reapply “at 30 day intervals if mosquito larvae are detected”, at one dunk per 6 square metres of water surface (100 square feet on the U.S. label). Heavy rain or water turnover may reduce effectiveness — check after major rain events. For a rain barrel or pond through the summer, put a fresh dose in each month from May through September while larvae keep appearing.
Do mosquito dunks actually work?+
Yes — BTI dunks are evidence-backed and used by mosquito control districts across North America. Published field research reports high mortality of mosquito larvae within 24–48 hours of contact. However, dunks only kill larvae in the water you treat. Adult mosquitoes flying in from neighbours' yards or natural water bodies (creeks, ravines, ponds nearby) are unaffected. For yard-wide control, combine dunks (larvae control) with professional barrier spray (adult control).
Mosquito Dunks vs Mosquito Bits — what is the difference?+
Same active ingredient (BTI), different format. Dunks are slow-release tablets ideal for long-term water sources (rain barrels, ponds — 30 days per dunk). Bits are quick-release granules that work within minutes, but they have no Canadian registration. In Canada, use Dunks (Reg. No. 28888), or a registered BTI granule such as AquaBac 200G (Reg. No. 27374) or My-Crobe 200G (Reg. No. 34631) where you would have used Bits; a small bird bath needs fresh water twice a week (Health Canada), not a product, and the granule labels rule out wading pools.
Where to buy mosquito dunks in Canada?+
Look for Summit Mosquito Dunks with REG. NO. 28888 on the label at hardware chains (Canadian Tire, Home Depot Canada and RONA, in-store and online), garden centres and pond shops. In Health Canada’s pesticide registry it is the only BTI briquet with a domestic registration, so a box with only a U.S. EPA number is not the Canadian product. We do not publish prices, because they go stale. Stock up at the start of mosquito season (April–May) — supplies often run low by mid-summer.
How much do mosquito dunks cost in Canada?+
We no longer publish a price: shelf prices moved within the 2026 season, and a stale price is worse than none, so check the retailer’s own site and compare the price per dunk between pack sizes. What decides the cost is the dose. On the Canadian label a rain barrel takes a quarter dunk (0.1 to 3.0 square metres of water), reapplied at 30-day intervals if larvae are detected, so one 6-pack covers several barrels for a May–September season. Buying in April–May also beats the mid-summer shortages.
Are mosquito dunks BTI-only or do they have other ingredients?+
Mosquito dunks hold one active ingredient, BTI (Bacillus thuringiensis subspecies israelensis, strain BMP-144, 10.31% on the Canadian label), with the rest of the tablet made up of other ingredients that allow the slow 30-day release. The BTI is the only active ingredient — it specifically targets mosquito and black fly larvae. There are no pyrethroids or other synthetic insecticides in the tablet. That narrow target list is why dunks are used in ponds and rain barrels; follow the label for use sites. The Summit Chemical brand sold most widely in Canada carries Canadian PCP registration number 28888.
Can I use mosquito dunks in a swimming pool?+
Only in a NEGLECTED or unused pool that has standing water. Mosquito dunks are not meant for active, chlorinated swimming pools — chlorine kills larvae anyway, and pool filtration would render the dunk ineffective. If you have an inactive pool that has accumulated rainwater, drop in a dunk to prevent it from becoming a mosquito breeding site until you can drain it. For active pools, just maintain proper chlorine levels.
Where can you buy mosquito dunks near you in Canada?+
You can buy mosquito dunks near you at most Canadian hardware and garden retailers — Canadian Tire, Home Depot and RONA, alongside independent garden centres and Amazon.ca; check that the box shows REG. NO. 28888. The quickest way to check local stock is to search 'mosquito dunks' on the Canadian Tire or Home Depot website and filter by store availability using your postal code. Stock is most reliable in April–May; shelves often thin out by mid-July, so buy early.
How do you use mosquito dunks for fungus gnats?+
To use mosquito dunks against fungus gnats, break a dunk into pieces and either float a chunk in your watering can overnight or steep it in a few litres of water, then use that BTI-infused water on your houseplants. The BTI kills fungus gnat larvae in the soil the same way it kills mosquito larvae. Repeat with each watering for 2–3 weeks to break the breeding cycle. Mosquito Bits are often easier for potting soil because you can sprinkle the granules right on top. That is U.S. practice: in Canada, fungus gnats are not on the Mosquito Dunks label (Reg. No. 28888), which covers mosquito larvae in water only, and Bits are not registered.
