A device to kill harmful insects, such as mosquitoes and similar, comprising two or more grids (6A, 6B, 6C) having shaped openings and at least one being electrically charged, applied to one or more surfaces of an object; the grids (6A, 6B, 6C) are applied, at least partially, at at least one wall (9A, 9B, 9C) of a space (8A) of a building or of a cage (7A, 7B, 7C) where it is present at least one human being or animal (8), while at the remaining surfaces of the building or cage (7A, 7B, 7C) a structure is present that does not allow passing through of insects, such as a structure of glass, plastic, wood, masonry and/or mosquito-nets (131).