Sept 8 (Reuters) - Wall Street"s main indexes opened flat on Wednesday on concerns that the spread of the Delta coronavirus variant could slow economic growth and on uncertainty over the timeline for the Federal Reserve to pull back its accommodative policies. The Dow Jones Industrial Average (.DJI) fell 12.55 points, or 0.04%, at the open to 35,087.45. The S&P 500 (.SPX) opened lower by 1.94 points, or 0.04%, at 4,518.09, while the Nasdaq Composite (.IXIC) dropped 13.98 points, or 0.09%, to 15,360.35 at the opening bell. (Corrects headline to say "opens" from "set to open")