Are Mosquito Dunk Buckets Actually Safe?

Mosquito dunks, containing the bacteria Bacillus thuringiensis israelensis (Bti), are becoming a popular way to control mosquito larvae. Female mosquitos are attracted to the standing water and lay their eggs in the bucket. The eggs hatch into larvae and the Mosquito Dunk kills the larvae. But, while many environmental organizations are telling us to go all in with this mosquito killer, it may be harming our ecosystems.

It may be tempting to spray or fog your own backyard with insecticides for mosquitos, however it is important to know that pyrethroid pesticides used to kill mosquitoes are broad spectrum and will kill any insect they contact including pollinators such as bees and butterflies. If your goal is to create wildlife habitat or grow a vegetable garden in your yard, broadcast spraying of non-selective insecticide during daylight hours will not support those goals. That’s what makes these “dunks” so attractive.

The discovery of the toxicity of Bacillus thuringiensis israelensis (Bti) to mosquito larvae in 1976 rapidly revolutionized the control of these insects all around the planet. This “bioinsecticide,” which is very selective and has low toxicity to non-target species, was nothing like the chemical products used until then. In fact, chemical insecticides generally have little or no selectivity. When the right dose is applied, they eliminate all insects.

How Does Bti Work?

Killing mosquitos in the larval stage rather than adult mosquitos is much more effective (100%) that’s why these dunk buckets can work well.

It’s important to understand the mosquito life cycle:

• All mosquitoes need water to complete their life cycle

• Some lay eggs on surface of water, eggs hatch in 24-48 hours

• Others lay eggs on sides of tree holes or discarded containers that will later fill with water.

• Larvae (called “wrigglers”) live about 7-10 days before transforming into pupa, then the adult emerges in 3-4 days

Bti often marketed as Dunks® (larvicide) are effective on mosquito and blackfly larvae, and have been recommended for use in backyard ponds or fountains where fish, frogs, tadpoles, and other aquatic animals live.

Larvicide doesn't instantly kill mosquito larvae. It's a growth inhibitor that stops the larvae from fully maturing. You'll still see them wriggle around but you won't see them turn into full on mosquitoes.

How Do You Use Bti?

You should avoid getting material from Dunks® in your eyes, and wash your hands after handling.

Homeowners can also create Larval Traps to encourage adult mosquitoes to lay eggs in water that has a larvicide in it.

One Dunk® can treat 100 square feet of surface water (10’ x 10’ area). This is equal to about 8 bathtubs, or the size of a car parking space.

Dunks can and should be broken up if the area that needs treatment is less than 100 square feet. Dunks float, and that is normal. The floating action slowly dissolves the dunk, releasing the bacteria. Since mosquito larvae filter their food in the water they also take in the particles with Bti.

Dunks begin killing mosquito larva within hours; you should start to notice a difference in about 48 hours. Dunks® last for 30 days. Reapplication may be needed throughout the mosquito season.

Impacts of Bti on Ecosystems

In recent years increasing evidence points to Bti-induced effects on the emergence dynamics of non-target dipterans (flies). Some studies have seen a 50% loss of chironomidae or midge larvae, which do not bite and others have shown declines in crustacea and dragonflies.

Midges are important becasue they emerge as adults in large numbers and are food for many animals nearby. Chironomids are very abundant and a significant source of food for many aquatic and terrestrial animals such as dragonflies, predaceous diving beetles, amphibians, fish, shrews, spiders, birds, and bats. In spite of the additives, Bti spores and toxins inevitably end up on the bottom of marshes where they have a particularly strong impact on the chironomids that feed there. They also live in the mud and help filter the water.

A study on House Martins in the Camargue showed that midges and mosquitoes are a major source of food given to nestlings. When this prey disappears the birds are fed harder to digest foods like ants.

Bti may impact other species durectly and indirectly. A study in the journal Environmental Science & Technology, revealed that VectoBac (Bti) may affect frog metamorphosis — the transition from tadpole to young frog. In wood frogs and American toads, VectoBac altered the time it took a tadpole to metamorphose, either delaying metamorphosis by nearly five days or advancing it by one day, depending on the treatment. Studies on frog ecology have established that early metamorphosis reduces a frog’s chance of survival, which could ultimately reduce population sizes.

Lastly Bti can last for multiple years in the environment and build up over time. In one study, Bti was present at concentrations 22 to 500 times higher in the mosquito-control sites compared to the control sites.

Safer Alternatives to Bti

The most effective way to control mosquitoes in your yard is to be vigilant about water sources. The reason you have mosquitoes coming to your dunk bucket is because you have another source of water nearby.

Homeowners can control mosquitoes in their own backyard by eliminating breeding sites for mosquitoes. “Tip and toss” any standing water where mosquitos can lay eggs. Birdbaths, buckets, and plant saucers are common culprits. Mosquitoes can breed in small pockets of water—as small as a bottle cap! To minimize mosquito breeding:

Before using Dunks, look at your property for areas where there is standing water, or where water collects after rain. These are places where mosquitoes, including mosquitoes such as Aedes albopictus, may lay their eggs.

If at all possible, prevent these areas from collecting water: empty and turn over buckets, unused flower pots, canoes and wheelbarrows. Clean clogged gutters so water flows freely through them. Place lids on outdoor trash cans.

What to Know if You Must Use Bti

Once these areas have been addressed, Dunks can be used on areas where water collects and cannot be removed or drained. These include pool covers, ornamental ponds, fountains and rain barrels.

You don’t need much water in a bucket to capture mosquitoes - most of their eggs can survive without water for years!

If you add more than a few inches of water add a ramp for amphibians to escape and/or put chicken wire on top to prevent other animals and pollinators from getting inside.

Don’t add Bti to water bodies that contain water all year where possible as the Bti can accumulate and potentially have greater effects on other species.