10 Signs You Have Mice in Your House

A faint scratch inside the bedroom wall at 2 a.m. A black speck in the cutlery drawer. A cereal box in the pantry with a hole in the corner. On their own, each one is easy to shrug off. Together, they are signs of mice in the house, and on the South Shore at the end of summer they tend to show up in the same order every year.

This guide walks through the 10 signs, from the subtle ones to the ones you cannot miss, with the exact spot to check for each. Then it covers why September through November is the entry season in Brossard, Longueuil and the towns around them, how to tell a mouse from a rat in thirty seconds, what the real health risks are, and what to do (and not do) tonight.

What are the signs of mice in the house?

A mouse spends about 90 percent of its time hidden. You may not see one for weeks. But it feeds 15 to 20 times a night, leaves 50 to 75 droppings a day, and runs the same routes along the same walls every time. That habit is what gives it away.

Here are the 10 signs, roughly in the order people notice them. One sign is worth a closer look. Three or more, especially the combination of droppings, night noises and chewed packaging, almost always means a family has already moved in.

1. Droppings, and exactly where you find them

This is the most reliable sign and the first thing to look for. Mouse droppings look like black grains of rice, 3 to 6 mm long, pointed at both ends.

Where to look: the back of the kitchen cabinets, under the sink, the cutlery drawer, the top of the fridge, behind the stove, along the basement baseboards and next to the water heater. In the attic, check around the spots where wires and pipes come through.

One useful detail: fresh droppings are soft, dark and slightly shiny. Old droppings are grey, dry and crumbly, and date back days or weeks. Fresh ones mean the activity is happening now.

2. Scratching or scurrying at night

Mice are most active between dusk and dawn. Once the house goes quiet, you hear short, light scratching inside the walls, above the bedroom ceiling, under the floor or in the attic. It sounds like a fingernail on cardboard, sometimes with a burst of quick little footsteps.

A rat is heavier and slower. A squirrel makes noise in daylight, mostly early morning and late afternoon. If the sound is light, quick and strictly after dark, think mouse.

3. Gnaw marks on wood, plastic and wiring

A mouse's front teeth never stop growing, so it has to chew to wear them down. You will find small parallel marks, like two very fine grooves, on the corners of cabinet doors, on trim, on plastic bags and storage bins and, more worrying, on the insulation of electrical wires behind appliances or in the basement. Fresh gnaw marks are pale, the colour of raw wood; old ones are dark and match the surface around them.

4. Grease marks along the walls

A mouse's fur is oily and coated in dust. Because it hugs the wall and runs the same route every night, it eventually leaves a grey or brownish smear, a bit like a dirty fingerprint, along the baseboard, the bottom of the wall or the edge of a floor joist.

They are easy to mistake for ordinary grime. Check along the basement walls, around the holes where pipes pass through, and behind the washer or the chest freezer.

5. A nest, or shredded material

Mice build nests out of whatever is close: pink attic insulation, paper, cotton batting, fabric, string, dry leaves, furniture stuffing. The nest is a loose ball about 10 to 15 cm across, usually littered with droppings and smelling of urine.

Look in dark, undisturbed corners: behind the stove and fridge, under the basement stairs, in the storage bins in the garage. Finding shredded material without the nest, say bits of paper towel in a cabinet, means a nest is being built nearby.

6. A smell of urine or ammonia

Mice urinate constantly wherever they go, to mark their territory. Over time, enclosed spaces (the cabinet under the sink, the back of the pantry, one corner of the basement) take on an ammonia smell, a bit like a neglected litter box.

If the smell is obvious, the population has been established for weeks. A rotten smell rather than ammonia means a mouse has died inside a wall, which often happens after a do-it-yourself poisoning.

7. Your cat or dog starts acting differently

Pets hear and smell what you cannot. A cat that sits for hours staring at the stove or a baseboard, a dog that keeps sniffing under the dishwasher, scratching at the bottom of a wall or barking at the ceiling at night: they are pointing at the activity.

If your pet has a brand new obsession with one specific corner of the house, take a flashlight and look at that corner. You will often find sign number 1 there.

8. Tracks in dust or flour

A mouse leaves tiny footprints and a tail drag in fine dust. On a dusty basement floor or a garage shelf, you can pick out these trails running along the walls.

