Thing is, there is too many cows because of current livestock farmings.
We should reintroduce big lands almost free from human action. Nature is balancing itself with autoregulation. Have a look at what happened when they reintroduced wolves in Yellowstone park :
https://www.yellowstonepark.com/things-to-do/wolf-reintroduction-changes-ecosystem
About mosquitoes, I agree they help to spread diseases. But extermination is not a solution because they have an important role by cleaning up aquatic ecosystem.
It's very complex but humanity has caused a lot of damages and the consequences could be very dramatic. If you look around there is no other species than ours which endangers the ecosystem on which it lives.