A mosquito dunk keeps a rain barrel free of mosquito larvae for 30 days or more, and it is replaced every 30 days or as soon as it has fully dissolved, whichever comes first. Rain barrels are among the standing water sites the dunks are made for, and a barrel's small surface needs only part of a dunk.

Why does a dunk keep working for a month?

Because it releases its Bti slowly. The dunk floats at the surface and dissolves gradually, and the Bti, a naturally occurring bacterium that kills mosquito larvae as they feed, disperses through the water for at least 30 days.

How much of a dunk does a rain barrel need?

Usually only part of one. A whole dunk treats up to 100 square feet of surface, far more than the top of a rain barrel, and the dunks can be broken into smaller pieces for smaller areas. Breaking a piece off one dunk from a Mosquito Dunks 6 Pack and floating it in the barrel treats the water, with the rest of the pack kept for the months ahead.

Is rain barrel water still usable after a dunk is added?

For plants and animals, yes. The dunks are described as harmless to plants and can be used in drinking water for animals, though watering vegetables with treated water is not covered in the dunks' directions.

Frequently Asked Questions

Does heavy rain wash a dunk out of the barrel?

The directions do not say. Because the dunk floats, a quick check after a storm confirms it is still in the barrel.

Does a tall barrel need more than one dunk?

No. Each dunk works regardless of water depth, because its Bti disperses throughout the water.

Can one dunk be split between two barrels?

Yes. Pieces can be broken off for smaller areas, so a single dunk can be divided between barrels.

Does a dunk kill adult mosquitoes around the barrel?

No. It targets the larvae in the water, not mosquitoes that are already flying.