Baltimore pests follow the weather. Termite swarms and ants typically start in spring, mosquitoes and wasps build through summer, and stink bugs, yellowjackets and mice push toward the house in autumn. Roaches, bed bugs and rowhouse mice stay active indoors all year. Timing shifts with each warm or cold spell.
Key takeaways
- Baltimore has hot, humid summers and cold winters, so most outdoor pests have a clear active season, while indoor pests such as German cockroaches and bed bugs can be a problem in any month.
- Spring is when termite swarmers and carpenter ants typically become visible, usually on the first warm, damp days.
- Late summer is often the peak for tiger mosquitoes and for yellowjacket and paper wasp nests; autumn brings stink bugs onto sunny walls and rodents toward cellars as nights cool.
- Rowhouse walls, rear alleys and cellars give rats and mice a way indoors through the cold months, so winter is not a pest-free season in the city.
- Month bands in this guide are typical patterns, not dates: a mild winter or a wet spring can move a pest's arrival by weeks.
- A calendar tells you what to watch for; it cannot tell you what is already in your walls, so a sighting out of season is worth a call.
What pests are active in Baltimore each month?
Baltimore's pest year typically runs in four waves. Spring brings termite swarmers, carpenter ants and trailing ants. Summer brings mosquitoes, wasps and heavy ant foraging. Autumn brings stink bugs, aggressive yellowjackets and rodents moving in. Winter pushes mice and rats into walls and cellars, while roaches and bed bugs carry on indoors.
The Maryland Manual's weather records, taken at BWI airport just outside the city, put average annual precipitation at about 43.6 inches, with July as the warmest month (highs averaging in the mid to upper 80s F) and January as the coldest (lows averaging in the low to mid 20s F). That is a wide swing, and it is why Baltimore pests are so seasonal: insects wake, breed and hunt for shelter on that swing.
No calendar can promise a date. Pest activity depends on temperature, rain and food, and the harbour, the stream valleys and the heat held by brick and asphalt all nudge things around. Treat the month bands below as a guide to what is typical, then adjust for the year you are having.
| Typical months | Pests most often noticed | What it usually means for a house |
|---|---|---|
| Jan-Feb | Mice, rats, cockroaches, bed bugs; stink bugs waking on warm days | Indoor problems carry on; scratching in walls and droppings in cellars are common signs |
| Mar-Apr | Carpenter ants, odorous house ants, early termite swarmers; rats breeding through mild spells | First warm, damp days can bring winged insects indoors |
| May-Jun | Termite swarmers, ants, new wasp nests, first mosquitoes | Peak window to tell swarming termites from flying ants |
| Jul-Aug | Tiger mosquitoes, yellowjackets, paper wasps, American cockroaches after heavy rain, bed bugs | Standing water, gutters and eaves need attention; travel and moving season raises bed bug risk |
| Sep-Oct | Yellowjackets at their most aggressive, stink bugs on sunny walls, mice and rats moving in | Time to seal gaps before nights turn cold |
| Nov-Dec | Mice and rats in walls and cellars, squirrels in attics, bed bugs after holiday travel | Quiet noises at night usually mean something has moved in |
What shows up in winter, January and February?
In January and February, Baltimore's active pests are the ones already indoors: mice in rowhouse walls, rats using cellars and alleys, German cockroaches in warm kitchens and bed bugs in bedrooms. Cold does not clear them out. It mostly removes outdoor competition for food and sends rodents toward heat and shelter.
Rowhouses matter here. Attached houses share walls, rooflines and often cellars, so a mouse that gets in next door can travel along the party wall into yours. A technician working in a rowhouse looks at the whole run of the block: the rear alley, the cellar door, gaps around pipes and the line where the wall meets the floor.
Warm days in the middle of winter can wake stink bugs that settled into wall voids in autumn, and you may see them crawling slowly across a window. Mild spells can also keep rats breeding. None of this is dramatic, but it explains why a house that felt pest-free in December can feel crowded by February.
If you hear scratching, find droppings or smell something stale in a cellar, winter is a good time to deal with it. Nothing is hidden by foliage, and a crew can see the entry points more clearly. Rat control and mouse control both start with finding how they got in, not just setting traps.
What shows up in spring, March through June?
