Adults hibernate during the winter in crevices of trees and buildings. They may appear in swarms on sunny winter days. In spring, females lay eggs in cracks in tree bark. Eggs hatch in 10 to 14 days. Nymphs feed on flowers, fruits, foliage and tender twigs. Adults may migrate to orchards in late summer, shortly before fruit matures. There is one generation each year in the Northwest.
Adults suck juices from fruit as they feed, causing dimples and deformations. The flesh of fruit where the bug has fed is corky and white. The damage looks similar to late season stink bug damage.
Potential damage in orchards cannot be determined by monitoring populations on host plants. Large numbers of western boxelder bugs may be found, but they do not always move into orchards and feed on fruit. A beating tray can be used to monitor adults in the orchard. However, this is not a practical method in late summer because too many fruit are dislodged. Visual observations are probably the best method. Adults are very easy to detect if they are in the orchard in large numbers.
Problems with western boxelder bug are sporadic but can be persistent and annual in some locations. Most orchards are not threatened by this pest. When populations are high on native hosts that dry out in late summer, adults will migrate in large numbers to orchards in search of food. Many insecticides will kill adults, but repeated applications may be required to protect fruit if they continue to immigrate from outside sources. Removal of hosts plants near the orchard may help reduce problems in orchards that suffer annual damage.