Boxelder bugs are a nuisance when they gather in mass on the warm southern side of homes and trees, and they become even more annoying when they make their way inside. Outside, spray them heavily with soapy water (5 tablespoons of liquid detergent in one gallon of water) until they are thoroughly wet. Another option is to use an insecticide labeled for use against boxelder bugs. Adult boxelder bugs are not easily killed, so repeated applications may be necessary. Spraying will reduce the population substantially. To prevent them from entering your home, caulk or seal cracks where they may enter. If you find strays indoors, vacuum them up.