Spring is when Baltimore's wood-related pests become visible. Carpenter ants and odorous house ants resume foraging in March and April, and eastern subterranean termite swarms typically peak in May and June on warm, damp days. Mosquito season starts, wasps begin new nests and rats breed through mild spells.
The first warm, damp afternoon of spring is when many homeowners notice winged insects indoors for the first time. A cluster of them near a window sill or a basement door is worth looking at carefully. Termite swarmers and flying ants look alike at a glance, and the difference matters because only one of them is eating your house. Our guide to termite swarm season in Maryland shows how to tell them apart.
Carpenter ants are a separate story. They favour wood that has been damp, which in Baltimore usually means porch posts, window frames, and framing near a leaky roof or flashing. Detached houses with mature trees, such as those in Roland Park and Govans, tend to give them more places to nest. See carpenter ant control for what a technician looks for.
Mosquito season starts quietly. Female tiger mosquitoes lay eggs in tiny containers: a clogged gutter, a buried bucket, a saucer under a flower pot. Emptying them weekly from May on is the cheapest control there is.
What shows up in summer, July and August?
July and August are Baltimore's peak for Asian tiger mosquitoes, yellowjackets and paper wasps. Heat and humidity speed up every life cycle. American cockroaches may move indoors after heavy rain, ants forage hard, and bed bug cases often rise with moving and travel. Summer is the busiest month pair for outdoor pest calls.
Wasps are small in spring and enormous by late summer. A paper wasp nest starting under a porch rail in May can hold a crowd by August, and yellowjackets nesting in a wall void or a ground burrow can be a hazard to anyone mowing near them. A nest the size of a fist is a good time to act. A nest the size of a football is not a do-it-yourself job; see wasp and hornet nest removal.
Mosquitoes follow water. Tiger mosquitoes breed close to where you live, in small containers and clogged gutters. Along the harbour, Back River and the Patapsco, tidal-marsh mosquitoes add a second kind of biting insect. Mosquito-borne illness such as West Nile virus exists in the region, which is one reason many households take mosquito control seriously in the second half of summer.
American cockroaches are the large reddish-brown ones, often found in cellars and near drains. Heavy summer rain can push them up and into kitchens. German cockroaches, the small ones that live in kitchens and bathrooms, do not depend on weather at all, but heat can speed up their breeding. If you see roaches in daylight, the population is usually large. Cockroach control covers both.
What shows up in autumn, September through October?
Autumn brings Baltimore's overwintering and cold-weather pests: brown marmorated stink bugs gather on sunny walls, yellowjackets turn their most aggressive as colonies peak, and mice and rats begin moving toward cellars and walls as nights cool. September and October are the best months to seal gaps before the cold arrives.
Stink bugs do not breed indoors or eat your house, but they enter in numbers, looking for a crack to spend the winter in. Their habit is to line up on south- and west-facing walls on warm afternoons, then slip into soffits, window frames and attic gaps. Our guide to stink bugs and fall invaders in Baltimore covers the sealing work and the stink bug control visit.
Yellowjackets are often at their most irritable now, with colonies large and food sources shrinking. They are drawn to open soda cans, trash and fallen fruit, which is a common reason they end up around picnic tables and bins.
Rodents begin their move indoors as soon as nights get cool. In a rowhouse block, the route is often the same: from the rear alley to a gap around a utility line, then up through a cellar or along the party wall. Rowhouse pest proofing is about closing those routes across the whole run rather than one house.
What shows up in late autumn and early winter, November and December?
In November and December, mice and rats are typically moving into rowhouse walls and cellars, squirrels are using attics and chimneys, and holiday travel raises the chance of bringing home bed bugs. These are quiet pests, so the first sign is often noise at night or droppings, not a visible insect.
Squirrels and raccoons look for warm, dry, sheltered spaces, and a gap in a chimney cap or a loose soffit board is enough. Scratching in the early morning or at dusk usually means squirrels, and heavier thumping at night can mean a raccoon. Squirrel and raccoon removal is the right call for these, because trapping without sealing the entry point just invites the next one.
Holiday travel and visiting guests are the other November and December risk. Bed bugs hitch rides in luggage and coats. Keep suitcases off beds, and check hotel mattress seams on the way. If you find them at home, bed bug treatment is the next step, and shared walls in apartments and rowhouses can complicate it.
