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For most rainwater tanks, the best mosquito control is not a chemical. Keep mosquitoes from reaching the water with a tight lid, screened openings, and protected overflow pipes. If mosquitoes are already breeding in a tank that holds non-potable water, a mosquito larvicide containing Bti may be an option if its label specifically allows use in rain barrels or water-storage containers.
Bti stands for Bacillus thuringiensis israelensis. It is a bacterium used to kill mosquito larvae before they become adults. CDC lists rain barrels among the places where homeowners may use labeled larvicides.
Do not simply add bleach, household insecticide, oil, or another chemical to a rainwater tank. What is safe depends on what the water will be used for.
The Best Way to Stop Mosquitoes in a Rainwater Tank
Start by stopping adult mosquitoes from reaching the stored water.
CDC recommends tightly covering cisterns, rain barrels, and other water-storage containers. If an opening cannot be closed with a solid lid, cover it with mesh that has holes smaller than an adult mosquito.
Check these common entry points:
- Tank lid or access hatch
- Downspout inlet
- Overflow pipe
- Vent
- Loose pipe connections
- Gaps around filters or diverters
A tank can look closed while still having an open overflow or vent large enough for mosquitoes to enter.
For small rain barrels that can be emptied, CDC also recommends emptying them at least every 10 days if they cannot be effectively screened.
Large tanks and cisterns normally cannot be emptied that often, so good screening becomes much more important.
What Can You Put in the Tank?
Bti Larvicide for Non-Potable Rainwater
For water that will not be used for drinking, a properly labeled Bti mosquito larvicide is one of the more practical options.
Bti products may come as:
- Dunks
- Tablets
- Briquettes
- Bits or granules
- Pellets
The larvae eat the Bti, which prevents them from developing into adult mosquitoes. It does not work as a repellent and does not kill adult mosquitoes flying around the tank.
EPA says Bti is widely used for mosquito control and can be applied to standing water. However, the directions differ between products, so the label determines where and how a specific product can be used.
Do not guess the amount based only on tank size. Use the dose and replacement interval printed on the product label.
If You Use the Water for Drinking
Be much more careful.
Do not assume that a product described as "natural," "biological," or "safe around water" can be added to drinking water.
EPA notes that some Bti products are allowed for certain drinking-water applications, such as some cistern uses, while other Bti products are not. The individual product label matters.
For a tank supplying drinking, cooking, or other potable uses, the safer approach is to correct the mosquito entry problem rather than routinely adding a larvicide.
Potable means water suitable for drinking. A rainwater system intended to produce potable water requires more than mosquito control. Collection surfaces, debris control, treatment, maintenance, laboratory testing, and local health requirements all matter. CDC recommends regular testing for germs and chemicals when collected rainwater is used for drinking, cooking, or bathing.
Things You Should Not Casually Add
Several home remedies are suggested for mosquito-filled rain barrels. They are not good general solutions for a rainwater storage system.
Bleach
Do not pour an arbitrary amount of household bleach into a tank just to kill mosquitoes.
Chlorine can be part of a properly designed water-treatment process, but correct treatment depends on chlorine concentration, water condition, contact time, intended use, and other factors. Random dosing is not a substitute for fixing an open tank.
Cooking Oil or Other Oils
You may see advice to put a layer of oil on standing water.
Avoid doing this in a rainwater storage tank. Oil can contaminate the stored water, coat tank surfaces, foul equipment, and create problems if the water feeds irrigation lines, pumps, filters, or other equipment.
Household Insect Spray
Never spray a general household insecticide into stored rainwater unless that exact use is specifically permitted by its label.
Products designed for walls, gardens, or flying insects are not automatically suitable for use in stored water.
Detergent or Soap
Soap may affect insects in small containers, but it is not a suitable mosquito-control treatment for a rainwater tank. It can contaminate the water and cause problems for plants, pumps, and downstream uses.
If Mosquito Larvae Are Already in the Tank
Guidance on rain-barrel mosquito-larvae controls helps evaluate tank materials suited to the water’s intended use.
Small mosquito larvae often appear as tiny moving or "wriggling" shapes near the water surface.
