Are mosquito dunks safe in a bird bath?

The short answer

• A mosquito dunk is a floating cake of Bti, a soil bacterium whose toxins EPA says affect only the larvae of mosquitoes, black flies and fungus gnats. The EPA-accepted Mosquito Dunks label lists bird baths as a use.
• Use a quarter of a dunk, not a whole one. The label's table gives 1/4 dunk for 1 to 5 sq ft of water, and a 24 in round bath is about 3.1 sq ft. A whole dunk is 4 times the label amount.
• You rarely need one. CDC puts egg to adult at 7 to 10 days, so a bath emptied and refilled every 2 to 3 days never produces a mosquito. Scrub it weekly too, because Aedes eggs stick to the wall above the water.
• A dunk earns its place in a bath you cannot change for a week or more, such as while you are away.
• The one bird effect on record came from spraying Bti across wetlands in France, where house martins raised fewer young for lack of insects. That is a landscape program, not a bird bath.

Yes, if you use the dose on the label, and that dose is a quarter of a dunk for an ordinary bath, not the whole thing. The label filed with the US Environmental Protection Agency (EPA) names bird baths as a use. Its active ingredient kills mosquito larvae and, on the evidence below, does not harm the birds that drink the water.

But the better answer comes first. A bath you empty and refill every 2 to 3 days never makes a mosquito, so it has nothing for a dunk to kill. The dunk is for the bath you cannot get to.

What a mosquito dunk is#

A mosquito dunk is a small, flat, floating cake with a hole in the middle. Mosquito Dunks is a trade name, but people use the word for any cake of this kind. The label calls it a "floating sustained-release larvicide." A larvicide kills larvae, the young stage of an insect. Mosquito larvae live in still water, and CDC notes they are often called wigglers.

The working part is Bti, short for Bacillus thuringiensis subspecies israelensis. It is a bacterium found in soil. EPA says it makes toxins that "specifically affect the larvae of only mosquitoes, black flies and fungus gnats." The larvae have to eat it, so it does nothing to adult mosquitoes that are already flying.

The label we read is EPA Registration No. 6218-47. It says the cake releases Bti "for a period of 30 days or more," floating at the surface and slowly settling to the bottom. It lists "Bird Baths" on its front panel, next to flower pots, gutters and animal watering troughs.

Worked through: a 24 in round bath#

Take an ordinary pedestal bath with a round basin 24 in across. Here is how the label's numbers apply to it.

1. Find the surface area. The radius is 12 in, or 1 ft. Area is 3.14 x 1 x 1, so about 3.1 sq ft of water.
2. Read the main rate. The label says one dunk for "up to 100 sq. ft. of surface area, regardless of depth." Depth does not matter, only the surface.
3. Scale it. At that rate, 3.1 sq ft would get about a thirtieth of a dunk. Nobody can cut a dunk that small.
4. Read the household table instead. The label has a second table for small water near the house. For 1 to 5 sq ft it says 1/4 dunk. For 5 to 25 sq ft, 1/2 dunk. So this bath gets a quarter. A pond of 25 to 100 sq ft gets one whole dunk.
5. Compare with a whole dunk. A whole dunk is 4 times the quarter the label allows for this basin. Against the main rate, it is about 32 times as much.

The label states that using the product "in a manner inconsistent with its labeling" breaks federal law. A whole dunk in a 3 sq ft bath is past the label. So the safe amount and the legal amount are the same: a quarter.

Most baths fall in the same band. A 30 in round basin is about 4.9 sq ft, still a quarter. A small 16 in dish is about 1.4 sq ft, and the table still gives a quarter, since it starts at 1 sq ft.

Why you probably do not need one#

The Centers for Disease Control and Prevention (CDC) says a mosquito egg takes 7 to 10 days to become an adult. Our bird bath routine changes the water every 2 to 3 days. Each change pours out the larvae before any can finish. A dunk in a bath on that schedule is killing larvae that the next change would have thrown out anyway.