Which pests are a problem all year in Baltimore?
German cockroaches and bed bugs live inside heated buildings and can be a problem in any month, and mice and rats in rowhouse walls and cellars do not take a season off. Seasons change when you notice these pests, not whether they are there. Indoor pests are driven by food, water and warmth, not the weather outside.
Orkin's 2025 list of its top 50 bed bug cities ranked the Baltimore area No. 11, based on its own treatment counts from May 2024 to May 2025. That is one company's data and does not measure how many homes are infested, but it fits what residents in dense rentals and shared-wall housing often report: bed bugs are a recurring problem, not a seasonal one.
Norway rats are the other all-year pest. In 2015, Baltimore's Department of Public Works reported that its Rat Rubout crew had grown to 16 technicians, up from eight, and planned to inspect and treat every alley on a 20-day cycle instead of 40, to interrupt the roughly three-week gestation of Norway rats. That is a 2015 figure, not a current one, but the logic still holds: rats breed fast, and the control has to keep pace with them. See our guide to rats in Baltimore alleys for what a homeowner can control.
The practical upshot is that general pest control pays off year-round for houses with a history of problems. A quarterly general pest control visit lines up well with the seasonal waves above.
How can I use this calendar to get ahead of pests?
Work one season ahead. Check for termite and ant entry points in winter, empty standing water and fix gutters in spring, inspect for wasp nests in early summer, and seal cracks around utilities and windows in early autumn. A small fix before the pest arrives is almost always cheaper and calmer than a call after.
A calendar is most useful when it drives a short checklist. Pick the pest that has bothered your house before, and use its season as your deadline. If ants return every May, look in March. If mice arrive in November, seal in September.
For rentals, the same calendar works as a conversation starter with a landlord or building manager: point to the season, name the pest, and ask for an inspection before it gets worse. Our guide to renting in Baltimore and who handles the pests walks through the practical steps.
If you are unsure what you are looking at, take a clear photo of the insect or the droppings, note the room and the time, and call (855) 974-4733. A technician can usually narrow it down from a description, and the price is confirmed with you before any work starts.
- Winter: inspect cellars and attics, and note any scratching.
- Spring: fix leaks, trim vegetation off walls, empty containers weekly.
- Summer: check eaves and porches for nests; clean gutters.
- Autumn: seal gaps around pipes, vents and windows before nights turn cold.
Frequently asked questions
When is termite swarm season in Baltimore?
Eastern subterranean termite swarms typically appear in spring, often May and June, on warm days after rain. The exact timing shifts from year to year. Winged insects inside the house, or discarded wings on a sill, are worth a closer look at any time.
When do mosquitoes get bad in Baltimore?
Mosquito season typically starts in late spring and builds through July and August, when heat speeds up their breeding. Tiger mosquitoes breed in small containers and clogged gutters, so emptying water weekly is the simplest control. Near the harbour and tidal creeks, marsh mosquitoes add to the count.
Do pests go away in winter in Baltimore?
Outdoor insects mostly slow down, but pests that live indoors do not. Mice, rats, German cockroaches and bed bugs are active all winter. Rodents often move into rowhouse walls and cellars as the weather cools, so winter noises should be checked rather than waited out.
Why do I see stink bugs on my house in the fall?
Brown marmorated stink bugs gather on sunny walls in autumn looking for crevices to overwinter in. They are harmless to people and structure but can enter in large numbers. Sealing gaps around windows, siding and utility lines before they arrive is the most reliable prevention.
What is the best time of year to schedule pest control in Baltimore?
Early spring and early autumn are the usual windows, because they come before the main waves of ants and termites in spring and rodents and stink bugs in autumn. Active problems such as roaches, bed bugs or rats are worth a call in any month.
Sources
- Maryland Manual On-Line – Maryland at a Glance: Weather
- Baltimore City Department of Public Works – Bigger, Faster Rat Rubout Crew Takes to the Alleys (2015)
- Seeking Alpha / Orkin – Orkin's 2025 Top 50 Bed Bug Cities list
- Wikipedia – Brown marmorated stink bug
This guide is general information for Baltimore City homeowners, not a substitute for an on-site inspection. Prices are estimates. See our disclaimer.