Mice in Summer: Why You Can Still Get Them and How to Keep Them Out

TL;DR

• Yes. Mice do come in during summer, and some never leave: house mice with stable food and shelter can breed year-round (UC IPM).
• A house mouse can slip under a gap only about 6 mm tall, so the openings mice use in autumn stay open to them all summer.
• Summer mice are easier to miss because they are active mostly at night. Watch for droppings, gnaw marks, shredded-paper nests and a musky smell.
• Water may draw mice indoors more often in hot, dry weather. Dripping outdoor taps, leaky pipes under the sink and pet water bowls all count.
• Seal every opening larger than about 6 mm before autumn, with metal or concrete. Mice can gnaw through plastic, rubber, vinyl, foam and wood used on their own.
• Need help? Get a quote from a recommended local exterminator through Pest Agent's referral form.

Mice in summer catch a lot of Canadians off guard: the droppings turn up in a kitchen drawer in July, long after the autumn rush everyone warns about. The short answer is yes, you can get mice in the summer. House mice breed year-round wherever food and shelter stay steady, so a family living in your walls has no reason to leave when the weather warms.

Summer also has its own risks. Cottages, trailers and garden sheds sit closed for months, and deer mice move into them, which matters at opening time: deer mice are the main carrier of hantavirus in Canada, and house mice are not known to spread it.

Some of the mice you find in July never went outside at all, and the rest only need a small gap to get back in.

Do Mice Come in the House in Summer?

Mice do come in the house in summer, and some never leave in the first place. A house mouse can slip under a gap about ¼ inch (about 6 mm) tall, so the same openings mice use in autumn stay open to them all summer. Summer activity is just easier to miss, since mice are active mostly at night and many spend more time outdoors while food is easy to find.

Mice Already Living Indoors

Mice already living indoors keep breeding through the summer. According to UC IPM, house mice with plentiful and stable food and shelter can reproduce year-round, which may keep a population high for months or years. A kitchen with a steady supply of crumbs, pet food or birdseed is exactly that kind of stable resource.

Small Gaps

Small gaps are the main way mice get in. House mice can squeeze under a gap ¼ inch (about 6 mm) tall and through an opening 3/8 inch (about 1 cm) wide, the space around a pipe, a cable or a worn door sweep. Our guide to finding mouse entry points walks through the spots to check around the foundation, vents and utility lines.

Climbing Routes

Climbing routes give mice a way past ground-level defences. House mice can run up many rough vertical surfaces such as brick or stucco, travel along wires and ropes, and jump, so a gap near the roofline or around a wall-mounted cable can be just as useful to them as a crack at ground level.

Water

Water draws mice indoors more often in hot, dry weather. House mice do not need to drink to survive, since they normally get enough moisture from food, but UC IPM notes that they look for liquids when their food is too dry or when they live in a hot or dry environment. Dripping outdoor taps, leaky pipes under the sink and pet water bowls all count.

Where Do Mice Go in Summer?

Mice that live outdoors spend the summer in tall vegetation, wood piles, fields and outbuildings close to food. Indoor mice stay in sheltered spots in walls, cupboards and storage areas, out of sight. Which one you are dealing with depends on the species and on what surrounds your home.

House Mice Outdoors

House mice outdoors stay close to cover. UC IPM describes outdoor house mice as often associated with taller vegetation and less bare ground, such as hedgerows, and they live in and around homes wherever people store food. A thick shrub bed, a wood pile or a stack of patio stones against the foundation gives them shelter within a few steps of your walls. For more on the places they settle, see where mice live.

Deer Mice Outdoors

Deer mice outdoors live in forests, fields and grassland, nesting in tree hollows, stumps and under rocks and logs. A deer mouse usually has a white belly, white feet and a two-tone tail (white below, dark above), while a house mouse is often grey-brown all over with an almost hairless tail. Deer mice are rarely found in city neighbourhoods unless fields, forest or large parks are close by, which makes them a rural and cottage-country concern.

Seasonal Buildings

Seasonal buildings such as cottages, trailers and garden sheds are where many Canadians meet mice in summer. UC IPM notes that deer mice often move into vacated homes and cabins to nest and store food, and that sheds can provide refuge for some rodents; HealthLinkBC also names cottages and trailers as places that may harbour them. A shed can be protected with the same sealing methods as a house, and our guide to keeping mice out of a shed covers the details.