What are mosquito dunks made of?+
Mosquito dunks are made of dried Bacillus thuringiensis israelensis (BTI) — a naturally occurring soil bacterium — compressed with an inert binder into a slow-dissolving donut-shaped tablet. BTI is the only active ingredient; there are no pyrethroids or other synthetic insecticides. As the tablet breaks down in water it releases BTI spores that mosquito and black fly larvae eat. The Summit brand sold across Canada carries Canadian PCP registration number 28888.
Where do you buy mosquito dunks in stores?+
In stores, mosquito dunks are stocked in the garden, pest-control, or seasonal aisle at Canadian Tire, Home Depot and RONA, plus most independent garden centres and nurseries. Look for the Summit Chemical orange-and-white box with REG. NO. 28888 on the label, near the insect sprays and larvicides. If you cannot find them on the shelf, ask staff to check the seasonal stockroom — dunks are sometimes only put out once mosquito season begins in May.
Do mosquito dunks really work?+
Yes — in the water you actually treat. Mosquito control districts across North America run BTI larvicides as a standard tool, and published trials repeatedly show over 90% larval mortality within 24–48 hours. One dunk covers up to 6 square metres of water surface on the Canadian label (roughly 100 sq ft on the U.S. label). What a dunk cannot do is touch adult mosquitoes that are already flying, because those arrive from a neighbour's yard, a ravine, a creek, or a storm drain you do not own. That half of the problem is what a barrier spray on vegetation handles.
Do mosquito dunks kill anything else?+
Almost nothing. The U.S. Environmental Protection Agency states that BTI toxins affect only the larvae of mosquitoes, black flies, and fungus gnats. Adult mosquitoes, dragonfly nymphs, water beetles, snails, tadpoles, frogs, fish, birds, bees, and earthworms are not on that list, because BTI only becomes active in the specific alkaline gut of those larvae. That narrow spectrum is why the EPA also notes BTI is approved for pest control in organic farming operations.
What is the difference between mosquito bites and mosquito dunks?+
Two different things get mixed together in this question. Mosquito Dunks are a BTI larvicide tablet you drop into standing water to kill larvae before they hatch — they do nothing for a bite you already have, which is a topical antihistamine or hydrocortisone job. The comparison most people actually mean is Mosquito Dunks vs Mosquito Bits: same BTI bacterium, different format. Dunks are slow-release tablets lasting about 30 days; Bits are quick-release granules that start killing larvae within minutes.
Are mosquito dunks harmful to humans?+
No. The U.S. Environmental Protection Agency states that BTI has no toxicity to people, and BTI is registered for residential use in both countries — EPA Reg. No. 6218-47 in the United States and Health Canada PCP #28888 in Canada. The U.S. Summit label carries the signal word CAUTION for moderate eye irritation; the Canadian label reads “CAUTION EYE IRRITANT” and “POTENTIAL SENSITIZER”. Wash your hands after handling a dunk, keep the box out of reach of children, and do not deliberately treat household drinking water at home.
Are mosquito dunks safe for dogs?+
BTI acts in the gut of mosquito, black fly, and fungus gnat larvae (the U.S. EPA states its toxins affect only those larvae), and the Summit label lists animal watering troughs among its use sites, so use it at the label rate. Dogs do sometimes fish a floating dunk out and chew it. Store the box out of reach, and call your veterinarian if a tablet was swallowed or your dog seems unwell.
Are mosquito dunks safe for cats?+
The same points apply as for dogs: BTI toxins act on specific insect larvae, and the product should be used at the label rate. Cats are also far less likely than dogs to chew a tablet. For a bowl or fountain a cat drinks from, skip the larvicide (Mosquito Bits have no Canadian registration, and the registered BTI granules’ labels rule out drinking receptacles): a bowl you refresh every two or three days never gets the week of still water larvae need.
Are mosquito dunks safe for birds?+
Birdbaths, koi ponds, and water gardens are listed use sites on the Summit label, and birds are not among the larvae BTI toxins act on. For a small bird bath the simplest control is dumping and refilling every three days, which beats larvae on timing alone — mosquito larvae need more than a week of undisturbed water to reach a flying adult. In Canada, refilling is the answer: Health Canada advises replacing bird-bath water at least twice a week, and Mosquito Bits have no Canadian registration.
