If you have a pond, a rain barrel or a corner of the yard that never quite dries out, you have probably seen mosquito dunks at the hardware store. They promise to stop mosquitoes before they ever fly. But do they actually work in Ontario, and are they safe around kids, pets and fish?
Here is a plain-language guide to Bti, the active ingredient in mosquito dunks, and how to use it properly.
Quick Answer
Yes, mosquito dunks work, but only on mosquito larvae in standing water. They use Bti, a natural bacterium that kills larvae and is considered low risk for people, pets, birds and fish when used as directed. Dunks do nothing against the adult mosquitoes already biting you, so they are one part of a plan, not the whole plan.
What Is Bti?
Bti stands for Bacillus thuringiensis israelensis. It is a bacterium found naturally in soil.
When mosquito larvae eat it, Bti produces proteins that destroy their gut. The larvae die within a day or two, before they can become biting adults.
What makes Bti special is how targeted it is. It mainly affects the larvae of mosquitoes, black flies and fungus gnats. Fish, frogs, birds, pets, bees and people do not have the gut conditions that activate it.
Mosquito Dunks vs Mosquito Bits
Bti is sold in two main forms.
| Product | What It Is | How Fast It Works | How Long It Lasts | Best For |
|---|---|---|---|---|
| Dunks | Slow-release floating ring or tablet | Within 24 to 48 hours | About 30 days | Ponds, rain barrels, water you cannot drain |
| Bits or granules | Small granules sprinkled on water | Within 24 hours | About 1 to 2 weeks | Quick knockdown, small or shallow areas, plant pots |
Many homeowners use both: bits to clear an active problem fast, and dunks to keep it under control for the rest of the month.
Where to Use Bti Around Your Home
Bti only makes sense where water stays for more than a week. Good places include:
- Ornamental ponds and water features without a pump
- Rain barrels, especially those without a tight lid
- Low spots, ditches or swales that stay wet after rain
- Old tires, tarps or containers you cannot remove right away
- Unused pools and pool covers holding water
- Large planters or troughs
If water can simply be dumped, dump it instead. Our checklist of hidden mosquito breeding sites helps you find the spots most people miss.
How to Use Mosquito Dunks
- Read the label. The product label is the legal rule for how much to use and where.
- Match the dose to the water surface. A common rate is one dunk for about 100 sq ft of water surface, no matter how deep it is. Smaller containers need only a piece of a dunk.
- Let it float. The dunk slowly releases Bti over time. You can tie it to a string so it does not drift into a filter.
- Replace it about every 30 days, or sooner if heavy rain flushes the water.
- Start early. Put the first dunks out in May, when water warms up and the first larvae appear. Our Toronto mosquito season calendar shows what to expect month by month.
Is Bti Safe?
When used according to the label, Bti is considered low risk for people, pets, birds, fish and pollinators. That is why public health programs across Canada rely on Bti-based larvicides in wetlands and stormwater areas.
A few common-sense tips still apply:
- Store dunks and granules away from children and pets
- Wash your hands after handling them
- Do not exceed the label rate, more product does not work better
- Water treated with Bti in a rain barrel is fine for ornamental gardens, but check the label before using it on edible plants
What Bti Cannot Do
This is where many homeowners get frustrated.
- It does not kill adult mosquitoes. If mosquitoes are already biting, dunks will not stop them.
- It only treats your own water. Mosquitoes fly in from neighbouring yards, ravines, creeks and stormwater ponds.
- It does not repel mosquitoes. It has no effect on where adults rest or feed.
If your yard backs onto natural water or green space, Bti in your pond will help, but it will not solve the problem on its own. That is the main reason some yards have so many more mosquitoes than others.
Bti and Professional Mosquito Control Work Best Together
The most effective approach combines both stages of the mosquito life cycle:
- Bti and source reduction stop new mosquitoes from hatching on your property
- A mosquito barrier spray targets the adults resting in your shrubs, hedges and shaded areas
On a seasonal program, our technicians check for standing water and breeding sites at every visit, every 3 to 4 weeks from May through September. If you are weighing your options, our comparison of professional mosquito control vs DIY lays out the pros and cons.
Frequently Asked Questions
Can I put a mosquito dunk in my bird bath?
You can, according to most labels, but it is easier to change bird bath water twice a week. That removes any larvae before they become adults.
Will Bti hurt the fish or frogs in my pond?
When used at the label rate, Bti is considered safe for fish, frogs and other pond life. It targets mosquito larvae specifically.
How long does it take for mosquito dunks to work?
Larvae usually die within 24 to 48 hours. You will notice fewer new mosquitoes over the following week or two, as no new adults emerge from the treated water.
Do mosquito dunks work in a rain barrel?
Yes. Rain barrels are one of the best places to use them. Better still, fit the barrel with a tight, screened lid so mosquitoes cannot reach the water.
Want help finding and treating mosquito sources on your property? Call (416) 995-3383 for a free quote on mosquito control in Toronto and the GTA.