Brown marmorated stink bugs, yellowjackets, mice, rats and squirrels all look for shelter or food as Baltimore's nights cool in September and October. Stink bugs gather on sunny walls and slip into gaps; rodents follow warmth into cellars and party walls. Sealing entry points early is the most reliable defence.
Key takeaways
- Autumn pests are driven by the same thing: falling temperatures push insects and animals toward warm, dry shelter, and houses offer plenty.
- Brown marmorated stink bugs often gather on sunny, south- and west-facing walls on warm afternoons in autumn, then crawl into soffits, window frames and attics to overwinter.
- Stink bugs do not breed indoors, bite or damage the structure; the trouble is their numbers, the odour when crushed and the way they reappear on warm winter days.
- Yellowjackets are often at their most aggressive in September and October, and a nest in a wall void or ground burrow should be left to a technician.
- In attached rowhouses, mice and rats often move through shared walls and cellars, so sealing one house may not hold unless the entry points along the run are dealt with too.
- A fall sealing and inspection visit, carried out before the first cold nights, is usually easier and calmer than removing animals from walls in December.
Why do pests come inside in the fall?
Pests come inside in the fall because shelter, warmth and food get scarce outdoors. Stink bugs seek crevices to overwinter, yellowjacket colonies run short of natural food and turn to human food, and mice and rats move toward heat. A house is a warm, dry, protected place with plenty of small openings.
The timing follows the thermometer. Baltimore summers are hot and humid, and by the time nights fall into the cooler range, insects that have been feeding all summer start looking for somewhere to wait out the cold. Winter lows at BWI average in the low to mid 20s F in January, which is cold enough that animals without a shelter do not do well.
Houses leak. A modern drafty window, a gap around a pipe, a loose piece of siding, a missing chimney cap and an unscreened attic vent are all invitations. In Baltimore's older rowhouses, shared walls, rooflines and cellars add more routes, because a gap in one house can connect to the next.
The table below summarises the main fall invaders, what draws them and where they usually enter. It is a general guide, not a forecast, and individual houses vary.
| Pest | What draws it in | Where it often gets in | First sign you notice |
|---|---|---|---|
| Brown marmorated stink bug | Shelter to overwinter | Soffits, window and door frames, siding gaps, attic vents | Clusters on sunny walls and windows |
| Yellowjackets | Food scarcity, trash, sweet drinks | Wall voids, eaves, ground burrows near foundations | Many wasps entering one small hole |
| Mice | Warmth and food | Gaps around pipes and utility lines, party walls, cellar doors | Droppings, scratching at night |
| Norway rats | Food and shelter near alleys and trash | Cellar gaps, broken vents, damaged foundations | Gnawing, burrows and droppings near the back of the house |
| Squirrels and raccoons | Warm, dry attic or chimney space | Chimney caps, soffits, roof edges | Scratching or thumping overhead at dawn or dusk |
What are brown marmorated stink bugs and why are they so common in autumn?
The brown marmorated stink bug is a shield-shaped, mottled brown insect, about the size of a dime, that overwinters in sheltered cracks. In autumn it gathers on sunny walls in numbers, then slips into soffits, window frames, attics and wall voids. It does not bite or breed indoors, but it releases a strong odour when crushed.
Stink bugs are an introduced species that has settled across the Mid-Atlantic, and Baltimore homes are now one of the places they reliably show up. On warm afternoons in September and October you may see dozens on a south- or west-facing wall or around a window. By evening they may be gone, tucked away into a gap you cannot see.
Inside, they are mostly a nuisance. They come out slowly on warm days in winter, drawn to lamps, and they wander across ceilings and windows. They do not eat wood, fabric or food, and they do not bite. The smell they release when handled or crushed is the main complaint.
The best control is exclusion, which means sealing gaps before they enter. A technician looks at the same points you can check yourself: caulking around windows and doors, torn screens, gaps under siding, the gaps where utilities enter, and attic and soffit vents. Our stink bug control page covers what a fall visit typically involves.
