Boxelder bugs, found east of the Rocky Mountains in North America, feed primarily on the seeds of boxelder trees. They can also be found on ashes and certain species of maples. "These bugs are completely harmless to humans," says Scott Creary, an entomologist with Phipps Conservatory and Botanical Garden in Pittsburgh, Pennsylvania. "They do very little damage to the plants they feed on. They are more of a nuisance than a pest."
Much like the Asian lady beetle and the brown marmorated stinkbug, boxelder bugs congregate on sunny south-facing walls and enter houses through crevices, seeking a warm place to spend the winter. They tend to gather indoors for a few weeks in autumn, especially following hot, dry summers, when their population is often greater.
Though their gregarious habits might seem troublesome, there is no cause for alarm. "They couldn't hurt you even if they wanted too," says Creary. "If you don't like them, you can just pick or sweep them up and move them back outside."
Boxelder bugs feed by piercing the seed coat, injecting it with digestive enzymes, and then feeding on the liquefied seed. Occasionally, these insects will supplement their diet with the sugars and moisture of developing fruit, causing a mild deformation called catfacing. Not to worry, though, says Creary. Their population fluctuates with changes in environmental conditions, the growing season, and natural predator populations. Some years there are lots of them, while other years there's nary a bug to be found.
Boxelder bugs are related to stinkbugs, but are members of the "scentless plant bug" family and don't have stink glands. They do have defensive chemicals that are released from their leg joints if crushed. So as you collect any that manage to make it indoors, take care not to stain walls, carpets, or curtains with these excretions.
Oh, and one last thing: Even if you don't have a boxelder tree on your property, your house can still play host to the bugs' autumn gatherings. They will travel up to a mile from a host tree to find a suitable wintering site. Lucky you.
Photo: Joseph Berger, Bugwood.org