Not sure about that, but there was an account of something similar in India, I believe, where there was a chemical agent involved that buzzards were extremely susceptible to and wound up killing off most, if not all of the population. This led to carcasses being left to rot and then became vectors for disease and basically led to a chain reaction of events that caused a few million people to die.

I don't know if killing off mosquitos would have quite as a profound effect as this, but there are so many things I feel like we have almost no understanding of when it comes to the natural world to say either way.

I've read in a few comments here that there are studies saying that it wouldn't have a crazy effect - if anyone could link them so I can give them a read, I would appreciate it.