Some time ago, I wrote about what humans consider to be invasive species vs. the useful ones. Useful to humans, obviously.
As a friend pointed out, a lot of people have exactly the same attitude to migrant workers once they are not needed any longer. When they were invited, the host nation received them quite pleased and congratulated itself for having them – who else would do the work? – but then they dared to stay, to bring their wives, to have children. Not to mention that they had their own language, culture or even a different religion.