Mosquitoes are probably one of the most annoying things in today's
modern world, and I bet we've all
wished at one time or another that we could wipe them out altogether.
Unfortunately, although it would prove very nice for a while, our ecosystem is so fragile, by wiping out an entire species of insects would set of a chain reaction of extinctions which would most likely lead up to our extinction.
For example, one case of near extinction was when the Sea Otter was hunted for its pelt. Sea Otters ate Sea Urchins (its amazing how they ate them. They would balance a rock on their stomach then crack them open), and Sea Urchins ate kelp, and so did alot of other species of sea creatures, and alot of sea predators ate them. So when there were less Sea Otters, there were more Sea Urchins, and less Kelp, which meant less of the kelp eating fish, and less of the predators, and eventually less of the Sea Urchins, but still not enough kelp. Confusing, but it almost lead to several extinctions.
Moral of the story, don't wish for extinctions, they can only bring bad tidings.