Sept 20 (Reuters) - Wall Street"s main indexes tumbled at the open on Monday, as concerns about the pace of an economic recovery hit energy and banking shares at the start of a week in which the Federal Reserve will decide on potentially tapering its pandemic-era stimulus. The Dow Jones Industrial Average (.DJI) fell 125.16 points, or 0.36%, at the open to 34,459.72. The S&P 500 (.SPX) opened lower by 30.04 points, or 0.68%, at 4,402.95, while the Nasdaq Composite (.IXIC) dropped 285.83 points, or 1.90%, to 14,758.14 at the opening bell.