Boxelder bugs, sometimes called maple bugs, are relatively large insects, about 1/2” long, dark gray in color with red markings on their backs. The bugs are harmless. What makes them remarkable is their tendency to congregate on certain houses in the fall of the year, often in enormous numbers. It truly seems like an invasion - especially the first time it happens.
Here's what's going on. Boxelder bugs (see a color image at www.LivingWithBugs.com/boxelder.html) have one generation a year. Eggs are laid in early summer on maple trees (boxelder is a type of maple). Eggs hatch and the young nymphs feed on maple tree leaves during the summer. They do no apparent harm to the trees. The insects reach adulthood in fall and can fly. Adults abandon the trees on which they developed in search of a place to “overwinter”. Many insects overwinter. It is not really a state of hibernation, or diapause, but rather simply an inactive stage brought on by low temperature. Once warmed the insects resume activity. Insects seek protected places in which to overwinter.
Fall migrating boxelder bugs are attracted to large trees with dense, evergreen foliage like cedars. Trees with deeply fissured bark also are selected. Both must appear to be good overwintering sites. A single large tree might attract hundreds, or thousands, of boxelder bugs from the surrounding area. These bugs are strong fliers so may travel miles from the maple tree on which they developed. We now have a large tree with perhaps thousands of overwintering boxelder bugs - so far so good.
Let's put this tree, full of bugs, near the sunny, southwest side of a house. When the house siding heats up during the day all of a sudden the nice warm house seems like a better place to overwinter than the tree. Presto, the bugs move en mass to the house. Imagine, a thousand, or more (seems like millions!), 1/2” bugs congregating on the outside of your house, around doors, windows and on siding. This actually happens to many people each fall, year after year.
What to do. First realize that these bugs are harmless. They don't bite or sting, won't hurt your house nor will they set up permanent residence. But even people who know all this freak out when confronted with all these bugs. Here are some suggestions. The most important thing is to prevent the bugs from getting indoors. If they do gain entry they will likely be around until spring. Repair window screens, caulk cracks around window and door frames and screen soffit vents.
Next, wash down aggregating masses of bugs with water spray from a garden hose then clean the siding with soapy water from a hose-end sprayer. A weak solution of laundry detergent is fine for this application. There is some evidence that washing the siding helps to discourage other bugs from congregating. Don't use insecticidal sprays. Insecticides don't work well on these semi-dormant insects and can be messy and dangerous. You may need to repeat this procedure every few days until the number of new bugs starts to drop.
On occasion large numbers of bugs will get into the attic or exterior wall voids of a house. Again, these insects cause no real harm but may be a nuisance. They will find a way into the home's interior on warm winter days - probably the day of your dinner party. In this case it may be necessary to treat the attic and walls with insecticidal dust. Bugs found crawling around indoors should be removed with a vacuum cleaner. Don't reach for the can of insecticide!
You may have concluded that the solution to this invasion problem is to remove the big tree. Sometimes this works but sometimes it does not. Sometimes the big tree is in your neighbor's yard. Good luck!
Find related information at www.LivingWithBugs.com. Information about insecticidal dusts can be found at www.LivingWithBugs.com/use_dust.html
copyright 2005 http://www.LivingWithBugs.com
Jack DeAngelis, PhD