Boxelder bugs (Boisea trivittata) are commonly found trying to enter houses, especially if you have a boxelder tree nearby. These insects are about ½” long and black with orange or red markings. During the spring and summer, the adults will feed on leaves and seeds of boxelder trees (and occasionally maple and ash trees). When temperatures begin to cool in the fall, they will start to seek out warmer areas. They are most often attracted to buildings with large southern or western exposures because they are warmer than the surrounding area. As they aggregate on buildings, they will find cracks and other spaces to squeeze into. They will occasionally make their way indoors often end up in walls and attics until warm weather returns.
Boxelder Bug Habit
There are two generations per year with 2nd generation adults overwintering in protected sites, including buildings. During the summer, these bugs live on boxelder trees, where they feed on seeds found on the tree and on the ground. When cool fall weather arrives, they migrate to buildings for protection. They cluster on the sides of the house. They crawl into cracks and crevices and eventually get into the walls. On warm days in winter, they can be found, often in large numbers, on the south and west sides of the house in the sun. They may also move into the house interior at the same time.
Boxelder Bug Damage
During the fall and winter they can be annoying in the house. Boxelder bugs do not feed on food or clothing nor reproduce in the house. They may spot curtains and wallpaper with their fecal material. Also, they will leave a red or purple stain if smashed.
Boxelder Bug Control
Nonchemical: If found indoors, remove by vacuuming. Remember not to crush them, or they will leave a red stain. Caulk all cracks and crevices to reduce their chances of entering the home. They may still enter through doorways. Eliminating seed-bearing boxelder trees may help reduce the boxelder bug problem
Chemical: Contact your county Extension office for current pesticide controls.
Read more from Ken Johnson on the Good Growing blog: Fall House Guests