Why mice move indoors in the fall, and how to stop them at the door

If you've started hearing scratching in the walls at night, you're not alone. Fall is when many homeowners first notice mice.

Mice move indoors in the fall because they need warmth, food and a safe place to nest as the weather cools. Outdoor food gets harder to find, and a heated house with a pantry is an easy upgrade. The best way to stop them is to find and seal the gaps they use before they settle in.

Why fall is the turning point

Mice don't hibernate. They stay active all winter, so they need shelter that stays warm and dry.

In the northern half of the country, the first cold nights often bring a wave of mice toward houses, garages and sheds. In the South, the shift is milder and more gradual. Mice can show up indoors there any time of year, but cooler, wetter weather still pushes more of them inside.

Harvest season plays a part, too. Fields are cut, gardens die back and tall grass gets mowed. Mice that lived comfortably outside all summer lose their cover and start looking for something new.

How mice get inside

Mice are small and flexible. The CDC says a mouse can fit through a hole about the width of a pencil. That means a gap you'd never notice can be a front door for a mouse.

The most common ways in are:

• Gaps where pipes, cables and gas lines enter the house
• Spaces under exterior doors and garage doors with worn weather stripping
• Cracks where the foundation meets the siding
• Attic, roof and crawl space vents without sturdy screens
• Openings around dryer vents and window air conditioners
• Gaps at the roof line, including eaves and soffits

They also come in by accident. A box from the garage, a bag of birdseed or a stack of firewood can carry a mouse inside with it.

How to stop them at the door

The CDC calls this step "seal up," and its guide to sealing out rodents lists where to look and what to use. Here is a simple way to work through it.

1. Walk the outside of the house slowly, close to the ground. Use a flashlight, even in daylight.
2. Check every spot where something passes through the wall, such as pipes, wires, hose bibs and vents.
3. Fill small holes with steel wool, then hold it in place with caulk or spray foam.
4. Cover larger openings with hardware cloth, sheet metal or cement.
5. Add or replace door sweeps on exterior doors and the door from the garage.
6. Make sure vent covers are intact and fastened.
7. Repeat the check inside, especially under sinks, behind the stove and around the water heater and furnace.

Make your home less inviting

Sealing gaps is the main defense. Taking away food and nesting material helps it work.

• Store dry food, pet food and birdseed in glass, metal or thick plastic containers with tight lids.
• Don't leave pet food out overnight.
• Wipe up crumbs and spills, and keep the area under the toaster and stove clean.
• Use trash cans with lids that close, inside and out.
• Cut back clutter in the garage, basement and attic, especially cardboard and fabric.
• Keep firewood and woodpiles off the ground and away from the house.

Signs mice are already inside

Sometimes the mice beat you to it. Look for these signs.

• Small dark droppings in drawers, cupboards or along walls
• Chewed food packages or nibbled soap
• Shredded paper or insulation tucked into a hidden corner
• Scratching or scurrying sounds at night
• A musky smell in closed spaces

If you find droppings, don't sweep or vacuum them. The CDC advises wetting them with a disinfectant first and wearing gloves, because dry droppings can put germs into the air.

Traps work best once the gaps are sealed. Trapping alone, with the entry points still open, often leads to a steady stream of new mice. If you use any rodent bait, follow the label exactly and keep it in a tamper-resistant station away from children and pets.

When to call a professional

A few mice in a garage may be a weekend project. Call a licensed pest control company if:

• You keep catching mice for more than a week or two
• You find droppings in several rooms
• You hear activity in the walls or ceiling and can't reach the area
• You see signs of rats, which are larger and harder to control
• You can't find how they're getting in

A good company will inspect the whole house, show you the entry points it found and explain how it plans to seal them. Ask whether exclusion work, meaning the sealing of gaps, is part of the price. Ask whether follow-up visits are included, and what happens if mice come back.

You can compare rodent control companies near you and read reviews from homeowners who have dealt with the same problem. The rodent control listings show which companies other customers recommend in your area.

Fall is the right time to act. A few hours with a flashlight and a tube of caulk now can save you a winter of scratching in the walls.