How do I get rid of stink bugs without making a mess?
Vacuum them up with a hose attachment and empty the bag outdoors, or sweep them into a bucket of soapy water. Do not crush them, because that releases the odour. Sealing the entry points is what stops the next wave, since killing the ones you see does not keep more from entering.
A vacuum is the least smelly option, though the machine may carry the odour for a while, so a dedicated old vacuum or a bag you can throw out is handy. A small jar of soapy water under a lamp works on the ones that fall to the floor.
Zapping or spraying them one by one is a losing game in a bad year, because the supply of insects outside is far larger than the number inside. Repellent sprays on window frames sometimes slow them down but do not replace sealing. If a house sees large numbers each year, a technician can both seal and apply a treatment to the exterior surfaces where they gather.
Do-it-yourself sealing checklist, from the ground up:
- Replace damaged window and door screens and repair weatherstripping.
- Caulk gaps around window and door frames, siding joints and fascia boards.
- Cover attic, soffit and gable vents with fine mesh.
- Seal gaps where cables, pipes and AC lines enter the house.
- Check chimney caps and dryer vents for damaged covers.
Why are yellowjackets and wasps more aggressive in September and October?
Yellowjacket colonies reach their largest size in late summer and autumn, while natural food gets scarcer. Workers then scavenge for sugar and protein around bins, picnic tables and open drinks, and they defend the nest more fiercely. A large nest should be treated by a technician rather than by a homeowner with a can of spray.
Paper wasps and yellowjackets build quietly in spring and summer, and by September the colony can number in the hundreds. Late-season workers spend less time hunting insects and more time at sweet food, which is why they gather around soda cans, fruit and trash.
In Baltimore, nests turn up in eaves and porch ceilings, in wall voids behind rowhouse cornices and in ground burrows beside foundations and under steps. A nest in a wall void is the one that causes the most trouble, because workers may come indoors through light fixtures and gaps around pipes.
If you see a steady line of wasps going in and out of one small opening, mark the spot from a distance and keep children and pets away. Spraying an enclosed nest from close range can send wasps into the house. A technician can treat the nest and seal the entry afterward. See wasp and hornet nest removal. Wasp colonies die out with the first hard frosts, but the wasps are most dangerous before that.
Why do mice and rats move indoors as the weather cools in Baltimore?
Mice and rats move toward warmth, food and nesting material as nights cool. In Baltimore's attached rowhouses, they can travel between houses through shared walls, cellars and rooflines. A gap the width of a pencil can admit a mouse, so the fix is sealing the whole run of entry points, not just trapping the ones you hear.
Norway rats are the city's signature pest, tied to rear alleys, trash and waterfront and stream corridors. In 2015, Baltimore's Department of Public Works reported that its Rat Rubout crew had grown to 16 technicians, up from eight, with a plan to treat each alley on a 20-day cycle. That is a 2015 report rather than a current staffing figure, but it shows that the city itself treats rats as an ongoing problem, and autumn is when they start to test houses more actively. Our guide on rats in Baltimore alleys goes deeper.
House mice are the more common indoor visitor. They squeeze through very small gaps and are happy to nest in wall insulation and behind cellar boxes. Droppings, gnawed packaging, a stale smell and scratching in the walls at night are the usual signs. Mouse control and rat control both begin with finding how they entered.
In a rowhouse block, a single treated house can still be re-invaded from next door, because the party wall is a common route. Rowhouse pest proofing is about closing those routes along the run, and our guide on rowhouse party walls and pests explains why neighbours working together matters.
What about squirrels, raccoons and other attic visitors?
Squirrels and raccoons look for warm, dry attic and chimney space in autumn, often entering through loose soffits, damaged roof edges or uncapped chimneys. Scratching at dawn or dusk usually means squirrels. Trapping alone rarely works; the entry point must be repaired, and young animals should be checked for before any sealing.
Detached houses near parks and stream valleys, such as those in Roland Park, Mount Washington and Towson, often have mature trees whose branches reach the roof. A branch within a jump of the roofline is a bridge. Trimming branches back and capping chimneys are cheap and effective steps.
