Stink bugs in the house: where they are all coming from, how to get them out without the smell, and whether they will breed indoors

If your windows, curtains and lampshades have suddenly filled with slow, shield-shaped brown bugs, you are not alone and you have not done anything wrong. This is the time of year when the brown marmorated stink bug, card #109, stops eating and starts looking for somewhere to sleep through the winter. Your house is a very good somewhere.

Where they are all coming from

The brown marmorated stink bug is from East Asia. Penn State Extension records that it was first collected in North America in Allentown, Pennsylvania, in the late 1990s, and it had probably arrived a few years earlier. Since then it has spread across most of the United States and into southern Canada, and it has also become established in parts of Europe.

All summer it lives outdoors, feeding on fruit trees, ornamental trees, vegetables and crops. Penn State notes that the adults start searching for overwintering sites from September into October, and that they are drawn to the outside of houses on warm autumn days. A sunny south- or west-facing wall gathers them first. From there they walk into any gap they can find: around window frames, under siding and fascia boards, through vents, down chimneys.

Our own seasonal data shows the same wave. On card #109, iNaturalist research-grade records of this species climb through the summer and peak in September, when there are about five times as many records as in January. That autumn peak is people photographing them on walls and window frames.

Is it a stink bug, or a look-alike?

A brown marmorated stink bug is about the size of a fingernail, shaped like a shield, mottled brown. Two details settle it: pale bands on the antennae, and alternating light and dark patches around the edge of the abdomen, which is what “marmorated” (marbled) refers to. The University of Wisconsin and Utah State University Extension both use those two features to separate it from its look-alikes.

The western conifer seed bug also comes indoors in autumn and also smells when handled, but Wisconsin’s insect lab notes it is a nuisance and little more. The wheel bug is a predator that hunts other insects, stink bugs included, and it can give a painful bite if handled, so move it with a jar rather than your fingers. If you are not sure which one you have, a photo in the identifier will tell you.

Will they breed in the house?

No. This is the most useful thing to know. The University of Kentucky’s extension fact sheet on stink bugs in houses notes that the adults overwinter unmated and do not reproduce indoors. The ones you see in January are the ones that came in during the autumn. They are not multiplying in the loft.

What they do is wake up on warm winter days, mistake the warmth of the radiator or the sunshine through a window for spring, and wander out into the room. Then they fly clumsily to the nearest lamp. In spring the survivors try to get out again, which is why you see a second wave at the windows around March and April.

Getting them out without the smell

The smell is a defence. A stink bug releases it when it is squeezed, crushed or alarmed, so the whole trick is not to alarm it.

• One at a time: slide a sheet of paper under it, or put a jar over it and a card under the jar, and take it outside. They are slow and rarely fly when approached calmly.
• If you would rather it did not come back: tip it into a jar of water with a few drops of washing-up liquid. The soap breaks the surface tension and the bug sinks. Several extension services recommend this for an invasive species, and it does not set off the smell indoors.
• For a crowd: Kentucky’s fact sheet suggests stuffing a nylon stocking into the vacuum hose and fastening it over the end with a rubber band. The bugs collect in the stocking instead of the bag, and you can empty it outdoors. Without the stocking, the vacuum will smell for a long time.
• Not recommended: crushing, flushing large numbers down the toilet, or spraying insecticide indoors. Dead bugs in a wall void attract other insects that feed on them.

Front desk note: the stink bug’s smell is not poison, just a very convincing request to be put down. Treat it like a houseguest who has overstayed and open the door politely.

Keeping them out next year

Prevention is a job for late summer, before the September rush. Oregon State University Extension lists the usual entry points: cracks around windows and doors, gaps where utility pipes come in, and spaces under siding and fascia boards. Close them with silicone or silicone-latex caulk, foam sealant or weather stripping. Fit fine mesh over roof and soffit vents, repair torn window screens, and check that the chimney has a cap. Kentucky notes that older buildings in poor repair, with many cracks and openings, are the most vulnerable.

Insecticide sprays on outside walls are sometimes offered, but extension advice is consistent that they are short-lived and no replacement for sealing the gaps.

Are they harmful?

To people, no. They do not sting and are not known to bite people. They do not eat clothes, food or wood, and they do not damage the house. Outdoors it is a different story: Utah State University Extension describes the brown marmorated stink bug as a significant pest of fruit, vegetables, nuts, some field crops and ornamental plants. That is why it is watched closely by farmers, even though indoors it is only a nuisance.

If you find a whole collection of shield bugs and are not sure they are all the same species, the true bugs section of the Field Guide has a page for each, and our guide to bugs that come into the house in autumn covers the ladybirds and cluster flies that often arrive at the same time.

Sources

• Penn State Extension: Brown marmorated stink bug
• University of Kentucky Entomology: Stink bug infestation of dwellings (ENTFACT-654)
• Oregon State University Extension: Stink bugs and boxelder bugs everywhere?
• University of Wisconsin Insect Diagnostic Lab: Conifer seed bugs
• Utah State University Extension: Invasive insect look-alikes
• Seasonal pattern: iNaturalist research-grade observations, as shown on card #109