Overview
The house mouse (Mus musculus) is the most common rodent pest in Montreal homes. It can compress its body to fit through a gap as small as 6 mm — roughly the diameter of a pencil — making most homes surprisingly easy to enter. Once inside, mice do not leave on their own. They reproduce rapidly, contaminate food, gnaw on wiring and structural materials, and can transmit disease. Montreal's extreme climate makes this problem particularly acute, as mice are strongly motivated to seek indoor shelter from October through April.
How to Identify House Mice
- Size: 15–30 g body weight; 7–10 cm body length plus 5–10 cm tail
- Colour: Grey or brown on top, lighter grey or white underneath
- Head: Pointed nose — distinguishes it from the Norway rat, which has a blunt nose
- Tail: As long as or longer than the body, scaly, nearly hairless
- Ears: Large relative to body size, with fine hair
- Eyes: Large, black, prominent
- Droppings: 3–6 mm, dark, pointed at both ends — scattered throughout territory
Biology & Reproduction
- Litters per year: 6–8
- Pups per litter: 4–6
- Gestation period: 19–21 days
- Sexual maturity: 6 weeks
- Lifespan: 1–2 years
- Home range: 3–10 metres from the nest
- Diet: Omnivorous — grains, seeds, insects, and virtually anything organic
- Water: Can survive on moisture from food alone — does not need a separate water source
Health Risks
- Hantavirus: Transmitted through droppings and urine — potentially fatal respiratory illness
- Leptospirosis: Transmitted through urine-contaminated surfaces
- Salmonellosis: Food contamination through droppings and contact with food surfaces
- Electrical fire risk: Mice gnaw on electrical wiring, removing insulation and creating short circuits
- Structural damage: Gnaws through plastic pipes, insulation, and wood
- Secondary pests: Mice carry fleas, ticks, and mites
Signs of Infestation
- Droppings: 3–6 mm, dark, pointed at both ends — scattered throughout territory (unlike rats, which concentrate droppings along specific routes)
- Gnaw marks: Small, clean gnaw marks on food packaging, wood, and plastic
- Grease marks: Dark smudges along baseboards and at the base of walls from oil in the mouse's fur
- Sounds: Scratching and movement in walls and ceilings, especially at night
- Nests: Shredded paper, fabric, and insulation in hidden locations
- Tracks: Small footprints in dust or flour
Prevention
- Seal all gaps larger than 6 mm with steel wool and caulk — any gap a pencil fits through is large enough for a mouse
- Install door sweeps on all exterior doors
- Store all food in airtight metal or glass containers
- Keep garbage in bins with tight-fitting lids
- Eliminate clutter in basements, garages, and storage areas
- Keep firewood stacked at least 6 metres from the house
- Trim tree branches that overhang the roof
- Schedule annual perimeter inspections in September before mice enter for winter
Professional Treatment
Professional mouse control involves two phases: (1) Elimination — strategic snap traps and bait stations installed in confirmed activity areas; (2) Exclusion — sealing all entry points with steel wool, hardware cloth, hydraulic cement, and expanding foam after the existing population is eliminated. The best time for preventive exclusion is September, before mice enter for winter. All Blackline treatments include a 100-day guarantee.