When I did my Long Trail End-to-End hike in 1997, I started on June 27. The trail was generally in great shape, and the black flies were only bad near Goddard Shelter early in the hike. Mosquitos were annoying the first week, but after that were not bad at all. It obviously depends on the weather. However, in late May the trail is more likely to be quite wet, and I wouldn’t have wanted to be hiking the trail this past May (2006). I also find black flies more annoying than mosquitos, so a May start in any year would be problematical for me. Also, it is true that the trail is officially closed at high elevations until Memorial Day to help prevent serious erosion problems. If you want the best chance for good weather and few bugs, an August 1 start is a good choice. And one of these years I’m going to do the trail in October. No bugs, no crowds, cool weather, great leaf peeping, etc. But whenever you go, have a great hike!
Professor T