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.