Mosquito Dunks kill mosquito larvae where they breed before thy become a nuisance Place one of these donut-like mosquito larvacide "dunks" in standing water where mosquitoes breed: bird baths, rain barrels, ponds, pool covers, clogged gutters, flower pot dishes, tree holes. Mosquito dunks are each made of a natural, organic, non-toxic agent that kills mosquito larvae, but is harmless to humans, fish, birds, pets or vegetation. This mosquito larvacide with BTI is effective beyond a mosquito generation (approximately one to two weeks). Each of these mosquito dunks controls 100 sq. ft. of water surface for 30 days. You get 20 mosquito dunks per card.