Mosquitoes begin their lives in standing water bodies. Female mosquitoes lay their eggs in the water, and the mosquitoes remain there while they progress through the larval and pupal stages. Finding water on your property infested with mosquito eggs or larvae might seem like a bad thing, but it allows you to take care of the problem before it devolves into something far worse. It’s much easier to deal with larvae and pupa than adult mosquitoes.
There are simple, DIY solutions for killing mosquitoes while they’re still in the water. One of the most effective is washing up liquid. Adding just a tiny amount of the stuff will create a layer on top of the water. This surface layer prevents the young mosquitoes from breathing the air above and effectively suffocates them. A single drop of washing up liquid is enough for a gallon of water. You only need to use a minimal amount. If you don’t have any washing up liquid on hand, you can achieve the same thing using cooking oil. If you’re going to use either of these methods, be aware that the liquid layer you create will prevent oxygen exchange between the water and the atmosphere. This will prevent any fish and other creatures that also live in the water from breathing.
The above method is effective but not perfect; some mosquitoes will probably survive. On the other hand, larvicides are potent chemicals that are more or less guaranteed to kill all the young mosquitoes. However, larvicides are also toxic to humans. Don’t use larvicides in water destined for human consumption.
Finally, if the water body is small enough, you can deal with the problem by simply scooping the water up and dumping it on the ground.