On a sunny fall afternoon the side of the house is suddenly covered with stink bugs, orange beetles or sluggish flies, and a few days later they're on the windowsills inside. These are overwintering pests. They don't breed indoors, eat your food or damage the house. They're looking for a sheltered place to spend the winter, and a wall void or attic is perfect. The numbers can be startling: one Maryland homeowner collected 26,205 brown marmorated stink bugs inside a single house over six months. Here's how to tell the three main fall invaders apart, what the research says about where they gather, and what actually keeps them out.
Seal them out before they arrive. Penn State Extension calls mechanical exclusion "the best method." Caulk gaps around windows, doors, siding, utility lines and fascia, repair screens, and put fine mesh over attic vents in late summer.
Once they're inside, vacuum them up and seal the vacuum bag. Don't spray or dust inside walls. Penn State warns that dead bugs in wall voids feed carpet beetles, which then move on to wool and stored food. Exterior sprays give only "minor relief," lasting several days to a week.
How to Tell the Three Apart
| Brown marmorated stink bug | Asian lady beetle | Cluster fly | |
|---|---|---|---|
| Looks like | Brown shield, about 5/8 inch, light bands on antennae | Round, yellow-orange to red, 0 to many spots, black "M" or "W" on a white shield behind the head | About 7 mm, slower than a house fly, yellow hairs on thorax, checkered abdomen |
| Where it gathers | Darker houses in rural areas, most on north and east walls in one study | Sunlit, southwest-facing, light-colored walls with strong color contrast, near woods | Warm sides of buildings that stand out, like homes on hilltops |
| Problems | Odor when crushed, sheer numbers | Pinches, foul yellow fluid that stains, allergies | Swarms at windows, dead flies attract carpet beetles |
| Breeds | Outdoors on fruit, crops and trees | Outdoors, feeding on aphids | Outdoors, larvae live inside earthworms |
Brown Marmorated Stink Bugs
This invasive stink bug from East Asia was first collected in Allentown, Pennsylvania, in 1998, according to Penn State Extension, though it probably arrived earlier. Clemson Extension says it first appeared in South Carolina in 2011 and is now widely distributed in the state. Penn State describes adults as about 17 mm, roughly 5/8 inch, and notes they start looking for overwintering sites in September and October.
Which walls they gather on. Most websites say south- and west-facing walls. A citizen-science study in the Mid-Atlantic, where volunteers counted stink bugs on their own homes every day in fall 2013 and 2014, found the opposite: the most bugs were on north and east walls both years. Counts were also lower on white houses than on gray ones (and brown or tan ones in one of the two years), higher on wood, cement and stone exteriors, and higher in rural areas than urban ones. So check every side of the house, not just the sunny one.
On Reddit, homeowners in "SO. MANY. STINKBUGS" threads try everything from perimeter sprays to homemade light traps. A pest professional's advice in one thread was simpler: "A few tubes of caulk will go a long way."
Asian Lady Beetles
The multicolored Asian lady beetle was brought to the U.S. to eat aphids and is now common nationwide. Purdue Extension says it gathers on buildings with "abrupt longitudinal color contrasts" like dark shutters on a white house, on "highly illuminated or southwest-facing sides," and on buildings close to trees or woods. Unlike stink bugs, it favors light-colored, sunlit walls.
- Do they bite? They pinch. Purdue says they pinch when landing on bare skin on warm days, especially if you're sweating, and that the pinch "does not break the skin." On Reddit, people are split between "they stink, bite" and "they don't bite." Both are partly right.
- Stains and smell. When disturbed they release a foul-smelling yellow fluid that can stain paint and upholstery.
- Allergies are real. In a West Virginia allergy practice where infestations are common, 21% of patients tested were sensitized to the lady beetle, close to the rate for cats (24%). The beetle caused allergic rhinitis and eye symptoms in 8% of patients and asthma in 2%. Purdue notes dried beetle fragments can become airborne, so clean up dead beetles.
Cluster Flies
Virginia Tech's insect identification lab describes cluster flies as robust, bristly flies about 7 mm long with short yellow hairs on the thorax. Their larvae are parasites of earthworms, so they're most common around homes with large lawns, fields or pastures. In fall they cluster on the warm sides of buildings, and Virginia Tech notes homes on hilltops or that otherwise stand out in the landscape attract the most. Once inside they often turn up spinning on windowsills on warm winter days. They don't bite or carry disease, and they don't live long indoors.
What looks like one species is actually several. Researchers recognize six cluster fly species in North America, all once lumped together as Pollenia rudis. Control is the same for all of them.
How to Keep Them Out
- Seal in late summer, before they start gathering. Use silicone or silicone-latex caulk, as Penn State recommends, around windows, doors, siding seams, utility pipes, chimneys and under fascia boards.
- Repair screens on windows, doors and crawl space vents, and add fine mesh behind soffit, ridge and gable vents.
- Check every side of the house. Stink bugs favored north and east walls in the Mid-Atlantic study. Lady beetles favor sunny southwest sides and color contrasts. Cluster flies go for the warm side.
- Close up the attic and upper stories. Virginia Tech emphasizes sealing attics and upper areas for cluster flies.
- Consider an exterior treatment as a supplement, timed just before insects gather. Penn State is frank that it offers "minor relief" because sunlight breaks the insecticide down within several days to a week. It helps most where full sealing isn't possible.
If They're Already Inside
- Vacuum them up, then seal and discard the bag. Stink bugs will make the vacuum smell for a while, so some people use a dedicated shop vac or a bag over the nozzle.
- Find where they're coming out. Penn State says stink bugs usually emerge from gaps behind baseboards, around window and door trim, and around ceiling lights and exhaust fans. Seal those from the inside.
