Believe it not, micro-bats can eat up to 300 hundred bugs an hour! These bugs include mosquitoes, moths, locus, grasshoppers, etc. Such bugs can destroy crops and spread disease. American farmer's biggest pest is the Corn earworm moth. One bat can eat 20 female moths a night reducing the number of crop eating caterpillars.
If you are out in the evening, you have experienced the nuisance of mosquitoes. I'll bet you wished there was a few bats around to eat them all up.