[thinkharder]

I think the obvious big difference between microorganisms like polio or smallpox and insects like mosquitoes is that the microorganisms aren't really part of the food chain, they don't have predator species that rely on them for a food source. Mosquitoes, on the other hand, are food for many other species, birds, bats, other insects, and completely removing them from the environment, removes that food source from those other species. It's possible that the predator species would expand their use of other food sources (possibly, outcompeting other species for those sources), or that they'd continue on in reduced numbers. Or it's possible that removing mosquitoes starts a domino effect, where former predator species go extinct (and their predators do the same or switch food sources), or enter into more intensive competition for other food sources, resulting in competing species either going extinct, finding new food sources (there's the domino effect again), or reducing population numbers.

We've done it with other animals (passenger pigeon) without the world ending, but I smell unintended consequences... Something as ubiquitous as the mosquito is hooked into the ecosystem in a lot of ways.

[zaphar]

    microorganisms aren't really part of the food chain

That is not strictly speaking true. Microbes in the soil have a lot to do with the healthy/unhealthy growth of many plants. Microbes in our gut have a drastic effects on our ability to digest food. Just because we don't see them doesn't mean they don't have an effect.

I do believe however that the sheer variety speed of mutation in that portion of the food change insulates it from a lot of damage. Perhaps the same could be said for Insects as well I don't know.