Most mosquito species don't pray on humans. For anti-disease and anti-annoyance practice we would only need to kill those mosquito species that bite humans.
In additional humanity likely killed a few species with DDT that was used against mosquitos. I think it's plausible that the world loses less species if we would remove the mosquitos that bite humans.
I've been wondering about good new topics for LW, and prediction might be one of them.
The effect of the Zika virus-- and human reactions to it-- on birth rates has the combination of being hard enough to be interesting, not being heavily plowed over by partisans, and having a quantitative outcome.
There's a lot of evidence that Zika causes microcephaly, but this isn't confirmed. There's also some reason to think it increases the rate of miscarriages.
Human reactions cover a wide range, including trying to wipe out the mosquitoes, increasing access to birth control, abortions, asking people to put off having children, creating a less-mosquito-friendly environment....
My assumption is that zika will cease to be a serious problem in not too many years, as more women get the disease and acquire immunity before their child-bearing years, but admittedly, this is assuming that zika (or some other disease with a similar infection pattern) is the problem.
Any other good prediction questions?