Mosquito Dunks eliminate mosquito larvae before they become biting adults. Each donut-shaped tablet treats up to 100 square feet of standing water for up to 30 days. Safe for fish, birds, and beneficial insects—contains only Bacillus thuringiensis israelensis (Bti), a naturally occurring soil bacterium. Simply float or place in birdbaths, rain barrels, ponds, and other water sources. This 6-pack provides season-long protection for multiple areas around your yard or garden. No chemicals, no mess, no odor.