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Harmless

Boxelder bug

Boisea trivittata
Also called box elder bug, maple bug

Boxelder bugs are a fall nuisance only; they do not harm people, pets or the house, though crushed bugs can stain fabric.

Boxelder bug, photo 1
Photo: Alfonso Benítez-Mora, CC BY 4.0, via iNaturalist
Sizeabout 12–13 mm (1/2 inch) long
Where in the houseWindows and walls, attic, living room, outside near the house
WhenLate August through October as they move in, and again on warm days in late winter and spring.

Check these features

  1. Long, flat-backed bug, black to dark brown
  2. Three red lines on the area just behind the head, one down the middle and one on each side
  3. Thin red lines along the edges of the wings
  4. Young bugs (nymphs) are smaller, wingless, with a bright red body
  5. Clusters by the dozens or hundreds on sunny south or west walls

Bites and stings

Does not bite people. A rare poke from its beak has been reported, but it is not harmful.

What to do

  1. Vacuum up the bugs rather than crushing them, which can stain fabric and smell.
  2. Empty the vacuum or seal the bag right away, outdoors if you can.
  3. Skip indoor sprays; vacuuming and sealing work better.
  4. Note where they cluster outside and seal the nearby gaps before next August.

Look-alikes

Red-shouldered bug (Jadera haematoloma)It has red edges on the shoulders like a boxelder bug but no red lines on the wings.
Small milkweed bugIt has a bold red X-like band across its back and a white spot near the wing tips, which boxelder bugs lack.
Asian lady beetleLady beetles are round and domed with spots, not long and flat with red lines.

Why it came inside

In late summer and fall the adults gather on warm, sunny walls and squeeze into cracks to spend the winter in a sheltered spot. They cannot breed indoors, and they wander out on warm winter days.

Keep it out

  • By late summer, caulk gaps around windows, doors, siding and where pipes or gas lines enter.
  • Keep window screens snug and repaired, and screen vents and attic louvers.
  • Add door sweeps to exterior doors.
  • Rake fallen leaves and seeds away from the south and west sides of the house.

What helps

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Vacuum with a hose

Lets you remove bugs, webs and egg sacs without touching them. Empty the bag or canister outside right away.

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Caulk and sealant

Silicone or siliconized acrylic caulk closes cracks around windows, pipes and baseboards. Bigger gaps need foam backer rod or expanding foam first.

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Screen repair kit

Patches or replaces torn window and door screens, one of the most common ways insects get in.

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Door sweep

Closes the gap under an exterior door. If you can see daylight under the door, bugs can get in.

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Weatherstripping

Seals the gaps around doors and windows that let in drafts and bugs alike.

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Sources

  1. UMass Amherst Extension Landscape, Nursery & Urban Forestry Program
  2. Montana State University Extension Urban IPM
  3. Colorado State University, Department of Agricultural Biology

Checked October 18, 2026. Rating backed by 3 sources. How we rate danger.