Boxelder bugs, multicolored Asian lady beetles and brown marmorated stink bugs gather on sunny Davenport walls in September and October, then slip through gaps to spend the winter inside. None of them breed indoors or eat wood. Sealing gaps plus a perimeter treatment before they arrive works best, and runs $175–$450 (estimate).
Key takeaways
- All three are accidental invaders that hunt for winter shelter in September and October and do not reproduce inside the house.
- Boxelder bugs gather on sunny south sides of buildings; multicolored Asian lady beetles help in the garden but become a household nuisance in late fall and winter.
- Iowa State University confirmed live brown marmorated stink bug adults in Scott County in 2012, Iowa's first established population, and the bug has since been found in many Iowa counties.
- Bugs that appear indoors in January or March got into the walls the previous fall and are waking on warm days.
- Sealing gaps around windows, siding, soffits and utility lines before late September, plus a perimeter treatment, prevents far more than spraying inside.
- A fall overwintering-pest treatment (perimeter treatment + sealing) runs $175–$450 around Davenport, as an estimate approved before treatment.
How do you tell boxelder bugs, lady beetles and stink bugs apart?
Boxelder bugs are flat, about 1/2 inch long, and black with red-orange lines. Multicolored Asian lady beetles are round, about 1/4 inch, orange to red with a variable number of spots and a black M or W mark behind the head. Brown marmorated stink bugs are shield-shaped, mottled brown and about 5/8 inch.
Native lady beetles rarely come indoors in big numbers. If dozens are crawling across a window in October, it is almost certainly the multicolored Asian lady beetle.
Getting the ID right matters less for the house than for the garden, since all three are kept out the same way. Outside it does matter: stink bugs can scar garden crops, while lady beetles are worth having around for the aphids they eat.
| Feature | Boxelder bug | Asian lady beetle | Brown marmorated stink bug |
|---|---|---|---|
| Size and shape | About 1/2 inch, flat and narrow | About 1/4 inch, round and domed | About 5/8 inch, broad shield shape |
| Color | Black with red-orange edges and lines | Pale orange to deep red, from no spots to many | Mottled brown-gray, light bands on the antennae |
| Gathers on | Sunny south-facing walls, often near boxelder trees | Light-colored, sunlit siding, window frames and eaves | Sunny walls, window and door trim, attic vents |
| Indoor nuisance | Droppings stain curtains and walls | Yellow fluid that stains and smells; can pinch | Strong odor when crushed or handled |
| Outdoors | Feeds on boxelder and maple seeds | Eats aphids and other soft-bodied insects | Feeds on fruits, vegetables and ornamental plants |
Why do they cover Davenport siding every fall?
Shorter days and cool nights tell all three insects it is time to find winter shelter. They fly to sunny south and west walls and crawl into any crack they find. Older Davenport homes with wood siding, loose window trim and settling mortar give them plenty of openings, and warm afternoons after a cold snap bring the biggest swarms.
In the river-bluff neighborhoods, tall Victorian houses catch afternoon sun high on the walls, and the bugs work their way up to soffits, attic vents and third-floor window frames. Newer two-story homes on the north side see the same thing at upstairs windows and where siding meets the roofline.
Brick houses are not exempt. The bugs work into loose mortar around window sills, the gap behind exterior trim and the seam where a porch roof meets the main wall.
Boxelder bugs build up where seed-bearing boxelder trees drop seed, and those trees are common along Midwestern rivers and creeks. Lady beetles are not tied to one tree; they feed on aphids in fields, gardens and woods all summer, then head for the brightest, most sunlit building nearby.
Are boxelder bugs, lady beetles or stink bugs harmful?
Not to people or the house. None of the three carry disease, eat wood or breed inside over winter. The trouble is the nuisance: bugs on windows and ceilings, stains on curtains and walls, the stink bug's odor and the lady beetle's yellow fluid. Asian lady beetles can give a small pinch, and some people react to them.
Crushing them makes everything worse. A squashed lady beetle leaves a yellow-orange smear, a squashed stink bug fills the room with a smell that lingers, and boxelder bug droppings leave reddish spots on light fabric. Vacuum them instead.
Dead bugs piled in wall voids and attics can later draw carpet beetles and other scavengers, one more reason to keep them out in the first place.
When did stink bugs arrive in Scott County?
Iowa State University confirmed live brown marmorated stink bug adults in Scott County in 2012, the first established population in Iowa. The insect, originally from Asia, has since been found in many Iowa counties. In Davenport it can now turn up alongside boxelder bugs and lady beetles on sunny walls in the fall.
Outside, the stink bug is a plant feeder with a wide menu, including tomatoes, peppers, sweet corn, apples and ornamental shrubs, so gardeners may spot it before it ever reaches the house. Inside, it is only a smelly nuisance. It does not bite, and it does not lay eggs indoors over the winter.
Numbers swing a lot from year to year and from street to street. One house may see a handful while a neighbor with more sun-facing siding and older windows collects hundreds.
Some falls are simply worse for all three insects. Lady beetle numbers tend to follow aphid numbers, including aphids in the soybean fields around the Quad Cities, and a warm, dry fall gives every one of these bugs more good flying days to find your walls.
When do fall invaders show up, month by month?
The first boxelder bugs and lady beetles usually show up on sunny walls in September, peak on warm afternoons in late September and October, and disappear into the walls after the first hard cold snap. On warm days from December through April, some of those hidden bugs wander back out into living space.