- Don't treat wall voids or the attic. Dusts can kill hundreds of bugs, but Penn State warns, and Virginia Tech notes for cluster flies, that dead insects left in the walls attract carpet beetles, which then attack wool, stored dry goods and other natural products.
- Skip foggers. Penn State says foggers kill bugs on walls and ceilings but don't stop more from emerging once the room airs out, and crack-and-crevice sprays are "not a viable or recommended treatment." Some Reddit users report success with bug bombs in attics, but that leaves dead insects behind for carpet beetles.
- Try the turkey-pan trap for stink bugs. This one started on YouTube and held up in testing. Fill a foil roasting pan with about half a gallon of water and a little dish soap, set a desk lamp over it in a dark room and leave it on overnight. Virginia Tech researchers tested it in volunteers' homes and found it caught 14 times more stink bugs than store-bought traps costing up to $50. Their how-to video has about 281,000 views. It catches bugs that are already inside. It won't stop more from coming in.
When to Call a Professional
- You get hundreds every fall and can't find or reach the entry points
- They're coming in at the roofline, soffits or second-story gables
- Someone in the house has allergies or asthma that flare with lady beetles
- Dead insects in the attic are drawing carpet beetles
Our residential pest control includes exclusion advice and seasonal perimeter service across South Carolina, North Carolina and Virginia. For why fall weather sets off these invasions, see how weather changes drive pest activity.
Key Takeaways
- They don't breed or cause damage indoors. The problem is numbers, odor, stains and allergies.
- Seal before they arrive. Exclusion is the best method. Exterior sprays last days to a week.
- Check all sides of the house. Stink bugs favored north and east walls in a Mid-Atlantic study, while lady beetles favor sunny southwest walls.
- Asian lady beetles pinch but don't break skin, and they're a real allergen.
- Vacuum, don't crush, and don't treat wall voids, which feeds carpet beetles.
Sources: Penn State Extension, "Brown Marmorated Stink Bug" (updated 2026); P. A. Zungoli and E. P. Benson, "Brown Marmorated Stink Bug", Clemson Cooperative Extension HGIC (2014); D. B. Inkley, "Characteristics of Home Invasion by the Brown Marmorated Stink Bug", Journal of Entomological Science 47:125 (2012); T. J. Hancock, D. Lee, J. C. Bergh, W. R. Morrison and T. C. Leskey, "Presence of the invasive brown marmorated stink bug on home exteriors during the autumn dispersal period: Results generated by citizen scientists", Agricultural and Forest Entomology (2019); T. J. Gibb, "Asian Lady Beetle", Purdue Extension E-214 (2017); D. W. Goetz, "Harmonia axyridis ladybug hypersensitivity in clinical allergy practice", Allergy and Asthma Proceedings 28 (2007); P. L. S. Wooden, T. A. Dellinger and E. Day, "Cluster Fly", Virginia Cooperative Extension ENTO-661 (revised 2026); A. Jewiss-Gaines, S. A. Marshall and T. L. Whitworth, "Cluster Flies (Calliphoridae: Polleniinae: Pollenia) of North America", Canadian Journal of Arthropod Identification 19 (2012); Virginia Tech News, "Stink bugs beware! Homemade stink bug traps squash store-bought models" (2014), and "Stink bug trap", YouTube; homeowner accounts from Reddit (via Google search excerpts).
Frequently Asked Questions
Brown marmorated stink bugs look for sheltered places to spend the winter, starting in September and October according to Penn State Extension. Wall voids and attics are ideal. They don't breed or feed indoors, and they head back outside in spring.
Seal them out before they arrive. Penn State Extension calls mechanical exclusion the best method: caulk gaps around windows, doors, siding, utility pipes, chimneys and fascia with silicone or silicone-latex caulk, and repair screens. Exterior sprays give only minor, short-lived relief.
In a Mid-Atlantic citizen-science study, the most stink bugs were counted on north and east walls, not south and west. Darker homes, homes with wood, cement or stone exteriors, and rural homes had more.
Only a little. Penn State Extension says exterior insecticides offer minor relief because sunlight breaks them down, so they may not kill bugs much beyond several days or a week. Use them only as a supplement to sealing, applied just before bugs gather.
No. Penn State and Virginia Tech warn that dead insects in wall voids and attics attract carpet beetles, which then attack wool, stored dry goods and other natural products. Vacuum the bugs you can reach and seal where they emerge.
They pinch. Purdue Extension says they pinch when landing on bare skin on warm days, especially when you're sweating, but the pinch does not break the skin.
Look at the white shield behind the head. Asian lady beetles have a black M- or W-shaped mark there. They range from pale yellow-orange to red and can have no spots or many.
Yes. In a West Virginia allergy practice, 21% of patients tested were sensitized to the lady beetle, and it caused allergic rhinitis and eye symptoms in 8% and asthma in 2%. Purdue notes dried beetle fragments can become airborne, so clean up dead beetles.
Cluster flies are sluggish flies about 7 mm long with yellow hairs on the thorax. Their larvae parasitize earthworms, so they're common around homes with big lawns or fields. They overwinter in attics and walls and often appear on windows on warm winter days.
No. Penn State says stink bugs won't reproduce indoors or damage structures, and Virginia Tech says cluster flies don't bite, spread disease or breed indoors. The problems are odor, sheer numbers, stains from lady beetles, and carpet beetles feeding on dead insects.
Photo credits: Rhododendrites, CC BY-SA 4.0, via Wikimedia Commons; Alandmanson, CC BY 4.0, via Wikimedia Commons; Alvesgaspar, CC BY-SA 3.0, via Wikimedia Commons.