Oh, I don't think they're roosting and landing in the citrus. It sounds more like they are flying around the area, and swooping bugs in the tops of the trees. They might be roosting in the palm tree fronds, though.

Over in Phoenix proper, there is a large drainage tunnel that is a nesting roost for the Mexican freetail bat, and I understand there is a very large number of bats that roost there. There's also a large number of bat watchers that roost near by to see the bats swarm out at twilight.