I saw very different works during my researches. For some people, a species can be considered exotic in Europe when its native range is quite far from the continent, i.e. North America. Others prefer to consider it exotic when the native range falls in a different biogeographical region, i.e. a species from the Mediterranean Basin found in Germany.
If we are talking about phytophagous arthropods, things get worse, because their movement usually depends on their host plants, and therefore, on their introduction in different areas for gardening and horticulture purposes.