Philadelphia's humid four-season climate sets when most pests are active. Termite swarms usually appear from February to June, stink bugs seek shelter in September and October, and mosquitoes and ticks are active outdoors from spring into fall. The dates shift with the weather, so use this as a set of likely windows.
Key takeaways
- Eastern subterranean termite swarms usually appear in Pennsylvania between February and June, and shed wings are often the only sign.
- Brown marmorated stink bugs look for winter shelter from September to October, gathering on warm sunny walls and entering gaps near trim.
- Mosquitoes and ticks are active outdoors from spring into fall, and blacklegged ticks have been found across the city.
- Cold weather pushes rodents toward heated buildings, so fall is the season to check seals, trash areas and cellar openings.
- The calendar shows likely windows, not fixed dates, so a warm spring or a cold early summer moves the timing.
What does Philadelphia's climate change about the pest calendar?
Philadelphia has a humid four-season climate. National Weather Service normals for 1991 to 2020 put the July average high near 88 degrees Fahrenheit and the January average high near 41, with about 44 inches of precipitation a year. Warmth and moisture decide when most pests are active.
The city is hot and humid in summer, and the moisture carries into basements, kitchens and bathrooms in the warm months. Insects that need humidity do well in that period, and many indoor pests become harder to keep out once the weather turns warm and the windows stay open.
Winter brings the opposite problem. Cold outside pushes mice, rats and some insects toward heated walls and cellars, where they can stay active through the season. That is why the calendar for an older rowhouse runs in both directions: the work of keeping pests out happens in fall, and the work of spotting them happens through the year.
The climate figures come from the National Weather Service, and they describe the long-run average rather than any single year. A cold spring or a warm February will move every date on this calendar, so treat the months as a plan for when to look, not a promise of when something will appear.
| Season | What to watch for | Where it shows up |
|---|---|---|
| Late winter to June | Eastern subterranean termite swarms | Near doors and windows; shed wings on sills |
| Spring through fall | Mosquitoes and ticks active outdoors | Yards, parks and wooded edges |
| Summer | Heat and humidity often increase indoor cockroach and ant activity | Kitchens, bathrooms and cellars |
| September to October | Brown marmorated stink bugs seek winter shelter | Warm sunny exterior walls and gaps near trim |
| Fall and winter | Rodents move indoors as the weather cools | Wall voids, cellars and under sinks |
| Late summer and fall | Spotted lanternfly adults | Trees, fences and parked vehicles |
When do termite swarms appear in Philadelphia?
In Pennsylvania, eastern subterranean termite swarms usually happen from February to June. Winged reproductives gather near doors and windows, and the wings they shed are often the only sign that a colony is nearby.
A swarm is the reproductive stage of a colony, and it is a short event. Swarmers flutter toward light, land near windows and doors, and drop their wings within a short time. A swarm means termites exist nearby. It does not, on its own, tell you how large the colony is or how much wood it has reached.
If you see swarmers, collect a few, photograph them beside a coin and do not spray the area before a technician sees it. Ask for a termite inspection that looks at the foundation, the sill plates and the cellar walls. The termite swarm season guide covers identification in more detail.
Swarms in older Philadelphia houses deserve closer attention, because timber framing that sits on or near masonry and soil gives a colony a way in. A swarm in a cellar or on a ground-floor sill is a clear reason to schedule the inspection early.
What are mosquitoes and ticks doing from spring into fall?
Mosquitoes and ticks are active outdoors across much of the year in Philadelphia. The state tracks West Nile virus, which moves between Culex mosquitoes and birds, and blacklegged ticks have been found at many sampled sites in the city.
The Pennsylvania Department of Environmental Protection reports that in 2024 the state recorded 60 human West Nile cases across 20 counties, with Philadelphia and Montgomery County among the highest, and 3,035 positive mosquito samples. Those figures describe how widely the virus circulated that year, and they are a reason to take mosquito activity seriously in the yard.
The city's Health Department monitors mosquitoes and, after positive samples, has treated adult mosquitoes in parks and sewer inlets. Residents can reduce breeding sites by emptying standing water in containers, gutters and planters, and by keeping grass and brush short near the house. The mosquito control service covers larger yards and shaded areas.
Blacklegged ticks have been found at 29 of 41 sampled city sites since 2013. About 40 percent of pooled tick samples tested positive for the Lyme disease bacterium over the 2013 to 2025 period. Lone star ticks were first identified in the city in 2020. Check people and pets after time in wooded or grassy areas, and see the tick control service for property work.
