Trial wipes out over 80% of Australian disease-spreading mosquito
It is theorized there is no adverse effect from killing mosquito species.  There is always a counter-effect.  Typically it's unpredictable.  Mosquitoes FACILITATE transmission of bad stuff.  I believe that if bad stuff is not transmitted, species evolve more slowly against pathogens.  Species will still be exposed, maybe generations later through a scratch or otherwise.  At that point, having never evolved a strong immune system the same animals will be less prepared.  It's not whether the American Indians were to die from European diseases, but when.