By Gabriela Baczynska BRUSSELS (Reuters) - Migrants arriving by sea to Italy are now the main problem faced by the European Union in its efforts to stem mass immigration to the bloc, the head of the EU border agency said on Wednesday. Italy has now overtaken Greece as the main point of entry for migrants trying to reach Europe, figures from the U.N. refugee agency (UNHCR) show. More than 144,000 migrants have reached Italy so far this year, compared to the nearly 154,000 arrivals in the whole of 2015.