Don't confuse the "grass" or yard mosquito with the type that likes blood. The grass mosquito is harmelss to animals (including humans).
Its unusual to have that many of the other type around unless you live near water. Of course, with all the rain some parts of the country has been getting, its easy for there to be enough standing water around to create a small infestation. It doesn't take much water for mosquitos to breed.
I don't know how effective those devices are. But I won't spend my money on one.