It is one of the most repeated pieces of household folklore: when the weather turns, spiders come inside to get warm. It makes intuitive sense and it is almost entirely wrong.
Spiders are not warm blooded and gain nothing from a heated room. What actually happens over winter is more interesting, and knowing it changes where you look and what you do. Where numbers are genuinely high, spider control addresses the conditions supporting them.
House spiders never left
The species most people find in a house are adapted to living in one. The common house spider, cellar spiders and several others complete their entire life cycle indoors and have no outdoor population to migrate from.
They live in basements, crawl spaces, garages, attics, wall voids, utility areas and the corners of rooms nobody disturbs. In winter they are still there, simply less active and less visible because the household is less likely to be poking around a basement in January.
So a spider found indoors in December did not come in for the winter. It has been there the whole time.
The common indoor species complete their whole life cycle inside a building.
Outdoor spiders stay outdoors
The species that live outside are built for the season and do not seek buildings.
Many survive as eggs, with the adults dying off in fall after the female produces an egg sac that overwinters in a sheltered spot and hatches in spring. Orb weavers, the big web builders of late summer, work this way, which is why they vanish in November.
Others overwinter as adults or juveniles in a dormant state, tucked into leaf litter, under bark, in rock piles, under stones and logs, and in soil cavities. Their body fluids contain compounds that prevent ice forming, and they tolerate freezing conditions that would be fatal to a warm weather insect.
A heated house offers them nothing they need, and the dry air inside is actively worse for them than damp leaf litter.
What people are actually seeing
Three things get mistaken for a seasonal migration indoors.
Fall mating season. In September and October males leave their webs to find females and wander across floors and walls. This is the real cause of the fall spike, it happens before any cold weather, and it tails off well before winter.
Accidental entry. An individual spider wanders in through an open door, a gap under a threshold or on firewood carried inside. That is a handful of spiders a season, not a migration.
Noticing what was always there. People spend more time indoors in winter, go into basements for storage, move boxes and decorations, and find spiders that have lived in those corners for months.
Where they are in your house right now
If you want to find the indoor population rather than wait for it to find you, these are the places.
- Basement corners, joists and the sill area, which is usually the densest part of a house
- Crawl spaces, for the same reasons and with less disturbance still
- Garages, particularly behind stored material and along the door tracks
- Attics and storage areas, inside and between boxes
- Behind and under appliances, especially a basement washer, dryer or utility sink
- Window wells and frames, where insects accumulate
- Under decks and porches, which is outdoor but sheltered and heavily used
Undisturbed, slightly damp and close to a food supply is what they want.
Why winter is a good time to deal with it
Counterintuitively, the cold months are a sensible time to reduce next year’s spider population, for two reasons.
First, egg sacs. Removing an egg sac now removes dozens to hundreds of spiderlings that would otherwise hatch in spring. They are papery, off white and tucked into corners, webs and crevices, and a vacuum handles them.
Second, access. Basements and garages are easier to work through when the storage can be moved and the yard work is not competing for time, and sealing and drying work done in winter is in place before the spring insect wave arrives.
Why the dry air indoors matters
One detail explains a lot of what people observe in winter without realizing it.
A heated house in January runs very dry, and spiders lose water through their cuticle like any other arthropod. That is why the indoor winter population concentrates in the damp parts of a building, the basement, the crawl space and around a utility sink, rather than in the living space.
It is also why a spider found upstairs in midwinter is often found dead or dying in a bathtub or sink. It went looking for moisture, which is the one thing it genuinely needs, and the smooth basin trapped it. Not a spider coming in from outside, but one already inside running short of water.
The real lever is the insect supply
Spiders are predators, and the population a building supports is set by what there is to eat. A house with a persistent spider problem almost always has a persistent insect problem underneath it.
That is why killing spiders individually never changes much: the food supply is unchanged, so the population rebuilds. Reducing the insects reduces the spiders by itself.
- Switch exterior lights to warm or yellow bulbs, which draw far fewer insects to doorways
- Seal the gaps insects use: door sweeps, window seals, utility penetrations, vents and the siding to foundation joint
- Dry out the basement or crawl space, which cuts both the spiders and their prey
- Clear mulch, leaf piles, firewood and shrubs from against the walls
- Remove webs and egg sacs regularly, indoors and out
- Reduce stored clutter in basements, garages and attics
Firewood deserves a mention
Firewood is the one genuine route by which outdoor spiders arrive indoors in winter, and it is also how a lot of other things arrive.
A stack sitting outside through fall is prime overwintering habitat, holding spiders, beetles, ants and other insects in the bark and between the splits. Carrying an armful inside brings all of it into a warm room, where the warmth can wake it up.
The fix is simple: store wood off the ground and away from the house, bring in only what will be burned that day, and do not stack it indoors to season.
Wood stacked outside holds overwintering spiders and insects, and carrying it in brings both.
What to expect through the year
The pattern repeats annually. Spring brings egg sacs hatching and a wave of tiny spiderlings. Summer brings growth and the first noticeable webs outdoors. Fall brings the mating season, the wandering males, and the peak of indoor sightings. Winter brings a quiet indoor population that most households never look at.
Which means the fall spike that prompts most calls is a seasonal behavior rather than an invasion, and the work that reduces it happens in the months on either side. Why spiders appear every fall covers that side in detail.
Stewart Termite & Pest Control has worked on Pittsburgh spider problems since 1987. Call 412-822-7610 or request an estimate online.
Frequently Asked Questions
Do spiders come inside for the winter?
Not really. Spiders are not warm blooded and gain nothing from a heated room. The ones found indoors in winter are usually house species that live there year round.
Where do outdoor spiders go in winter?
Many die off after leaving an egg sac that overwinters and hatches in spring. Others go dormant in leaf litter, under bark and stones, and in soil cavities, with compounds in their body fluids that stop ice forming.
Why do I find spiders indoors in December?
They have been there all along. People spend more time indoors and in basements in winter, moving boxes and storage, which is where the indoor population lives.
Is winter a good time to deal with spiders?
Yes. Removing egg sacs now prevents a spring hatch, and sealing and drying work done in winter is in place before the spring insect wave arrives.
Can firewood bring spiders inside?
Yes, and it is the main genuine route in winter. Store wood off the ground away from the house and bring in only what will be burned that day.
Why do spiders keep coming back after I kill them?
Because the food supply is unchanged. Spider numbers track the insect population a building supports, so reducing insects is what reduces spiders.