There is one catch, and it is the reason to scrub. The common container mosquitoes, the Aedes group, lay their eggs on the inner wall "above the waterline," CDC says. The eggs "stick to container walls like glue" and can survive drying out for up to 8 months. Tipping the water out leaves them on the rim. A stiff brush around the wall takes them off. CDC's own advice is to empty and scrub items that hold water, birdbaths among them, once a week.

So the full routine is two parts:

• Every 2 to 3 days: tip out, refill. This removes larvae.
• Once a week: scrub the whole basin, rim included. This removes eggs, and the slime that carries disease between birds.

Do both and the bath stays mosquito-free without any product.

When a dunk makes sense#

A dunk earns its place in water you cannot change on that schedule. That means a bath you will leave for a week or more, which is long enough for eggs to reach adulthood. A vacation in July is the usual case, since summer is peak bath season.

Before you leave, scrub the bath and refill it. Drop in a quarter dunk. The label says it lasts 30 days or more, which covers most trips. When you get back, go back to changing the water.

The label also says that some larvae which hatch after a dunk goes in "may partially develop before dying," and to allow at least 48 hours. So a dunk dropped into a bath already full of wigglers is slower than tipping it out. Tip first, then treat.

The same goes for the water around the bath. A plant saucer, a blocked gutter or a rain barrel holds water for weeks and nobody changes it. Those are where a dunk does real work. The label lists all three.

Is Bti safe for birds?#

At the dose a bath uses, the sources agree it is. EPA says Bti "has no toxicity to people" and that its toxins "do not affect other types of insects including honey bees." A fact sheet from the National Pesticide Telecommunications Network calls Bt "practically nontoxic to birds and fish." The label goes further and says it "Can Be Used In Fish Habitats."

The bird question that matters is not poison. It is food. A study in the Camargue, a wetland in southern France, followed house martins, a kind of swallow, before and during 3 years of Bti spraying on marshes in part of the area. Brigitte Poulin and colleagues reported in the Journal of Applied Ecology in 2010 that Bti had "a significant effect on the diet and feeding rate" of the birds. That gave "significantly lower breeding success" at the sprayed sites. In 2009, nests at sprayed sites raised two young on average, against three at the unsprayed ones. Birds at the unsprayed sites ate mostly mosquitoes and midges. At the sprayed sites, flying ants made up a larger share of what they ate.

That was a program spraying marshes across part of a national park. Even there, the ecologist Pierre Mineau warned it was "just one species in one context." A bird bath of 3 sq ft is not a marsh, and it does not feed a colony of swallows. The study is a reason to question mosquito spraying across a whole wetland. It is not a reason to fear the bath.

Why the advice online splits#

You will read both "safe" and "never" about the same product. Both have a root.

The "never" comes from a sound general rule: put nothing in water birds drink. That rule is right for copper, bleach tablets, soap and algaecides. Our own bird bath page used to list dunks with them, and we have corrected it. Bti is the one item on that list whose federal label names bird baths.

The "safe" answers are usually right about the toxin and silent about the dose. Most of them say to drop a dunk in. A whole one is 4 times the label amount for a normal bath.

The NPTN fact sheet also says a drop in insects "will not greatly affect birds." It was reviewed in 2000. The Camargue study came ten years later and found an effect from spraying a landscape. The two are not about the same scale.

The quick table#

What this answer does not cover#

This page reads one label, Mosquito Dunks under EPA Reg. No. 6218-47. Other Bti products, including granules sold as bits, may give a different amount for a bird bath. Use your own label's table. If its active ingredient is anything other than Bti, nothing here applies.

A dunk does nothing for the bath's main hazard, which is shared water. Birds pass disease through it, and only changing and scrubbing deals with that. If you see a sick bird, take the bath down with the feeders, as set out in the disease guide. The same bleach dilution used on feeders applies to the basin.

We found no study that tested Bti on birds drinking from a treated bath. The safety case rests on EPA's statement of what the toxin affects and on the NPTN rating for birds. The label also bars use in finished drinking water meant for people. It says nothing about adult mosquitoes, which a dunk does not touch, or about wasps at the water, which it does not deter.

For the rest of the bath, the depth, the drip and the placement, see bird baths and water. For winter, when there are no mosquitoes but there is ice, see what a heated bath costs to run.

