Black-and-red bugs can suddenly turn the sunny side of a Colorado home into a moving patchwork of dark bodies and reddish markings. They may appear around window frames, crawl across glass, or cluster along siding after a stretch of cool mornings and bright afternoons. Because Colorado weather can shift quickly during fall, the change can seem almost overnight.
Those insects are often boxelder bugs (Boisea trivittata), a species closely associated with boxelder trees and certain maples. Their appearance is particularly noticeable against pale siding, stucco, brick, and brightly lit windows. Seeing dozens of them together can make homeowners wonder whether something is happening inside the walls, but their gathering usually begins with conditions outside the house.
Colorado’s intense sunshine is an important part of the story. Even when the air feels chilly, a wall or window receiving several hours of direct sunlight can become considerably warmer than a shaded surface. During autumn, that difference can create exactly the kind of warm refuge boxelder bugs seek as temperatures begin falling.
Why Boxelder Bugs Gather on Colorado Windows
Boxelder bugs are strongly influenced by temperature and sunlight. As summer gives way to fall, they become increasingly interested in places that provide warmth during cooler periods and protection from the elements…