Basement Spider Control
Spiders in Your Basement - Ontario Guide
Ontario basements are ideal spider habitat. Two species breed here year-round, and an untreated basement population seeds the entire home. Here is what is living in yours and how to deal with it.
Why Basements Are the Spider Epicentre
Of all the areas in an Ontario home, the basement has the highest spider density. This is not random - basements provide everything spiders need to thrive year-round:
Stable Cool Environment
Cellar spiders in particular prefer temperatures between 10 and 20°C - exactly the range in most Ontario unfinished basements. Unlike heated upper floors, basements maintain ideal conditions through the summer when upper-floor temperatures may be too warm for some species.
Moisture & Condensation
Spiders, like most arthropods, are sensitive to dehydration. Basement humidity - from condensation on cold walls, minor foundation seepage, and poor ventilation - keeps spiders alive and active in conditions that would desiccate them on upper floors.
Insects Drawn to Basements
Fungus gnats, springtails, silverfish, earwigs, and other moisture-loving insects concentrate in basements. Spiders go where the food is. A basement with a persistent insect population will always have spiders, because the insects are sustaining them.
The Two Basement Spider Species
Cellar Spider (Pholcus phalangioides)
The pale, long-legged species commonly misidentified as “daddy long-legs.” Cellar spiders build loose, irregular webs in corners, ceiling junctions, beam undersides, and along utility pipes. They are harmless - their chelicerae (mouthparts) are too small to puncture human skin. They reproduce continuously indoors, producing multiple egg sacs per female per year. An uncontrolled cellar spider population in an Ontario basement can number in the hundreds within a single season.
Common House Spider (Parasteatoda tepidariorum)
The brown, round-bodied cobweb spinner present in virtually every Ontario home. House spiders establish primarily in undisturbed corners - under stairs, behind storage boxes, along basement walls. Females produce 100 to 400 eggs per sac and multiple sacs per year indoors, making them the primary driver of whole-home spider populations. The spiders you see upstairs are typically males from the basement population roaming in search of mates.
How Spider Squad Treats Basement Spiders
Basement spider treatment requires a two-part approach: eliminating the existing interior population and blocking the exterior entry points that allow reinvasion.
Interior Perimeter Treatment
Our treatment is applied along all basement baseboards and wall-floor junctions, under stair stringers, along beam and joist lines in the ceiling, around sill plates, and at utility penetrations (plumbing, electrical, HVAC). Spiders in contact with treated surfaces are eliminated. The residual provides ongoing control as hidden populations emerge from voids and crevices over subsequent weeks.
Exterior Foundation Barrier
The exterior foundation is treated in a band extending 60 to 90cm up the wall and 30cm outward. This is the primary entry pathway for basement-level immigration - spiders entering through foundation cracks, weep holes, window wells, and gaps around utility lines. Without the exterior barrier, interior treatment becomes a repetitive exercise as new spiders continuously enter from outside.
Window Wells & Basement Windows
Basement window wells are prime entry and harborage zones. We treat the well frames, surrounding soil, and the exterior of basement window frames. Spider Squad also identifies window wells where accumulated debris or standing water is making the problem worse, and recommends corrective action.
What You Can Do Between Treatments
- Run a dehumidifier - Keeping basement relative humidity below 50% makes the environment less hospitable to both spiders and the insects they feed on.
- Seal obvious entry points - Caulk gaps around utility line penetrations, apply foam backer to door frame gaps, and install window well covers on basement windows.
- Reduce clutter - Cardboard boxes, fabric, and undisturbed storage provide ideal web-building and egg-sac attachment sites. Replacing cardboard with sealed plastic bins removes the most spider-friendly harborage.
- Fix moisture issues - Minor foundation seepage and condensation on cold walls creates the humidity that sustains the basement insect population that feeds spiders. Address the moisture source, not just the symptom.
- Install door sweeps on basement door - The gap under basement doors is a primary pathway for ground-level spiders moving from basement to upper floors.
Frequently Asked Questions
Why are there so many spiders in my basement?
Basements provide stable cool temperatures, high humidity, low light, and insects to eat - ideal conditions year-round. Cellar spiders and common house spiders breed continuously indoors in these conditions and can establish dense populations in a single season without treatment.
What spiders are in Ontario basements?
Primarily cellar spiders (Pholcus phalangioides) and common house spiders (Parasteatoda tepidariorum). Both breed year-round indoors. Wolf spiders also appear in basements seasonally, entering through foundation gaps and window wells in fall.
Are basement spiders in Ontario dangerous?
No. Cellar spiders cannot bite through human skin. Common house spiders and wolf spiders can bite but are not aggressive and their venom poses no medical risk to healthy adults.
Do basement spiders come upstairs?
Yes. The spiders you see on upper floors typically migrated from the basement population, especially male house spiders roaming in fall. Controlling the basement population is a prerequisite for reducing whole-home activity.
Basement Spider Control
Eliminate the Source Population
Interior perimeter treatment + exterior barrier. Call to book.
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