Frequently asked questions#

Will a mosquito dunk hurt birds that drink from the bath?

Not at the label dose, on the evidence we found. EPA says Bti toxins affect only the larvae of mosquitoes, black flies and fungus gnats and have no toxicity to people. The National Pesticide Telecommunications Network calls Bt practically nontoxic to birds and fish. The label itself lists bird baths and says the product can be used in fish habitats. The known harm to birds comes from wiping out insects across a whole landscape, which one bath cannot do.

How much of a mosquito dunk do I put in a bird bath?

A quarter of one for a typical bath. The label's household table gives 1/4 dunk for 1 to 5 sq ft of water surface, 1/2 for 5 to 25 sq ft, and 1 dunk for 25 to 100 sq ft. To find your area, measure across the basin in feet, halve it, square that, and multiply by 3.14. A 24 in round basin comes to about 3.1 sq ft, so a quarter dunk. Break the dunk along a straight line and keep the unused pieces dry and sealed.

Do I still need to clean the bath if I use a dunk?

Yes. A dunk only kills mosquito larvae. It does nothing about droppings, algae or the germs birds pass to each other at shared water. Change the water every 2 to 3 days and scrub weekly, as CDC also advises for bird baths. If you keep to that, the dunk has nothing left to do.

Are Mosquito Bits the same thing?

This page reads one label, the Mosquito Dunks label filed with EPA. Products sold as granules or bits may use the same active ingredient at a different dose. Read your own product's label for its bird bath amount and its active ingredient. If the active ingredient line names anything other than Bti, the answer on this page does not apply to it.

Can I put a mosquito dunk in a bath my dog drinks from?

The label lists animal watering troughs among its uses, at the same dose by surface area. Its only drinking-water restriction is for treated, finished water meant for people. Pet safety beyond what the label says is outside what we checked, so ask your vet if your pet has a health condition.

Sources and further reading

1. Summit B.T.I. Briquets (alternate brand name Mosquito Dunks), EPA Reg. No. 6218-47, accepted label, 27 March 2008 US Environmental Protection Agency, Pesticide Product Label System Bird baths listed; 1 dunk per 100 sq ft regardless of depth; 30 days or more; can be used in fish habitats; 48 hours for control; not for finished drinking water for people; use inconsistent with labeling violates federal law
2. Mosquito Dunks, EPA Reg. No. 6218-47, label of 23 February 1993 US Environmental Protection Agency, Pesticide Product Label System Household dosage table read here because the 2008 scan is illegible in one cell: 1/4 dunk for 1 to 5 sq ft, 1/2 for 5 to 25, 1 for 25 to 100
3. Bti for Mosquito Control (January 2025 snapshot) US Environmental Protection Agency Toxins affect only larvae of mosquitoes, black flies and fungus gnats; no toxicity to people; can be used in bird baths
4. Bacillus thuringiensis General Fact Sheet National Pesticide Telecommunications Network (Oregon State University and US EPA), hosted by New Jersey Department of Agriculture Bt practically nontoxic to birds and fish; israelensis targets mosquitoes and flies; not likely to multiply in water; reviewed 24 October 2000
5. Life Cycle of Aedes Mosquitoes US Centers for Disease Control and Prevention Eggs laid on inner container walls above the waterline, stick like glue, survive drying up to 8 months; egg to adult 7 to 10 days
6. Preventing Mosquito Bites US Centers for Disease Control and Prevention Once a week, empty and scrub, turn over, cover or throw out items that hold water, including birdbaths
7. Effect of mosquito control on breeding birds Tour du Valat Summary of Poulin, Lefebvre and Paz 2010, Journal of Applied Ecology: significantly lower house martin breeding success at Bti-sprayed Camargue sites, through diet and feeding rate
8. Mosquito spray affects bird reproduction (Nature News, 15 June 2010) BioEd Online, Baylor College of Medicine Two fledged chicks per nest at treated sites against three at control sites in 2009; diet shifted from mosquitoes and midges toward flying ants; Mineau: one species in one context