Mosquito dunks vs bits

Same active, different delivery

Both products contain BTI — Bacillus thuringiensis subspecies israelensis, a naturally occurring soil bacterium that produces a protein toxic to the larvae of mosquitoes, blackflies and fungus gnats, and to essentially nothing else.

A mosquito larva eats it, the protein disrupts its gut, and it dies. Fish, birds, pets, people, bees and beneficial insects are unaffected — the EPA's assessment of BTI for mosquito control is linked at the foot of this page.

This matters because it makes BTI the rare pest product with almost no trade-off: it attacks the stage of the life cycle that actually determines how many mosquitoes you get, and it does so without collateral damage.

The difference is release rate

Dunks are compressed donuts. They float or sit submerged and release BTI slowly over about thirty days. One dunk treats roughly 100 square feet of surface water regardless of depth.

Bits are granules. They release rapidly — most of the effect within about 24 hours — and last around seven to fourteen days. They can be scattered across a surface, mixed into wet soil, or steeped in water.

Everything else follows from that.

Which to use where

A rain barrel, a water butt, an ornamental pond margin, a livestock trough, a blocked ditch → dunks. Stable water, set it and replace monthly.

A bog garden, a tarp fold, a tree hole, a plant saucer, a shallow depression that fills after rain → bits. A dunk cannot sit properly in two centimeters of water; granules can be scattered anywhere.

An active bloom you want knocked down now → bits first for speed, then a dunk to hold it.

Fungus gnats in houseplants → bits, and only bits. Sprinkle on the soil or steep in water and use it to water the plants. This is the crossover that makes bits worth keeping in the cupboard year-round — fungus gnats in houseplants covers the method.

A septic or gray water area, or a French drain → follow the label, which will specify.

Where to actually apply it

The point of BTI is to treat the water mosquitoes are breeding in, which means you have to find it. Walk the yard after rain:

• Plant saucers and pot trays
• Blocked gutters and downspout splash blocks
• Tarps, covers and anything with a fold
• Bird baths and pet water bowls
• Buckets, watering cans, wheelbarrows, children's toys
• Tires, including a spare left outdoors
• Corrugated extensions on downspouts, which hold water by design
• Tree holes
• Low spots that hold water for more than a few days
• Rain barrels — screen the top as well

Tip out what you can. Emptying a container beats treating it, every time. BTI is for the water you cannot remove.

Timing and repetition

• Start in spring, before the season, not after the first bites.
• Dunks: replace roughly every 30 days through the season, and after any flood event that flushes the water.
• Bits: reapply every one to two weeks while the water is present, and after heavy rain.
• Keep treating until the water is gone or the season ends. A gap of three weeks is a full generation.

What it does not do

Nothing to adult mosquitoes. The ones biting you tonight are unaffected. BTI lowers the next generation, which means the benefit shows up in one to two weeks, not the same evening.

That is why the honest yard plan pairs it with something that works tonight — a repellent zone — rather than expecting a larvicide to do a repellent's job. The full plan is in how to get rid of mosquitoes in your yard.

It will not cover a neighbor's property. If the breeding source is off-site, treat what is yours and protect the space you use.