Are mosquito dunks safe for fish?+
Koi ponds and water gardens are listed use sites on the Summit label, and BTI is used routinely in stocked ornamental ponds. Fish eat mosquito larvae themselves, so a well-stocked pond often needs no dunk at all. Where a dunk earns its place is a pond with no fish, or the shallow, plant-choked margins and bog shelves that fish do not patrol — that is where larvae survive in an otherwise stocked pond.
Are mosquito dunks safe for bees and other pollinators?+
BTI is active only in the gut of mosquito, black fly, and fungus gnat larvae (the U.S. Environmental Protection Agency states its toxins affect only those larvae), so bees, butterflies, fireflies, ladybugs, and dragonflies are not its target. The EPA also notes BTI is approved for pest control in organic farming operations. This specificity is the main reason BTI is preferred over broad-spectrum larvicides in ponds, rain gardens, and pollinator plantings.
Are mosquito dunks safe for plants?+
BTI is a soil bacterium, not a herbicide. Gardeners pour water steeped with a piece of dunk on houseplants and vegetable beds — that is exactly how it is used against fungus gnats. Because the EPA states BTI is approved for pest control in organic farming operations, water from a treated rain barrel can still go on an organic vegetable garden.
Do mosquito dunks expire?+
Yes, by the Canadian label: “Store Mosquito Dunks in the original closed container in a cool (25°C or less), dry place and use them within one year from date of manufacture to ensure microbial purity and potency.” The Canadian label has a date-of-manufacture line for that reason, so leftovers carry over to the next season only while they are inside that year. What ruins a dunk fastest is moisture. One that has been wetted, dried, and re-wetted, or stored damp in a shed floor puddle, has already given up part of its BTI and should be replaced rather than trusted.
Do mosquito dunks still work if they sink?+
Yes. A dunk keeps releasing BTI whether it floats on the surface or sits on the bottom, and Summit says as much on the label. Mosquito larvae are filter feeders that graze through the whole water column, so they take up the spores regardless of where the tablet ends up. The only real drawback of a sunk dunk is that you cannot see it, so put the monthly replacement on a calendar instead of relying on spotting the tablet.
How do you make a mosquito dunk bucket?+
Fill a 5-gallon bucket about two-thirds with water, add a handful or two of straw or grass clippings, and let it steep for a few days until it smells foul — that stink is what attracts egg-laying Culex females. Then add one quarter of a mosquito dunk and refresh it every 30 days. Drill overflow holes about six inches below the rim, cover the top with hardware cloth, and lay a stick inside as an escape ramp. This build follows the instructions published by K-State Research and Extension in Johnson County, Kansas.
How many mosquito dunks do I need for a season?+
It depends where you live, because the season length does. In Canada the label sets a rain barrel (0.1 to 3.0 sq. metres of water) at a quarter dunk, reapplied at 30-day intervals if larvae are detected, so a May-to-September season takes about five quarters and one 6-pack covers several barrels. In the northern United States, budget about five dunks a barrel for the same season. In the Mid-Atlantic and Midwest, budget six or seven for roughly April to October. In the Southeast and Southwest, eight or nine for roughly March to November. On the Gulf Coast and in Florida, Hawaii, and Puerto Rico, mosquitoes breed close to year-round — plan on about twelve, which means a 20-pack, not a 6-pack.
Where can you buy mosquito dunks in the United States?+
Summit Mosquito Dunks are stocked nationwide at Home Depot, Lowe's, Walmart, Ace Hardware, Tractor Supply, Target, and independent garden centres, plus Amazon.com. Look for the orange-and-white Summit box in the garden or pest-control aisle. Before you buy, it is worth searching your county name plus "mosquito abatement district" — many U.S. districts inspect standing water and hand out free BTI dunks to residents, a service that has no equivalent in Canada.
Are mosquito dunks EPA registered?+
Yes. Summit Mosquito Dunks are registered with the U.S. Environmental Protection Agency under EPA Reg. No. 6218-47, held by Summit Chemical Company of Baltimore, Maryland. The EPA has registered five BTI strains across 48 pesticide products for residential, commercial, and agricultural use. In Canada the same product carries PCP registration number 28888. The EPA also reports no documented resistance to BTI as a larvicide, even in mosquito populations treated for decades.
