They’re red, black and almond-shaped — and chances are, you’ve seen a lot of them lately. Boxelder bugs are out in full force this time of year.
Most often, Boxelder bugs are found around maple, ash and boxelder trees. They don’t harm trees, people or animals, though they do feed by sucking out the middle of the trees’ seeds.
One of their more unpleasant traits: when smashed, their feces can leave a red stain.
So why do we see so many of them in the fall? They’re simply looking for a warm place to spend the winter — a process known as “winterizing.”
“They like to land on south-facing or west-facing sides of buildings, which tend to be warm and sunny areas,” said Sarah Hughson, arthropod diagnostician at the Michigan State University Plant and Pest Diagnostics Lab. “We don’t know exactly why they prefer warm, sunny locations, but those are places where they like to aggregate.”
Their flattened bodies allow them to slip through the tiniest crevices around doors and windows, making it easy for them to crawl indoors — and stay rent-free all winter.
When Boxelder bugs gather, or “cluster,” it may be annoying to homeowners, but it serves a purpose.
“For the adult insect, aggregating can help them find mates easily because they’re all in one place already,” Hughson said. “They don’t have to expend energy searching for that mate.”
You may also notice a mix of adults and tiny baby Boxelder bugs, known as nymphs. That mix can be helpful for the young insects, Hughson said, because adults sometimes pierce seeds, making them easier for the nymphs to feed on.
Although harmless, Boxelder bugs can still be a nuisance once they move indoors.
“If you find them moving around the house, you can vacuum them up or catch them and toss them outside,” Hughson said. “But I would suggest not smashing them because they could leave that little red stain behind.”
For prevention, Hughson recommends sealing entry points.
“Some of the best management strategies for Boxelder bugs include sealing gaps and caulking around windows and doors,” she said. “Putting screens over vents can also help reduce their access to those locations.”
Overall, Hughson encourages a “live and let live” approach when possible.
“Many of our insects are an excellent food source for other animals, so they have an important role in the food chain,” she said.
Boxelder bugs can be found across the United States, with both eastern and western varieties present.