Mid-August brings peak aggression from yellow jacket wasps (Vespula species) as colonies reach maximum size with hundreds to thousands of individuals. These social wasps have been building nests and raising young all summer, and by mid-August, colonies shift from rearing workers to producing next year's queens. This change in colony dynamics, combined with dwindling natural food sources and maximum population, makes yellow jackets increasingly aggressive and desperate for food. They become nuisances at picnics, garbage cans, and outdoor gatherings, attracted to sweet drinks, meats, and other human foods. While yellow jackets are beneficial predators that consume large numbers of pest insects through summer, their late-season behavior makes them unwelcome visitors. The first hard frosts will kill most of the colony, with only newly-mated queens surviving to overwinter and start new colonies next spring.

Huckleberry picking moves into its best window across the mid-elevation Cascades—this is the traditional high-point of the harvest, coinciding with late summer hiking season. First golden chanterelles appear in earnest in western Washington and Oregon Cascade foothills after late-summer rains. Plums from Yakima Valley orchards begin ripening. Columbia River fall Chinook fishing is active.