Every September and October the same thing happens. Spiders start turning up in bathtubs, crossing living room floors in the evening and sitting on walls where nothing was sitting a month ago.
The usual explanation is that they are coming inside to escape the cold. That explanation is mostly wrong, and the real one is more useful because it tells you what to do about it. If the numbers are genuinely high, spider control works on what is feeding them.
They were already in the house
The common house spider species are exactly that. They live their entire lives indoors, in basements, crawl spaces, garages, attics, wall voids and the undisturbed corners of rooms, and they do not come in from anywhere.
What changes in fall is not their location but their visibility. Spiderlings that hatched in spring have spent the summer growing, and by September they are large enough to notice. A spider that was a speck in June is a recognizable spider in October.
So the fall increase is partly just an increase in size. The population did not arrive, it grew up.
Most of the spiders people see indoors in fall have lived indoors their whole lives.
The males are out looking for females
This is the main driver, and it explains the specific pattern people see.
For most common house spiders, fall is the mating season. Males leave their webs and wander in search of females, covering a great deal of ground and crossing open floor to do it. Females stay put.
That is why the fall spider is almost always seen walking rather than sitting in a web, often across a floor or a wall, and frequently in the evening. It is also why they end up in bathtubs and sinks: a wandering male falls into a smooth sided basin and cannot climb out.
Once you know this, the behavior makes sense. The spider in the tub is not thirsty and did not come up the drain. It walked in, fell in, and is stuck.
What is not happening
Three beliefs are worth retiring.
They are not coming in from the cold. Spiders are not warm blooded and do not seek heat. Outdoor species handle winter outdoors, with antifreeze compounds in their body fluids, in egg sacs, or in sheltered spots like leaf litter and bark. A warm house is not a draw.
They did not come up the drain. Drain traps hold standing water that a spider cannot cross. Anything in a tub or sink came over the edge.
It is not about cleanliness. Spider numbers track food supply and harborage, not housekeeping. A tidy house with a damp basement and an insect problem will hold more spiders than a cluttered one without.
The food supply explains the rest
Where spiders settle is determined almost entirely by what there is to eat.
Fall concentrates insects in and around buildings. Stink bugs, cluster flies, boxelder bugs and lady beetles mass on sunny walls and push inside through gaps. Outdoor lighting pulls moths and flies to doorways through the warm evenings. Crickets and other ground insects move toward foundations.
Spiders follow all of it. A house with heavy insect pressure in October will have heavy spider pressure in October, and a persistent spider problem is usually an insect problem being reported under a different name.
That is the lever worth pulling, because reducing the food supply reduces the spiders in a way that killing individual spiders never does.
Spiders go where the insects are. The insect population drives everything.
Where they concentrate indoors
Different species want different conditions, which is why they sort themselves into predictable parts of a house.
Basements, crawl spaces and garages suit species that want damp and dark, and these are usually the highest density areas in a home.
Window frames, corners and ceilings in living space suit web builders catching flying insects.
Attics and storage areas suit species that want undisturbed space, and stored boxes provide exactly that.
Around exterior doors and lights, where insects gather at night, which is why a porch light is often ringed with webs by October.
What actually reduces them
- Cut the insect supply. Switch exterior lights to warm or yellow bulbs, which attract far fewer insects, and move lighting away from doors where practical
- Seal the gaps insects use: door sweeps, window seals, utility penetrations, vents and the joint where siding meets the foundation
- Clear vegetation and debris from against the walls, including mulch depth, shrubs touching siding, firewood and leaf piles
- Dry out the basement or crawl space, which reduces both the spiders and the insects they eat
- Remove webs regularly inside and out, with a vacuum or a brush. Egg sacs removed now are spiders prevented next spring
- Reduce storage clutter in basements and garages, which is harborage
- Treat the exterior before the fall wave, which is what a preventive service is timed around
The ones worth identifying
Almost every spider in a Pennsylvania home is harmless and is, on balance, doing useful work by eating other insects.
Two species get asked about constantly. The brown recluse is not established in Pennsylvania; the vast majority of suspected sightings are other brown spiders, most often one of the common house or grass spiders. Black widows do occur in the state but are uncommon, generally found outdoors in sheltered spots like woodpiles, meter boxes and under stored items, rather than wandering across a living room.
The practical advice is to photograph rather than guess, and to wear gloves when moving long stored material in a garage or shed.
Webs around lights and doorways mark where the insects are arriving.
When it is worth calling someone
An occasional spider is normal and not a sign of anything. What justifies a call is a pattern: spiders appearing daily in multiple rooms, webs rebuilding within days of being cleared, egg sacs in several locations, or a heavy concentration in a basement or garage.
Each of those usually points at a food supply or a moisture condition rather than at the spiders themselves, which is where the work goes. Where spiders actually spend the winter covers the seasonal side further.
Stewart Termite & Pest Control has handled Pittsburgh spider problems since 1987. Call 412-822-7610 or request an estimate online.
Frequently Asked Questions
Why do I see more spiders in the fall?
Mostly because males leave their webs to look for mates and wander across floors and walls, and because spiderlings that hatched in spring are now large enough to notice. They are not coming in from the cold.
Do spiders come indoors to escape the cold?
No. Spiders do not seek warmth, and outdoor species overwinter outdoors in leaf litter, bark and egg sacs. The ones indoors in fall mostly lived indoors all along.
Why are there spiders in my bathtub?
A wandering male fell in and cannot climb the smooth sides. They do not come up the drain, since the trap holds standing water they cannot cross.
Does having spiders mean my house is dirty?
No. Spider numbers track the insect supply and available harborage, so a clean house with a damp basement and heavy insect pressure will hold more than a cluttered one without.
How do I get rid of spiders for good?
Reduce what they eat. Switch exterior lights to warm bulbs, seal the gaps insects use, clear vegetation from the walls, dry out the basement and remove webs and egg sacs regularly.
Are there dangerous spiders in Pennsylvania?
Very few. The brown recluse is not established here and most suspected sightings are common house spiders. Black widows occur but are uncommon and usually outdoors in woodpiles and sheltered spots.