The mosquito and black fly hatch generally starts at the end of May or the beginning of June.
Black flies hatch in cold running water.
They can be especially numerous if we have unusually warm weather during spring runoff, or when heavy rains keep streams high into summer. Large numbers of black flies are usually gone by July. Some years, the island has relatively few. In a heavy year, a few may linger until frost. The first frost may come as early as the end of August or as late as the end of September.