Boxelder bugs are bright red or black with narrow reddish lines on the back. They feed principally by sucking juices from the Boxelder tree, but are sometimes found on other plants. Boxelder bugs build up a large population and then invade a home. They are usually pests only by their presence, although their piercing-sucking mouthparts can sometimes puncture skin and cause irritation. Boxelder bugs do very little damage to the trees they attack. Adult Boxelder bugs will enter structures in the fall, seeking winter shelter. Once Boxelder bugs have invaded a home, there aren't many treatment options. The trusty vacuum cleaner is the best option by that point. It is important to consider treatment before this happens.
To be effective, barrier treatments should be applied before the bugs enter buildings over winter. In Wisconsin, the proper timing for such treatments is typically late-September thru the end of October although the optimal period will vary with seasonal conditions.