Do mosquito dunks kill fungus gnats?+
Yes. The U.S. Environmental Protection Agency lists fungus gnat larvae as one of only three insect groups BTI affects, alongside mosquito and black fly larvae. The BTI kills the larvae living in damp potting soil; it does not kill the adult gnats already flying around your windows, which is why the population seems unchanged for the first week. A fungus gnat cycles from egg to adult in roughly two to three weeks, so keep the treated water going for at least three weeks to break the cycle. In Canada, though, fungus gnats are not on the Mosquito Dunks label (Reg. No. 28888), which covers mosquito larvae in water only.
How do you use mosquito dunks in potted plants?+
Do not bury a whole tablet in the pot. Break off about a quarter of a dunk, drop it into a gallon jug or watering can of water, let it steep for 24 to 48 hours, then water your plants with that BTI-infused water. Repeat at every watering for three weeks. Mosquito Bits are easier for potting soil because you sprinkle the granules straight onto the top inch and water them in, which also avoids leaving a tablet sitting in a decorative pot. Both methods are U.S. practice; the Canadian Mosquito Dunks label covers mosquito larvae in water only.
Can you break a mosquito dunk into pieces?+
Yes, and for small containers you should. The Canadian label says dunks “can be used whole or in pieces” and puts 0.1 to 3.0 square metres of water at a quarter dunk; Summit's U.S. rate is one dunk per 100 sq ft of water surface, which is far more BTI than a 5-gallon bucket, a plant saucer, or a watering can needs. K-State Research and Extension uses one quarter of a dunk per 5-gallon bucket. Score the tablet with a knife or snap it by hand — the BTI is distributed evenly through the dunk, so a quarter tablet treats a quarter of the volume and a 6-pack stretches to 24 small treatments.
Do mosquito dunks work for a lawn?+
Not in the way people usually mean. A dunk is a water treatment, not a turf treatment — scattering dunks across a lawn does nothing, because there is no standing water for the BTI to dissolve into. Where dunks pay off is the water that lawn drains into: the low spot that stays wet for a week after rain, the catch basin, the ditch at the property line, the clogged gutter overhead. Adults already flying over the lawn are a barrier spray problem, not a larvicide problem.
Can you use mosquito dunks in animal watering troughs?+
Yes. Summit's label lists animal watering troughs among its use sites, alongside rain barrels, tree holes, flower pots and saucers, planter reservoirs, roof gutters, water gardens, koi ponds, and unused swimming pools. Use it at the label rate. Refresh the dunk about every 30 days, and top the trough up rather than draining and refilling while the tablet is still working.
Are mosquito dunks approved for organic gardening?+
Yes. The U.S. Environmental Protection Agency states that BTI is approved for pest control in organic farming operations, and Summit markets Mosquito Dunks as approved for use in organic production. In practice that means water from a treated rain barrel can be used on an organic vegetable garden without breaking the rules of home organic growing. If you are certified organic commercially, confirm the specific product against your certifier's input list, since approved-input lists are program-specific.
Can you use mosquito dunks in Canada?+
Yes. In Canada, Mosquito Dunks are a domestic-class larvicide, Reg. No. 28888 under the Pest Control Products Act, with registrant Summit Chemical Company and Bacillus thuringiensis subspecies israelensis strain BMP-144 at 10.31% as the active ingredient; the Canadian label rate is one dunk for up to 6 square metres of water surface, and it limits use to standing or containerized water on your own property that cannot be drained, emptied or changed at least weekly, not gutters, ditches or drains that flow off the property.
Are mosquito dunks available in Canada?+
Yes. Mosquito Dunks are registered in Canada as a domestic-class product, Reg. No. 28888, registrant Summit Chemical, with the registration current to December 31, 2028 in Health Canada’s pesticide registry (checked October 23, 2026). Buy the box that shows that number. Health Canada warns that an unregistered import may “not be properly labelled or packaged to ensure safe use, even if the imported pesticide has the same name as a pesticide registered in Canada,” so a U.S. box with only an EPA number is not the Canadian product.
How much of a mosquito dunk goes in a rain barrel?+
A quarter, on the Canadian label. The label doses by water surface, not volume: “Use one (1) Mosquito Dunk for up to 6 sq. metres water surface, regardless of depth”, with ¼ dunk for 0.1 to 3.0 sq. metres and ½ dunk for 3.0 to 6.0. A rain barrel’s water surface sits in the first row. The label also asks you to “Allow a minimum of 48 hours for control” and to reapply at 30-day intervals if larvae are detected. Screen the barrel too: Health Canada says to empty a barrel “if the water is over a week old, unless it’s protected with a fine screen”.