The heaviest days come right after a cold snap breaks. A chilly week followed by a sunny 70-degree afternoon in October is the classic setup for a wall covered in lady beetles.
| Month | What the bugs are doing | What you can do |
|---|---|---|
| August | Feeding outdoors; boxelder bugs clustering on seed-bearing trees | Walk the house and list gaps; book a fall treatment |
| September | First groups on sunny south and west walls | Finish sealing; the perimeter treatment goes on now |
| October | Peak swarms on warm afternoons, then into cracks | Keep doors and screens shut; vacuum what gets in |
| November | Settled in wall voids, attics and window frames | Note where bugs appear inside; leave the walls alone |
| December–February | Mostly dormant; a few wake on sunny days | Vacuum strays instead of crushing them |
| March–May | Leaving the walls and heading back outdoors | Mark the gaps they use for next fall's sealing |
How do you keep fall invaders out of the house?
Seal the house before mid-September, when the bugs start gathering. Caulk gaps around window and door trim, repair torn screens, screen attic and soffit vents, seal where pipes, wires and air conditioning lines enter, and replace worn door sweeps and weatherstripping. Start on the sunny south and west sides.
Work from the top down on tall houses. Bugs that land high on a sunny wall crawl upward, so soffits, attic vents and the gaps around upper-floor windows matter more than the foundation for these insects. Ladder work like that is often easier to hand off along with the treatment.
Old wood windows in the historic districts are the hardest to tighten up, and storm windows that close fully make a bigger difference there than any spray. Check these spots on a dry day in late summer:
- Gaps where siding meets the foundation and at outside corners
- Window air conditioner frames and storm window edges
- Fascia, soffits and the seam where the roof meets the wall
- Chimney flashing and attic vent screens
- Openings around outdoor faucets, cable lines and dryer vents
- Loose trim on porches and around old wood-frame windows
What should you do when they're already inside?
Vacuum them up, empty the bag outdoors and wait it out. Bugs that got into the walls last fall will keep appearing on warm, sunny days through winter, then try to get back outside in spring. Spraying insecticide indoors rarely helps, because most of the bugs are hidden in wall voids and attics where sprays don't reach.
A knee-high stocking stretched inside the vacuum hose and held with a rubber band catches the bugs without filling the vacuum with stink bug smell. Empty it into soapy water or a sealed bag.
Note where they show up. Clusters at one window, one outlet or one attic corner point to the gap you need to seal before next September. Light traps sold for attics catch some lady beetles, but they will not stop the next wave.
Keep the attic in mind as well. Lady beetles sometimes gather by the hundreds in attic corners and around vent openings, then find their way down through light fixtures and ceiling gaps in spring. Sealing around ceiling fixtures and the attic hatch keeps more of them out of the bedrooms.
How does a fall overwintering-pest treatment work?
The technician inspects the sunny sides of the house, seals accessible gaps and applies a perimeter treatment to the exterior surfaces where boxelder bugs, lady beetles and stink bugs gather, such as eaves, window frames and siding. Timing is everything: late August through September, before the bugs move in. Expect $175–$450, approved before treatment.
Products are applied according to their labels, and the technician explains how long kids and pets should stay away from treated areas. A three-story house on the bluff costs more than a ranch because of the height and the amount of trim to cover. You get a written service report showing what was treated and where.
Treatment and sealing work as a pair. Closing every gap on an older house is rarely possible, and a perimeter treatment alone leaves open doors for the bugs that get past it, so the visit covers both and the report notes any gaps that still need attention. Apartment buildings and offices with big sunny walls get the same approach on a larger scale, usually as part of a commercial pest control plan.
Each insect has its own page if you want details: boxelder bug control, Asian lady beetle control and stink bug control. The Quad Cities pest calendar shows how fall invaders fit into the rest of the year.
Frequently asked questions
Why are lady beetles still showing up in March?
They spent the winter in your walls or attic and are waking up as the sun warms the house. They head toward light, which brings them to windows and ceiling fixtures. The flow stops once they have found their way back outside, usually by late spring.
Will cutting down my boxelder tree stop boxelder bugs?
It may help a little, but it rarely solves the problem. Boxelder bugs fly in from trees on nearby lots, and they also feed on maples. Sealing the house and treating the sunny walls in early fall usually does more than losing a shade tree.
Do Asian lady beetles bite?
They can pinch, especially when crowded indoors, but it rarely breaks the skin. Some people are sensitive to the beetles or to the fluid they release, and allergies to them have been reported. Vacuuming is safer than picking them up.
Can stink bugs damage my garden?
They can. Brown marmorated stink bugs feed on many fruits and vegetables, including tomatoes, peppers and apples, leaving pitted or discolored spots. Garden damage is a separate issue from the fall invasion of your house, which is only a nuisance.
Is it too late for a treatment once the bugs are inside?
For this winter, mostly yes. An exterior treatment works by stopping the bugs before they enter, so once they are in the walls, vacuuming and spot-sealing are the best options. Make a note to book next year's treatment for late August or early September.
Sources
- Iowa State University Extension and Outreach – Horticulture and Home Pest News
- U.S. EPA – Introduction to integrated pest management
- U.S. EPA – Introduction to pesticide labels
This guide is general information for Scott County homeowners, not a substitute for an on-site inspection. Prices are estimates. See our disclaimer.