Mice in the Fall: How to Keep Them Out Before Winter

Late October, early November: nights drop below zero and mice look for a warm, dry place close to food to spend the winter. This is when we get the most calls about noises in the walls, in Montreal as well as in Laval, on the North Shore and on the South Shore. The good news: much of the problem can be avoided by blocking entry points now.

Why mice come inside in the fall

House mice do not handle the cold well. As fields, parks and alleys cool down, they follow foundation walls and the heat escaping from your home. An opening of about 6 mm, roughly the width of a pencil, is enough for them to squeeze in. Once settled, a female can have several litters a year: a few mice in November can become a real infestation before spring.

The most common entry points

In Montreal plexes and older buildings

• cracks in stone or concrete foundations;
• gaps around pipes and wires in the basement and behind appliances;
• garage, cellar or shed doors that do not seal at the bottom;
• gaps where balconies and outdoor staircases meet the wall.

In houses in Laval and on the North and South Shores

• around the garage door and the joint between the garage and the house;
• basement windows, dryer vents and other vents without mesh;
• siding that is poorly sealed at ground level;
• properties near fields, woods or new construction sites, where mice are displaced in the fall.

Signs they are already inside

• Droppings 3 to 6 mm long, black and pointed, in cupboards, under the sink or along baseboards;
• scratching in the walls or ceiling, especially in the evening and at night;
• chewed cereal, rice or cookie packages;
• a urine smell (ammonia) in a closet or basement;
• small nests of paper, fabric or insulation.

Prevention: what to do this week

1. Walk around the house at ground level with a flashlight and note every hole or crack.
2. Plug gaps with steel wool or metal mesh, then seal with exterior caulk. Spray foam alone is not enough: mice chew through it.
3. Install door sweeps and check the garage door weatherstrip.
4. Cover basement windows and vents with mesh.
5. Store food (including birdseed and pet food) in hard, sealed containers.
6. Keep firewood, dead leaves and debris away from the foundation.

Traps: useful, but not enough

A few snap traps set along walls, perpendicular to the baseboard, can handle a single mouse. But if you catch several, if the noises continue or if you find droppings in several rooms, there is an established population and entry points are still open. Do not leave poison where children or pets can reach it: a poisoned mouse can also die inside a wall and cause strong odours.

When to call an exterminator

A technician finds the entry points that are hard to see, installs secure, locked bait stations and comes back for a follow-up. At ExtermZone, mouse treatment comes with a 6-month guarantee. We serve Montreal, Laval, the North Shore and the South Shore, 24/7: (514) 295-7696. See also our article on mouse smell in the walls.