How to use this guide
Occasional invaders are species that don't establish permanent breeding populations indoors under normal conditions, but enter homes in response to weather events, seasonal temperature changes, or structural moisture. They're distinguished from true indoor pests (German cockroaches, bed bugs, house mice) by the fact that environmental management — rather than ongoing chemical treatment — is the primary control lever. This guide is organized by the most common encounter scenario: when homeowners see something new and want a fast identification. Each entry includes the key field ID feature, the most common season or trigger for indoor appearance in Metro Vancouver, and the single most important action. For detailed treatment protocols, the species-specific articles in this cluster provide full guidance.
| Species | Size & key feature | Season / trigger | Indicates | Primary action |
|---|---|---|---|---|
| Silverfish (Lepisma saccharina) | 13–25 mm, silver, 3 tails, fast | Year-round, peaks in wet season | Humidity > 75% — moisture problem | Dehumidify to 40–50% RH |
| Firebrat (Thermobia domestica) | 13–19 mm, mottled grey, 3 tails | Near heat sources, year-round | Heat + humidity near furnace or bakery | Remove food + moisture near heat sources |
| Earwig (Forficula auricularia) | 12–15 mm, dark brown, pincers | Summer entry (dry spells), autumn rain | Weather event or adjacent harborage | Perimeter granules + sealing |
| Pillbug (Armadillidium) | 10–15 mm, rolls into ball, grey | Year-round, peaks after rain | Basement/crawlspace humidity > 60% | Moisture management + sealing |
| Sowbug (Oniscus asellus) | 10–15 mm, cannot roll, 2 tail appendages | Year-round, peaks after rain | Same as pillbug | Same as pillbug |
| House centipede (Scutigera coleoptrata) | 25–50 mm, very long banded legs | Year-round in basements | Prey (silverfish, cockroaches) present | Treat prey, not the centipede |
| Springtail (Collembola) | 1–2 mm, jumps when disturbed | After rain (48-hour window) | Saturated soil adjacent to foundation or wet houseplant soil | Drainage improvement or watering correction |
| Booklouse (Liposcelis) | 1–2 mm, pale, slow | New construction, damp bathrooms | Surface mould — humidity > 60% | Ventilation upgrade |
| Cluster fly (Pollenia rudis) | 8–10 mm, golden thorax hairs, slow | September–October entry | No sealing on south wall / attic vents | Seal before September |
| Boxelder bug (Boisea trivittata) | 12–14 mm, black + red | September–October entry | Adjacent boxelder or maple tree seed crop | Seal + exterior residual September |
| Asian lady beetle (Harmonia axyridis) | 6–8 mm, orange, variable spots, M-mark on pronotum | October–November entry | Nearby agricultural land or orchard | Seal + exterior residual September |
| Brown marmorated stink bug (H. halys) | 14–17 mm, mottled brown, banded antennae | October entry, spreading in BC | Agricultural area adjacent; expanding range | Seal + September exterior treatment; do not crush indoors |
| Carpet beetle (Anthrenus verbasci) | 2–3 mm, mottled scales (adult); bristly larva | Year-round, adults enter on flowers in spring | Undisturbed natural-fiber item as food source | Find and remove food source; vacuum intensively |
| Drain fly (Psychoda) | 2–4 mm, very hairy, moth-like wings | Year-round near slow drains | Organic biofilm in drain pipe | Drain brush + enzymatic cleaner |
| Fungus gnat (Bradysia) | 2–4 mm, dark, slender, long legs | Year-round from houseplants | Overwatered houseplant soil | Allow soil to dry; BTi soil drench |
The moisture tier: BC's primary occasional invader driver
Eight of the fifteen species above are directly or primarily driven by moisture: silverfish, firebrat (indirectly), pillbugs, sowbugs, house centipedes, springtails, booklice, and drain flies. In Metro Vancouver's wet climate, moisture management is therefore the highest-leverage single investment any homeowner can make for occasional invader prevention. The moisture management infrastructure — bathroom ventilation, basement dehumidification, crawlspace vapour barrier, foundation drainage, structural sealing — is detailed in the [moisture management guide](/guide/occasional-invader-prevention). A home that maintains 40–50% RH throughout and has no standing water adjacent to the foundation will see dramatically lower occasional invader activity across all moisture-dependent species simultaneously.
The thermal tier: fall-overwintering home invaders
Four species in BC's occasional invader profile are heat-seeking overwintering species that enter buildings in autumn as temperatures drop: cluster flies, boxelder bugs, Asian lady beetles, and brown marmorated stink bugs. These are structurally distinct from moisture-driven species — the control lever is building envelope exclusion timed to September, not moisture management. The September exclusion window addresses all four simultaneously. Caulking soffit-fascia gaps, replacing attic vent screens, sealing window frame perimeters, and applying an exterior pyrethroid residual in early September intercepts all four species before their peak autumn migration. One investment, four species addressed. For more detail on the autumn compound event when moisture-driven and thermal-driven species appear simultaneously, see the [48-hour rain pattern guide](/guide/bc-rain-driven-invasions-48hr).
The food-fabric tier: carpet beetles, pantry moths, clothes moths, and weevils
Three occasional invader categories are driven by food sources rather than moisture or weather: carpet beetles, clothes moths, pantry moths, and grain weevils. These are distinguished from other occasional invaders by the fact that they can establish persistent indoor populations as long as food is present — and the food sources (stored natural fibers, dry pantry goods) are stable year-round. These species don't respond to moisture management or structural exclusion because their driver is different. The control lever is food source removal and protective storage. Annual inspection of stored wool and natural-fiber items plus sealed container storage for all pantry goods are the primary preventives. For full protocols see the [carpet beetle guide](/guide/carpet-beetle-fabric-damage), [pantry moth guide](/guide/pantry-moths-flour), and [clothes moth guide](/guide/clothes-moth-heirloom-protection).
BC's seasonal occasional invader calendar
- March–April: spring emergence. Cluster flies, boxelder bugs, and Asian lady beetles that overwintered in wall voids become active and move toward windows. Drain flies become more active as temperatures rise. Clothes moth adults begin flying as indoor temperatures warm.
- May–June: carpet beetle adults enter on cut flowers, screen-seeking begins for summer. Earwig populations build outdoors. Fungus gnats from houseplants peak in spring from overwinter-accumulated overwatering.
- July–August: earwig pressure on gardens peaks. Silverfish populations in damp basements peak with summer humidity. Cluster fly adults develop and begin seeking building entry sites for autumn.
- September: the critical exclusion window. Cluster flies, boxelder bugs, and BMSB begin seeking overwintering sites. September sealing work is the single highest-value pest prevention investment of the year for Metro Vancouver homeowners.
- October: the compound event. First autumn rains displace springtails, pillbugs, earwigs. Temperature drops drive thermal-tier species into buildings. Homes without September exclusion work experience multiple simultaneous entry events.
- November–February: wet season persistence. Moisture-driven species (silverfish, booklice, pillbugs) continue year-round in poorly managed homes. Cluster flies, boxelder bugs overwinter in wall voids. Clothes moth and carpet beetle damage accumulates silently in undisturbed storage.