If you find them, first work out how adult mosquitoes reached the water.
Inspect the lid, inlet, vent, and overflow. Repair loose screens and seal gaps that should not be open.
For a small non-potable rain barrel, you may be able to empty and clean it. CDC recommends regularly emptying and scrubbing water-holding containers because mosquito eggs can remain attached to their surfaces.
For a large cistern or tank, draining hundreds or thousands of gallons may not be practical. A labeled larvicide may make more sense if the stored water is non-potable.
Do not climb inside a tank to clean it. Tanks and cisterns can present confined-space, fall, and air-quality hazards. Cleaning that requires entering the tank should be handled by someone trained and equipped for that work.
Check the Overflow Pipe
The overflow is one of the easiest mosquito entry points to miss.
When a tank fills, extra water leaves through this pipe. Because the pipe needs to stay open for water flow, you cannot simply cap it.
Instead, use a suitable insect screen or another mosquito-resistant overflow fitting that still allows the tank to discharge water freely.
Keep the screen clean. Leaves, algae, sediment, and insects can clog fine mesh. A blocked overflow can cause water to back up or escape somewhere it should not.
The same idea applies to tank vents. They must still perform their intended job while keeping insects out.
Screen the Water Inlet
Water entering from the roof can also provide an opening.
A screened inlet helps keep mosquitoes and larger debris out of the tank. The screen should fit securely without creating an easy bypass around its edges.
Roof runoff can carry leaves, dirt, bird droppings, insects, and other material. Keeping this material out also makes tank maintenance easier.
A first-flush diverter can help with water quality as well. It sends the first portion of runoff from a storm away from the storage tank because that early runoff often carries more contaminants from the roof. It is useful for rainwater management, but it does not replace mosquito screens.
Do You Need to Treat the Whole Tank Forever?
Usually not.
Larvicide can control larvae, but the long-term solution is preventing mosquitoes from getting inside.
A well-protected tank should have:
- A secure lid or access hatch.
- A screened inlet.
- A protected overflow.
- A screened vent where needed.
- No unexplained gaps around pipes or fittings.
Inspect these parts from time to time, especially after storms, maintenance work, or moving pipes.
Screens can tear. Lids can shift. Overflow fittings can loosen. A small opening may be enough for mosquitoes to turn stored water into a breeding site.
Frequently Asked Questions
Can I put mosquito dunks in my rainwater tank?
For non-potable rainwater, a mosquito dunk containing Bti may be appropriate if its product label specifically allows use in rain barrels, tanks, or similar water containers. Follow the label for dose and replacement timing. For drinking-water systems, do not use a product unless its label specifically permits that use.
Is Bti safe for rain barrels?
EPA and CDC recognize Bti as a mosquito larvicide for suitable standing-water applications. However, individual product labels differ. Some products allow uses that others do not, so always follow the label for your specific product.
Can I put bleach in my rain barrel to kill mosquitoes?
Do not use an improvised bleach dose as a mosquito-control method. Prevent mosquito access with screens and a secure cover. If non-potable standing water cannot be emptied or protected, use an appropriately labeled mosquito larvicide instead.
How do mosquitoes get into a closed rainwater tank?
Common entry points include uncovered overflow pipes, vents, downspout openings, loose lids, damaged screens, and gaps around fittings. Inspect every opening connected to the tank.
Will a screen stop mosquitoes?
A properly fitted screen can greatly reduce mosquito access. CDC recommends mesh with openings smaller than an adult mosquito for water-storage containers that cannot be fitted with solid lids.
How often should I empty a rain barrel to prevent mosquitoes?
CDC advises emptying a rain barrel at least every 10 days if mosquito access cannot otherwise be prevented. A securely screened and covered storage system is usually more practical when you need to keep the collected water.
Can I use rainwater containing mosquito larvae on my garden?
Mosquito larvae themselves do not mean the water should simply be left untreated as a breeding site. Fix the opening that allowed mosquitoes into the tank. If treatment is needed, choose a larvicide whose label allows use in water intended for irrigation and follow its directions.
