Explanation:
Larval mosquitoes breath through trachea in their siphons. This is a structure analogous to the snorkel on a diesel electric submarine. It allows for gas exchange with the atmosphere while the remainder of the insect is submerged. The opening of the siphon is hydrophobic so it won’t get wet and blocked by water. This works well to keep the siphon and the trachea open under normal conditions. Oil when poured on water forms a thin film. When there are mosquitos in that water, when their siphons contact the oil layer, the oil wets and blocks their siphons and suffocates the mosquitoes. This works against most, but not all mossies, as evolution is an amazing process. Some mosquitoes (Mansonia, Coquilletidia) have a siphon designed to penetrate the air vessels in aquatic plants and they don’t need to come to the surface to breath like other mossies. So oil won’t work on these genera.