Do not seal an opening until you know no animals are inside. Closing a gap on an adult can trap it in the house, and closing it on a mother can leave her young to die in the wall. A technician checks the attic, identifies the species and sets up a humane exit before the repair is made. Our squirrel and raccoon removal page describes the usual steps.
If you hear something overhead, note the time of day and the sound. A light, scurrying noise at dawn and dusk suggests squirrels, a heavier thump at night suggests a raccoon, and scratching inside walls at night suggests mice or rats.
What should I seal, trim and clean up before the first cold nights?
Work from the roof down. Cap the chimney, screen vents, repair soffits and fascia, and trim branches back from the roof. Then caulk windows and door frames, seal gaps around pipes and utility lines, install door sweeps and fix cellar door gaps. Finish with food storage, trash and leaf piles away from the house.
Most of the work is simple and cheap, and it is easiest in early autumn, while the weather is dry and the pests are not yet inside. A morning on a ladder and an afternoon in the cellar can prevent the problems that follow. Because every house is different, a visit by a technician can still find gaps that are hard to see from the ground.
Food and water matter too. Store pet food and birdseed in metal or hard plastic containers, use trash cans with tight lids, and clean up fallen fruit. In alleys and yards, stacked wood, clutter and leaf piles give rats and mice cover close to the building.
If you are tackling the job yourself, use steel wool or copper mesh with caulk for small holes, and hardware cloth or metal flashing for larger ones. Foam alone does not stop rodents, which chew through it. If you want a hand with a particular spot, call (855) 974-4733 and a local technician can talk through what you are seeing.
| Area | What to check | Typical fix |
|---|---|---|
| Roof and chimney | Chimney cap, soffits, fascia, roof edges | Install or repair cap, replace damaged boards, trim branches |
| Attic and eaves | Vents, gable openings, torn screens | Cover with metal mesh or hardware cloth |
| Windows and doors | Weatherstripping, caulk, screens, door sweeps | Replace worn seals and screens, recaulk |
| Walls and siding | Utility penetrations, siding gaps, cracks | Seal with caulk, mesh and metal flashing |
| Cellar and foundation | Pipe gaps, cellar door, cracked masonry, drains | Seal gaps, repair cracks, keep the area dry |
| Yard and alley | Trash, stacked wood, leaf piles, pet food | Lidded bins, raise wood off the ground, clear clutter |
Frequently asked questions
Do stink bugs damage my house or furniture?
No. Brown marmorated stink bugs do not eat wood, fabric or stored food, and they do not breed inside. The problems are their numbers, their smell when crushed and their habit of reappearing on warm winter days after hiding in wall voids.
Why do I see stink bugs in the house in winter?
Stink bugs that settled into walls or attics in autumn often wake on warm winter days, and the heat of the house draws them toward windows and lamps. They are not new arrivals. Vacuum them up and plan to seal the gaps next fall.
Will pest control keep stink bugs out for good?
No treatment gives a guarantee. A technician can seal gaps and treat exterior surfaces where stink bugs gather, which usually reduces the number that get inside. Neighbouring trees, fields and houses mean new bugs can arrive each autumn.
When should I seal my house for fall pests?
Early autumn is the usual window, before nights turn cold and before insects and rodents begin moving in. Late summer is not too early. If you already see stink bugs on the walls or hear scratching, do not wait.
How do I know if I have mice or rats in my walls?
Scratching at night, droppings behind appliances or in cellar corners, gnawed packaging and a stale smell are the common signs. Rats tend to leave larger droppings and gnaw marks, and are more likely to be linked to burrows near the back of the house.
Sources
- Wikipedia – Brown marmorated stink bug
- Baltimore City Department of Public Works – Bigger, Faster Rat Rubout Crew Takes to the Alleys (2015)
- Maryland Manual On-Line – Maryland at a Glance: Weather
This guide is general information for Baltimore City homeowners, not a substitute for an on-site inspection. Prices are estimates. See our disclaimer.