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Harmless

Brown marmorated stink bug

Halyomorpha halys
Also called BMSB, stink bug

It does not bite or spread disease; it just smells bad when crushed and can show up indoors in large numbers.

Brown marmorated stink bug, photo 1
Photo: Emmett Collins-Sussman, CC BY 4.0, via iNaturalist
Sizeabout 17 mm (about 5/8 inch) long
Where in the houseWindows and walls, attic, living room, bedroom, outside near the house
WhenLate September and October when they move in, then on warm days in winter and spring until they leave by about May.

Check these features

  1. Shield-shaped, mottled brown to gray-brown bug
  2. Antennae with alternating light and dark bands (look for a pale band near the tip)
  3. Edges of the abdomen, sticking out past the wings, banded light and dark
  4. Smooth, rounded shoulders with no spines or teeth
  5. Gives off a strong odor when handled or crushed

Bites and stings

Does not bite people.

What to do

  1. Vacuum the bugs up gently or sweep them into a bag, taking care not to crush them, which releases the smell.
  2. Seal the bag or empty the vacuum right away and put it in the outdoor trash; a vacuum may smell for a while afterward.
  3. Find and seal the gaps they are using, such as around window air conditioners, pipes, wires, and window frames.
  4. Skip indoor sprays and foggers; bugs that die inside walls can smell and attract carpet beetles.

Look-alikes

Native brown stink bugs (such as rough stink bugs)Native look-alikes usually have pointed or toothed shoulders and plain antennae, while the brown marmorated has smooth shoulders and white-banded antennae.
Western conifer seed bugThe seed bug is long and narrow rather than shield-shaped, and its back legs have a flat, leaf-like flare.

Why it came inside

In fall the adults hunt for protected places to wait out winter and gather on the sunny sides of buildings, then crawl through gaps into attics and walls. Some wander into living space on warm winter or spring days.

Keep it out

  • In late summer, caulk cracks around windows, doors, siding, pipes and chimneys.
  • Keep window screens and door sweeps tight and in good repair.
  • Cover attic and crawl space vents with fine screen mesh.
  • Remove window air conditioners before fall or seal around them.

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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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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Screen repair kit

Patches or replaces torn window and door screens, one of the most common ways insects 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. Washington State University Extension (PestSense)
  2. Utah State University Extension
  3. UConn Home & Garden Education Center
  4. Penn State Extension (for the seed bug comparison)

Checked October 18, 2026. Rating backed by 2 sources. How we rate danger.