Are Mice More Active in Summer or Winter?

Mice are active in both seasons, but house mice mostly move into homes in autumn, when nights turn colder. Summer is breeding season outdoors, so the number of mice near your home tends to build through the warm months.

Summer Breeding

Summer breeding is fast. A female house mouse can have around 5 to 10 litters a year of about 5 or 6 pups each, with young usually born about 18 to 21 days after mating and often ready to breed about 5 to 6 weeks after birth. Deer mice usually stop breeding in winter, so their young are mostly born in the warm months. The full cycle is explained in how often mice reproduce.

Autumn Move Indoors

The autumn move indoors is the main seasonal shift. UC IPM says that where outdoor house mice exist, they frequently enter homes in autumn when nighttime temperatures become colder, and a pest control expert quoted by CBC said mouse activity tends to peak in the fall. What happens once they are inside is covered in our article on mice in winter.

The reasons some years bring far more mice are in why mice are so bad this year.

What Are the Signs of Mice in Summer?

Signs of mice in summer are often droppings, gnaw marks, shredded-paper nests and a musky smell, the same clues as in any other season. Look in the places where food is stored and along walls, where mice travel. The complete list is in our guide to the signs of mice.

Droppings

Droppings are usually the first sign. They show up in cupboards, drawers, under the sink and along baseboards, often near food packages. Fresh droppings in a spot you cleaned a few days earlier mean mice are still active there.

Gnaw Marks

Gnaw marks appear on food boxes and bags, baseboards and cables. According to the University of Kentucky, house mice gnaw through electrical wiring, so chewed wiring in a basement, attic or vehicle is worth taking seriously.

Nests

Nests are balls of finely shredded paper, fabric or other fibrous material, tucked into sheltered spots such as the back of a drawer, behind appliances or in stored boxes. Finding one indoors in summer means mice are breeding inside, not just passing through.

Musky Odour

A musky odour tends to be typical of house mice in large or long-term infestations. A stale, musky smell in a closed cupboard or a cottage that has sat empty since autumn points to mice living there.

How Do You Keep Mice Out in Summer?

Keeping mice out in summer starts with sealing every opening larger than ¼ inch (about 6 mm) before autumn pushes outdoor mice inside. Summer work gets ahead of the autumn move indoors, while walls and foundations are easy to reach.

Seal Openings

Sealing openings works best with materials mice cannot chew. UC IPM recommends metal or concrete for foundation cracks and gaps around pipes, vents and utility cables, and steel wool as a temporary plug. Plastic, rubber, vinyl, insulating foam and wood are unsuitable on their own, since mice gnaw through them. Check that doors, screens, pet doors and garage doors close tightly.

Yard Shelter

Yard shelter near the house gives mice a place to wait for an opening. HealthLinkBC advises reducing rodent shelter, such as thick bushes and wood piles, along with food and garbage, around your home. Keep firewood away from the walls and tall grass trimmed along the foundation.

Food and Garbage

Food and garbage are what keep mice close. Store pet food, birdseed and pantry staples in hard sealed containers, and pick up fallen fruit and spilled seed under feeders. Keep garbage and compost bins closed with tight lids.

Traps

Traps are the recommended first step if you already see signs indoors. HealthLinkBC recommends spring-loaded traps to remove rodents from buildings. Our step-by-step guide on how to set a mouse trap covers bait and placement.

Cottage Opening Cleanup

Cottage opening cleanup needs extra care, since deer mice carry hantavirus and favour buildings left empty. HealthLinkBC advises airing the building for 30 minutes before you start, wearing rubber gloves, a well-fitting mask and goggles, and pouring a disinfectant or diluted bleach (a mixture of 1 part bleach to 9 parts water, as the Public Health Agency of Canada also advises) onto droppings and nests. Let it soak 10 minutes, wipe up with paper towels, and do not sweep or vacuum. The full method is in our guide to cleaning mouse droppings.

When to Call a Professional

Calling a licensed pest control professional makes sense when you keep finding fresh droppings after trapping, when you find nests inside walls or chewed wiring, or when you cannot find how mice are getting in. You can get a quote from a recommended local exterminator through Pest Agent's referral form.

The mice guides cover identification, prevention and treatment in more depth.