Tell your friend that there are about 5,000 known species of mosquito, of which about 100 feed on mammalian blood, and out of those 9 are disease carriers. If we can target those specific 9 or some subset thereof, like Aedes aegypti as Oxitec is doing, we at least reduce the misery index while preserving 99.9% of the prey items consumed by birds, dragonflies, and other predators. Also, there's at least one study that suggests total eradication of mosquitoes would not harm the ecosystem because of the availability of substitute food sources.
