Unfortunately wiki says "Indiscriminate eradication of mosquitoes is likely to have effects undesirable to humans. Entomologist Phil Lounibos of Florida Medical Entomological Laboratory (FMEL), Institute of Food and Agricultural Sciences (IFAS), University of Florida, says that eradication "is fraught with undesirable side effects", as mosquitoes are a significant food source for birds and bats and, as larvae, fish and frogs, parts of the food chain affecting many species. If mosquitoes are eradicated, they may also be replaced by other species, possibly more undesirable."

I really wonder what could be more undesirable though.