Uhm....I think it also has to do with the climate. Here in The Netherlands I've never seen a single cookroach. Whereas in Australia you nearly trip over the damn things. Spotted them in Corsica too. I usually take my holidays in Scandinavia and I haven't seen them there either. That would suggest they only really thrive in warm(er) climates?
BTW I have a big insect/roach-phobia so it's for the better I don't see them in middle/northern Europe ;-).