The technician's trick: sprinkle a thin layer of flour or cornstarch in a 30 cm strip along the suspect wall, or in front of the suspect hole, and check the next morning. Footprints a few millimetres across with a fine line between them means a mouse.

9. Holes in food packaging in the pantry

A mouse does not need to open a box. It goes straight through it. Bags of rice, flour, nuts or dog food chewed open at the corner, cereal and cracker boxes with a clean hole the size of a dime, oatmeal packets torn open with crumbs around them. The bag of pet food in the garage is a classic target.

A chewed package gets thrown out, not rescued.

10. A mouse seen in broad daylight

If you watch a mouse run along a wall, vanish under the dishwasher or cross the basement floor, the identification is done. A house mouse is 7 to 10 cm long without the tail, greyish brown, with large ears and a tail nearly as long as its body.

What matters is the timing. A mouse seen in full daylight, in a lit and occupied room, usually means one of two things: the population is large enough that competition pushes some mice out at risky hours, or the nest was just disturbed. Either way, it is not one lone mouse, and it is time to act.

Why do mice come inside in September on the South Shore?

The calendar works against you. As long as the nights stay warm and the fields are full, a mouse has no reason to come inside. With the first nights below 10 degrees, usually the second or third week of September, it starts looking for a heated shelter with food. That is a description of your house.

On the South Shore, two things speed this up. First, the harvest. Around La Prairie, Chambly, Carignan and Saint-Hubert, the corn and soy fields empty out from September into October. Every pass of the combine flushes out hundreds of mice that head for the nearest residential streets. If you live within a kilometre of a field, you are on the front line every fall.

Second, the age of the housing. The 1950s and 1960s bungalows in Greenfield Park, Saint-Hubert and old Brossard have foundations that have shifted, sill plate joints that have dried out, dryer vents with no screen, and garage doors with worn weatherstripping. A mouse fits through a 6 mm gap, about the width of a pencil.

That is why most of our rodent calls in Saint-Hubert and the surrounding streets come in between late September and mid-November. Mice that get in during October spend the whole winter with you and breed the entire time. Our upcoming guide on how mice get into your house goes through every entry point and how to close it.

Recognized three or four of these signs and want to know how serious it is? Extermination BETA is based in Brossard and offers same-day response in Longueuil, Saint-Lambert, Greenfield Park, La Prairie, Chambly and across Greater Montreal. The first inspection is free. Call 514-553-2070 and describe what you found.

Mouse or rat: how do you tell them apart quickly?

The treatment is different, so the question matters. Three clues settle it in most cases.

• The droppings. Rice grains, 3 to 6 mm, pointed: mouse. Fat raisins, 12 to 20 mm, blunt: rat.
• The noises and the holes. Light scurrying and clean holes under 2 cm: mouse. Heavy movement, deep tooth marks and holes of 5 cm or more in wood or plastic: rat.
• The location. A mouse climbs everywhere and settles in kitchen cabinets, walls and the attic. A rat stays closer to the ground, in the basement, the crawl space, or near sewers and the compost bin.

On the South Shore, mice account for the vast majority of residential cases. When in doubt, keep one dropping in a sealed bag to show the technician.

Are mice dangerous to your health?

Yes, but not in the way most people picture. The risk is not a bite, which is very rare. The risk is contamination.

Salmonella and other bacteria. A mouse walks through its own droppings and urine, then across your counter, through your cutlery and over your food. Rodent-related food poisoning almost always comes from that. Any opened food in a visited cabinet should be thrown out, and the surfaces disinfected before you use them again.

Allergens. Mouse urine contains a protein that lingers in household dust long after the animal is gone. For children with asthma and people with allergies, a house with mice can make breathing symptoms worse, especially once the infestation has gone on for months.

Hantavirus. You hear about it, but it needs to be put in perspective. Hantavirus is carried by the deer mouse, a species of woodlots and fields, not by the house mouse found in cities. Human cases in Quebec are extremely rare. The risk comes mainly from sweeping or vacuuming dry droppings in an enclosed space (a cottage, a shed, an attic) and breathing the dust. That is why you never clean droppings dry: wet them with a disinfectant, wait five minutes, pick them up with paper towel, gloves on and a window open.

Electrical wiring. Not a health risk, but the most expensive one. A mouse chewing the insulation off a wire inside a wall or behind a stove creates a genuine fire risk.

