While the contrast of their bright colors is alarming, according to Tim Johnson, the director of horticulture for the Chicago Botanic Garden in Glencoe, IL, boxelder bugs are pretty harmless to plants and to property. Common in the midwest, Chicago exterminators don't need to respond to too many calls for these little critters. Although they are unsightly, they only are known to damage property by staining walls with their excrement (eew). They feed on ash, maple, and boxelder trees, but they don't cause significant damage to the trees. Nobody, however, likes to see any kind of bugs swarming in his house. Boxelder bugs do migrate inside when the weather turns cold. Prevent them from entering your house by sealing up structural openings.