The movement pattern of a migrating male spider
Understanding the exact movement pattern of a migrating male house spider explains why some entry points matter more than others. Male European house spiders (Tegenaria domestica) and giant house spiders (Eratigena atrica) mature in late summer. When they leave their webs to find mates, they navigate primarily by pheromone — female spiders leave chemical traces in their silk and on surfaces they walk across. The male follows these traces toward the source.
Males move at ground level, typically staying within the lower 30–50 cm of surfaces. They're nocturnal — most migration activity occurs after dark, peaking between 8 pm and 2 am in Metro Vancouver's late summer conditions. They travel fast: a large male giant house spider covering 50+ metres per night is not unusual. They follow the path of least resistance, which typically means following wall edges and corners where female silk traces concentrate.
The primary entry points by order of frequency
Based on exclusion inspections across thousands of Metro Vancouver properties, the entry points for fall-migrating spiders rank in this order:
- Door bottoms (highest frequency): the gap between the bottom of the exterior door and the threshold is the single most common spider entry point. A 4–5 mm gap (which develops in most Metro Vancouver homes as door sweeps compress over 5–10 years of use) is easily accessible. Doors facing west and south see the most pressure because these are the sun-warmed faces where female spiders establish webs over summer.
- Foundation cracks and gaps: any crack in the foundation wider than 3–4 mm at or below grade is accessible. Homes with poured concrete foundations have fewer of these; homes with block or brick foundations typically have multiple accessible gaps. Look particularly at the foundation-sill plate junction where the wood frame meets the concrete.
- Window frame junctions: the seal between window frame and adjacent siding degrades over time on Metro Vancouver's aging housing stock. Ground-floor windows are the most relevant — migrating males travel at ground level.
- Utility penetrations: water, gas, and cable penetrations through the foundation are consistent spider-entry points if the fill material has cracked or compressed. Even gaps of 3–5 mm around a plastic-jacketed cable are accessible.
- Crawlspace vents and accesses: deteriorated crawlspace vent mesh (common on 1950s–1980s Vancouver homes) provides direct basement access from the crawlspace, which is outdoor habitat. A failed vent mesh is a large opening — effectively an open door for any ground-level invertebrate.
- Garage door bottom: the rubber seal on the bottom of garage doors deteriorates and develops gaps, particularly in the centre section where the door sags. A garage with an accessible bottom gap becomes an indoor harborage for the garage population, with secondary entry into the house proper.
Why timing matters: the August window
The migration begins in Metro Vancouver when nighttime temperatures consistently drop below 15°C — typically in the last week of August, varying by a week or two depending on the year. In warmer years (2024 being a recent example), the migration start can push into the first week of September. The peak is mid-September. The migration is essentially over by mid-November.
This timing creates a specific intervention window. Structural sealing is effective at any time — gaps sealed in January are sealed in September. But chemical perimeter treatment needs to be in place before the migration begins to function as an interception rather than a reactive measure. Treatment applied in early August establishes a residual barrier at entry points 3–4 weeks before the first migrating males approach the building. Treatment applied in late September is treating spiders that have already crossed or are crossing, which is less efficient.
| Entry point | Priority | DIY-fixable? | Timing |
|---|---|---|---|
| Door bottom gap | Highest | Yes — replace door sweep | Any time, ideally before July |
| Foundation cracks >3mm | High | Yes — exterior caulk or hydraulic cement | Spring or early summer |
| Window frame junction | Moderate-high | Yes — exterior caulk | Dry weather, summer |
| Utility penetrations | Moderate-high | Yes — mesh wool + foam | Any time |
| Crawlspace vent mesh | High for basement access | Yes — hardware cloth replacement | Spring |
| Garage door seal | Moderate | Yes — door seal replacement | Any time |
The pheromone trail problem
One factor that makes fall spider entry difficult to fully stop with structural measures alone: pheromone trails. If a female spider has established a web inside your home — in the basement, behind appliances, in a utility room — she's leaving pheromone traces that are detectable to migrating males from outside the building. The males are following a chemical signal that says 'female spider inside.' This is a powerful motivator. Males will probe multiple entry points repeatedly, trying to follow the trace to its source.
This is one reason the perimeter chemical treatment is valuable as a complement to structural sealing: even if a male reaches the door bottom, the pyrethroid residual at the entry point intercepts him before he can push through. The combination of a structural barrier (sealed door bottom, caulked gaps) and a chemical barrier (perimeter treatment) performs significantly better than either alone. The structural measure prevents easy passive entry; the chemical measure intercepts determined active probing.