I am encouraged by the stupidness of those aliens. We have been successfully defeating mosquitoes for a century. Humans have spectacular antimosquito tech. The USA eradicated malaria in the space of 4 years once it was decided to do that. The same is true for Europe.
These efforts used a lot of DDT (which still works great!), but 50 years prior, a dedicated mosquito control effort without DDT allowed the building of the Panama Canal.
http://ocp.hul.harvard.edu/contagion/panamacanal.html
In constructing the Panama Canal, American planners and builders faced
challenges that went far beyond politics and engineering. The deadly
endemic diseases of yellow fever and malaria were dangerous obstacles
that had already defeated French efforts to construct a Panama Canal
in the 1880s. The crippling effects of these diseases, which
incapacitated many workers and caused at least 20,000 to die, led the
French to abandon their goal in 1889.
For the later American effort, William Crawford Gorgas was appointed
chief sanitary officer. His task was to prevent yellow fever and
malaria infection among the laborers—a task that proved critical to
American success.
We have even more tools now. Spray trucks and planes reduce mosquitoes to limit diseases like West Nile. Methoprene can be added as briquettes to water where mosquitoes breed, interefering with hormones and trapping them as juveniles - which is nice because fish eat them.
The problem with mosquitoes is now a third world problem, and as with most third world problems, really a problem of poverty and chaos. If mosquitoes were the advance guard of an alien invasion, there would be no shortage of money and political will. Concern about pelican eggs and wetland health would fall by the wayside. Those mosquitoes would be toast.