What do rodents and cockroaches do as the seasons turn?
Cold weather often pushes rodents into heated buildings, and that is when indoor signs commonly appear. Cockroaches can stay active indoors year round in heated buildings, and their numbers track moisture and food rather than the calendar.
Norway rats are the most common rodent in the city, according to a Health Department program assessment from 2015. They travel along walls, cellars and alleys, and fall is when many people first notice droppings or noise in the wall. Sealing gaps and tidying trash before the cold arrives is the most useful step on this part of the calendar.
Cockroaches do not follow the same pattern. A heated apartment gives German cockroaches warmth year round, so the work is less about the season and more about reducing the water, grease and clutter that keep them there. The cockroach control service covers that work, and the German cockroach apartment guide explains where they hide.
Mice are often reported in fall for the same reason as rats: the first cold nights send them looking for shelter. A gap the width of a pencil is enough for a mouse, so a fall check of the cellar and the foundation is a good use of an afternoon.
When do stink bugs and spotted lanternflies show up?
Brown marmorated stink bugs look for winter shelter from September to October, gathering on warm sunny exterior walls and entering gaps near trim and baseboards. They are a nuisance rather than a structural pest. Spotted lanternfly is a plant pest that adults are most visible in during late summer and fall.
Stink bugs gather on the sunny sides of houses before they move into the walls. They enter through gaps near window trim, soffits and baseboards, and then appear indoors on warm days over the winter. Penn State Extension notes that they do not cause structural damage, so the concern is the nuisance and the smell when they are crushed.
Sealing the gaps around trim, vents and utility penetrations before September is the best time to act. Vacuuming the insects up is simpler than crushing them. The stink bug control service handles the gap work and the removal in one visit.
Spotted lanternfly is a different kind of problem. It feeds on plants and is most visible on trees, fences and vehicles in late summer and fall. The spotted lanternfly control service and the spotted lanternfly guide cover identification and reporting.
How should a homeowner use this calendar?
Use the calendar to decide when to inspect, seal and ask questions, not as proof that a pest is or is not present. Spring is the time for a termite check and a yard review, and fall is the time to seal gaps and tidy trash before the weather turns.
A practical year starts in late winter with a look at the cellar, the foundation and any wood near the soil, because the swarm window opens before most people think about inspections. Spring brings mosquito and tick checks in the yard. Early summer is the time to reduce moisture in kitchens and bathrooms, and to look at the trash and food areas before heavy use starts.
Fall is the most important season for sealing. Check the gaps around pipes, vents, and the door thresholds, and look at the cellar openings. Stink bugs and rodents will test those gaps in the cold months, so an inspection in September or October pays off.
If a pest appears outside its usual window, do not assume it is a different problem, and do not assume it is the same one. Note the date, the place and what you saw, and ask a technician to identify it. A calendar is a guide to what to look for, and the identification is what decides the next step.
Frequently asked questions
When is the best time to call about termites?
Swarm season, February to June, is when people most often notice termites. A swarm is a sign, not a full picture. A technician can inspect the structure at any time, whether or not you have seen wings, and the inspection tells you whether a colony is in the building.
Do ticks stop in winter in Philadelphia?
Ticks are generally least active in the coldest months but can be active on mild days. The timing is not fixed, so treat tick checks as a habit for the whole warm period and check people and pets after time in grassy or wooded areas.
Why do mice show up in October?
Cold nights push mice indoors looking for warmth and food, and fall is when many people first notice them. The usual cause is a gap large enough to pass through, which is why a sealing check belongs on the fall list.
Are stink bugs a health risk?
The main concern with brown marmorated stink bugs is nuisance rather than structural damage, according to Penn State Extension. They release an unpleasant smell when crushed, so vacuum them up and seal the gaps they use to get inside.
What should I do if I think I see a lanternfly?
Photograph it with a coin for scale, note the place and date, and report the sighting to the state Department of Agriculture. Check vehicles and outdoor items before moving them out of any quarantine area, and confirm the current county list first.
Sources
- National Weather Service - Philadelphia climate normals
- Penn State Extension - Eastern subterranean termites
- Pennsylvania Department of Environmental Protection - Mosquitoes
- City of Philadelphia Department of Public Health - Tickborne diseases advisory
This guide is general information for Philadelphia County homeowners, not a substitute for an on-site inspection. Prices are estimates. See our disclaimer.