Without quantification, this position is basically unfalsifiable. Plus, there's a trivial contrapositive that is just as valid: "what if allowing mosquitoes to live causes an outbreak of a new disease that is transmitted by mosquitoes and kills billions?". Sure, that could also happen. As with total food-chain collapse from reduction of population of a non-keystone species, it seems very unlikely. Contrast that against the almost-certain probability that 0.5m will die next year (0.1 Holocausts per year, or one Holocaust per decade) from malaria.

If we don't attach a probability to these outcomes there's no way of having any sort of meaningful conversation about them.

edit: Seriously, wouldn't it be best to solve existing problems with direct threat to food production than to go off tinkering with new ideas and unknown side-effects?

Their presence in the US is itself an example of manipulating the environment to our own ends.