The red and black bugs you’re seeing in Michigan are most likely boxelder bugs, a half-inch-long insect that swarms sunny walls in fall and slips indoors to hibernate through winter. They’re harmless, but they show up in large enough numbers to be genuinely annoying. A few similar-looking species also live in the state, so knowing the differences helps you figure out exactly what you’re dealing with and whether you need to do anything about it.
Boxelder Bugs: The Most Common Culprit
Adult boxelder bugs are flat, blackish insects about half an inch long with bright red to orange markings along their wings and thorax. The red lines form a distinct “X” pattern on their backs when their wings are folded. If you’re seeing dozens or hundreds of these on the south-facing side of your house on a warm fall afternoon, this is almost certainly what you have.
Their young look different enough to cause confusion. Boxelder bug nymphs are bright red all over, smaller, and wingless. They hatch from eggs in about two weeks during summer and gradually darken as they mature. So if you spot tiny all-red bugs crawling around a maple or boxelder tree between June and August, those are juvenile boxelder bugs, not a separate species.
Other Red and Black Bugs in Michigan
A few other insects share the same color scheme and can be easy to confuse with boxelder bugs.
Red-shouldered bugs (golden rain tree bugs) look strikingly similar but are very dark gray to black with only two bright red lines on the thorax, compared to the boxelder bug’s more extensive red markings. Their nymphs are bright red with black legs and antennae, much like boxelder bug nymphs. These bugs feed on golden rain trees rather than maples or boxelders, so checking the nearest tree often settles the identification. They also invade homes in fall to overwinter, and if you crush one, it leaves a red stain on surfaces.
Large milkweed bugs are about the same size but have a broad red or orange band across their back with black patches. They feed almost exclusively on milkweed seed pods. If the bugs you’re seeing are clustered on milkweed plants, that’s your answer. They don’t typically enter homes in large numbers.
Small milkweed bugs are slightly smaller with a red and black pattern that includes a heart-shaped mark. Like their larger relatives, they stick to milkweed plants and aren’t home invaders.
Why They Swarm Your House in Fall
Boxelder bugs and red-shouldered bugs are both overwintering insects. As days shorten and temperatures drop in September and October, they start looking for sheltered spots where the temperature stays between 40 and 50 degrees Fahrenheit through winter. That range is warm enough for hibernation but cool enough that they don’t burn through their stored energy and die before spring.
Your home’s south- and west-facing walls absorb the most afternoon sun, which is why the bugs congregate there first. They work their way into attics, wall cavities, and gaps around windows and doors. On warm winter days, some will wake up and wander into your living space, which is how you end up finding them on curtains and windowsills in January. They aren’t breeding indoors. They’re just confused by the warmth and trying to get back to sleep.
Do They Bite or Cause Damage?
Boxelder bugs do not sting, do not transmit disease, and are generally not known to bite, though there are rare reports of defensive biting if one is pressed against skin. They won’t damage your home’s structure, your furniture, or your clothing.
They do have two mildly annoying habits. Their droppings can leave small brownish stains on light-colored surfaces like curtains, walls, and windowsills. And smashing them releases an unpleasant odor, similar to the defensive smell of stink bugs. Red-shouldered bugs add a third problem: crushing them leaves a noticeable red stain.
Outdoors, boxelder bugs feed on the leaves, flowers, and seed pods of female boxelder trees, silver maples, and ash trees. They occasionally feed on fruit trees like apple, cherry, peach, and pear, where their feeding punctures can cause minor deformities in the fruit. For backyard fruit trees this is rarely significant, and they do little damage to ornamental trees.
How to Keep Them Out of Your Home
The most effective strategy is physical exclusion, done before the fall invasion begins. Walk the exterior of your home in late August or early September and seal the gaps they use to get inside:
- Caulk cracks around window frames, door frames, and where siding meets the foundation.
- Replace worn weatherstripping around doors and windows.
- Cover vents and soffits with fine mesh screening.
- Check utility entry points where pipes, cables, and wires enter the house.
Once they’re already inside the walls, there’s no practical way to kill them without creating a bigger problem (dead insects in wall cavities can attract carpet beetles and other pests). The simplest approach for the ones that wander into your living space is vacuuming them up with a hose attachment and emptying the bag or canister outside. This avoids the staining and odor you get from crushing them. For red-shouldered bugs, gentle vacuuming is especially worthwhile since squishing them stains walls and fabric.
Reducing Numbers Long-Term
Boxelder bugs depend heavily on female boxelder trees, which produce the seed pods that are their primary food source. If you have a female boxelder tree close to your house and the bug numbers are overwhelming year after year, removing that tree or replacing it with a male boxelder (which doesn’t produce seeds) dramatically cuts the local population. Silver maples also attract them, though typically in smaller numbers.
Spraying the exterior walls where they congregate in fall can reduce the number that make it inside, but timing matters. These bugs are more vulnerable to contact sprays in late spring and summer when they’re still actively feeding and growing. By fall, when they’re clustering on your siding, they’re hardier adults with more resistance. A residual insecticide applied to south- and west-facing walls in mid-September, before the main migration, is more effective than spraying after hundreds have already arrived. For most homeowners, sealing entry points works just as well without any chemicals.