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.
Mosquitoes usually get off to a quick start and then slowly taper down as the summer dries out.
In a rainy summer, they will stay at high numbers. At least some mosquitoes will hang on until a heavy frost finally ends their season. Insects are part of the natural scene at Isle Royale and they can be annoying. The number of bugs and when they arrive varies considerably from year to year. Insect numbers also vary from one location to another on the Island due to micro-climate effects and from day to day due to weather changes.