Well, mosquitos are a natural part of ecology on the planet. They've been around for long before humans. As with most other animals, mosquitos are predator to other living organisms, and that helps to keep the prey population in balance. Humans are part of their prey, and they're evolutionarily good at scenting humans out. Nobody questions our ability to wipe out human "predators" like mosquitoes, just that there is obviously a cascading ecological effect since it's not just humans they go after. On the topic of leeches to the planet: let's present the elephant in the room, the Human.