Sudden-Death Mosquitoes
Some mosquitoes were engineered as a way to fight malaria. This disease causes a million deaths annually as well as infecting additional 300 million people. These malaria fighting mosquitoes are able to resist the plasmodium parasite, which means that it is almost impossible for them to become infected with the disease. The thing, however, is that plasmodium parasites can evolve quickly, leading to some people wondering if we would be better off by killing mosquitoes.
To deal with this option, some scientists created sudden-death mosquitos which pass the relevant gene to their offspring. This gene means that the baby mosquitoes would die (naturally of old age) before reaching sexual maturity. The issue, however, is that without mosquitoes, the entire ecosystem would be affected with facing extinction of bats and other predators.