Immigrant species are bad – they have done great harm all over.
Safe rule of thumb – if people can (and do) eat the species, its ok. For example no fruit trees have become pests big time even though we import all kinds of food.

This is a comment made in response to Mark Davis‘ Sept 25 2009 article in  New Scientist, Immigrant Species Aren’t All Bad (thanks to Marco Antonio Castro Cosio for sending).