Allegiant Air has announced twice-weekly nonstop flights between Des Moines International Airport and Destin-Fort Walton Beach airport, Fla., beginning in May. In addition, Des Moines airport officials announced that Southwest Airlines will begin its seasonal service to Phoenix on Aug. 1, earlier than usual. On the downside, the Des Moines Register reports that Frontier Airlines will end its flights between Des Moines and San Francisco, leaving the Des Moines airport without direct service to that market. Des Moines airport officials said they had not been notified of that change.