What should you do tonight if you found signs of mice?

What you do in the first 24 hours matters. Here is what helps, in order.

1. Lock up all the food. Cereal, flour, rice, pasta, nuts, pet food: into glass, metal or hard plastic containers with lids. The bag of kibble in the garage too. With no food within reach, mice turn to the traps.
2. Clean up crumbs and standing water. Under the toaster, behind the stove, the dog's water bowl overnight.
3. Run the flour test from sign 8 in the two or three most suspicious spots. By tomorrow morning you will know where the activity is real.
4. Set a few classic snap traps along the walls, trigger side facing the wall, with a pea-sized dab of peanut butter. In the lower cabinets, behind the stove and along the basement baseboards. Snap traps are the safest tool in a house with kids or pets, and they give you a sense of how big the problem is.

And here is what not to do.

• Do not put down over-the-counter poison in a house with children, a dog or a cat. Rodenticide blocks are attractive to pets, and a cat that eats a poisoned mouse can be poisoned in turn. On top of that, a poisoned mouse usually dies inside a wall, and the smell lasts two to three weeks. A professional uses locked, anchored bait stations placed out of reach, which is nothing like a block left under the sink.
• Do not seal the holes tonight. It is the most common reflex and the worst one. If you close the entry with the mice still inside, they do not leave. They chew a new route, often into a living space, or they die in the walls. Eliminate first, seal after.
• Do not buy an ultrasonic repeller. They have never shown a lasting effect, and mice get used to them within days.

When is it a job for an exterminator?

One lone mouse, caught within two days by a snap trap, with no other signs, you can handle yourself. The rest, honestly, is a professional's job. Call if:

• you find fresh droppings in more than one room, or on more than one floor;
• the traps catch a mouse a night for several days, which means the nest is producing faster than you are catching;
• you hear noise in the walls or the attic, where traps cannot reach;
• you have seen a mouse in daylight;
• there is a baby, an asthmatic or an immunocompromised person in the home;
• you rent and the problem affects more than one unit. In that case notify the landlord in writing, since in most cases it is their responsibility to treat.

What a professional does beyond setting traps: finds the actual entry points, identifies the species, places stations and traps where the mice really travel, comes back to check, and tells you what to seal so it does not start over in January. Extermination BETA's rodent control service follows exactly that sequence, with certified technicians who hold the Quebec CD pesticide permit.

On cost, in Quebec you generally pay 250 $ to 700 $ for a basic mouse treatment, including the inspection, setting up the stations and traps, and a follow-up visit. Sealing the entry points is billed on top, often 200 $ to 1,000 $ depending on how many openings there are and the state of the foundation. Our guide to exterminator prices in Quebec breaks it down by situation. Every house is different, so call for an exact quote.

Frequently asked questions

Is one mouse a big deal?

A single mouse is rare. Mice come in as small groups, and a female can have a litter of five or six pups every month from about six weeks of age. If you saw a mouse in October and did nothing, expect several dozen by January. An inspection now costs less than a full treatment in the spring.

Do mice leave on their own in the spring?

No. Some of the population heads back outside once it warms up, but the ones that found food and a comfortable nest stay put. A heated house with a pantry is better territory than a field in any season.

How can you tell if droppings are old or fresh?

Fresh droppings are black, soft and slightly glossy. After a few days they turn grey, dry out and crumble when you press them with a pencil. To settle it, clean one spot (wet, with gloves) and check again 48 hours later. New droppings mean the activity is ongoing.

Can a mouse climb all the way to the attic?

Easily. A mouse climbs brick, vinyl siding, a downspout or the cable line, and it can jump about 30 cm. Noises above an upstairs ceiling are nothing unusual. High entry points (soffits, roof vents, the joint between roof and wall) are part of the inspection.

Spotted signs of mice? Do not let the fall slip by

Three or more signs from this list is an active infestation, not a maybe. And every week you wait between September and November adds mice to the house for the winter. Run the flour test tonight, lock up the food, and if the results confirm what you suspect, call.

Extermination BETA serves Brossard, Longueuil, Saint-Hubert, Saint-Lambert, Greenfield Park, La Prairie, Chambly and Greater Montreal, with same-day response and a real 24/7 emergency line. Call 514-553-2070 or reach us through the contact page. The first inspection is free, and you get 15 percent off your first service.