Mice Move In When It Gets Cold: How to Keep Rodents Out of Your Rental This Fall
Every fall, the first cold nights in the Black Hills send two things looking for a warm place to spend the winter: people and mice. The National Weather Service office in Rapid City puts the median first freeze in late September, so by mid-October the season has already turned.
For a rental owner, that's the signal to look at the house the way a mouse does. A mouse doesn't need much. According to the CDC, a mouse can fit through a hole the width of a pencil, about a quarter inch. Here's how to find those gaps, close them, and skip a winter of traps, droppings, and tenant calls.
Nothing here is legal advice. Laws change, and your lease and situation matter. Talk to a South Dakota attorney before you rely on any of this.
A Quarter Inch Is All It Takes
Most owners picture a mouse problem starting with an open door or a hole you could put your fist through. It usually starts smaller: a gap around a gas line, a worn garage door seal, or the spot where the siding stops a little short of the foundation.
The South Dakota Department of Health recommends sealing every entry hole a quarter inch or larger around roofs, attics, basements, windows, doors, foundations, vents, air conditioners, and under sinks and other pipes. It also makes the point that matters most: if you trap inside but don't seal the entry holes, new rodents will keep coming in.
Seal first. Trap second.
Walk the Outside First
Grab a flashlight and a pencil, and walk the whole perimeter. If the pencil fits, so does a mouse.
Where pipes and wires go in. Gas lines, AC line sets, dryer vents, hose bibs, and cable lines all pass through the exterior wall. Check the gap around each one.
The foundation line. Look for cracks in the foundation and gaps where siding meets concrete.
Garage doors. A garage door that doesn't seal tight to the slab is an open invitation, and an attached garage leads straight into the house. The CDC calls out sealing garages and outbuildings, too.
Exterior doors. If you can see daylight under a closed door, a mouse can see the way in. A new door sweep is one of the cheapest fixes on this list.
Vents and crawlspace openings. Torn or missing screens need to be replaced, not taped.
What's stacked against the house. The Department of Health recommends clearing brush, grass, and junk from around the home and, where possible, keeping woodpiles 100 feet or more from the house and 12 inches off the ground. Firewood stacked against the siding is a mouse hotel with a heated lobby.
Seal It the Right Way
The CDC's guidance is practical: fill small holes with steel wool, then hold it in place with caulk or spray foam. Use hardware cloth, lath screen, metal sheeting, or cement for larger openings, cut to fit around pipes.
One upgrade worth knowing: University of Nebraska–Lincoln Extension notes that plain steel wool will eventually rust, and recommends copper or stainless steel wool for gaps that can't be closed another way. It also notes that mice can quickly gnaw through rubber or vinyl weather stops. Foam and weatherstripping help, but they aren't a barrier on their own.
These are building repairs, so they're the owner's side of the ledger. It's also work where vetted vendors earn their keep: a sloppy seal job looks finished right up until the first mouse shows up in the pantry.
Inside the Unit: Who Handles What
When a tenant reports mice, the first question is usually "Whose problem is this?" South Dakota law gives a starting point. Under SDCL 43-32-8, a residential landlord has to keep the premises in reasonable repair, fit for human habitation, and in good and safe working order, except when the disrepair was caused by the negligent, willful, or malicious conduct of the tenant or someone under the tenant's control. The statute also says the parties can't waive that duty in the lease.
In plain English: gaps in the building are the owner's job. Housekeeping is the tenant's. Food left out, garbage without a lid, and pet food on the floor overnight will draw mice into even a well-sealed home, which is why the Department of Health's list includes rodent-proof food containers, tight garbage lids, and putting away uneaten pet food.
Spell that out at move-in and again in a fall reminder. And when a tenant calls, treat it as urgent. Pests made our list of the complaints we actually hear because not knowing who's handling it becomes its own complaint.
Sealing and trap checks mean entering an occupied home. Under SDCL 43-32-32, except in an emergency, 24 hours' written notice is presumed reasonable, and the notice should give the date, a time window during normal business hours, the purpose, and a way to reschedule.
Trap Smart
Once the entry points are closed, traps handle whoever is already inside. The CDC's trapping guidance recommends snap traps set along walls with the baited end against the wall. Check them daily, keep them away from children and pets, and keep trapping until a full week passes with no catches and no new signs.
The CDC reserves poison and bait stations for infestations that persist. In a home with kids and pets, that's a job for a pest professional.
Clean Up Like It Matters, Because It Does
This is the part owners and tenants most often get wrong. In South Dakota, the Department of Health says two species, deer mice and white-footed mice, are known carriers of the Sin Nombre strain of hantavirus. It's rare and it doesn't spread person to person, but it's serious, and people catch it by breathing in particles from rodent urine, droppings, or nesting material.
The CDC's cleanup steps are straightforward:
Air it out. Open doors and windows for 30 minutes before cleaning, and leave the area while it ventilates.
Never sweep or vacuum droppings. That's how the particles get into the air.
Soak first. Wearing rubber or plastic gloves, spray droppings and urine with a disinfectant or a fresh bleach solution (1 part bleach to 9 parts water) until very wet, and let it soak for 5 minutes.
Wipe, bag, and wash. Wipe up with paper towels, double-bag dead mice and nesting material, wash your gloved hands, then wash again once the gloves are off.
For a heavy infestation, especially in a unit that sat vacant, both the CDC and the Department of Health point to professionals. Put "don't sweep it, soak it" in your fall tenant letter.
Why This Matters
Mice are cheap to keep out in October and expensive to get rid of in January. By then you're dealing with contaminated cabinets, chewed insulation, and a tenant who no longer feels the home is clean. That's how a good tenant starts thinking about moving.
It's also why we inspect every property twice a year. A torn vent screen or a garage seal with daylight under it is a quick fix on an inspection and a winter-long headache when nobody's looking. Our Black Hills winterizing checklist covers the rest of the fall list, and this belongs right next to it.
Whether your rental is in Rapid City and the Black Hills, Sturgis, or out on the prairie in Huron, the fall routine is the same.
Frequently Asked Questions
How small a hole can a mouse get through?
About a quarter inch, roughly the width of a pencil, according to the CDC.
Who is responsible for mice in a South Dakota rental?
It depends on the cause and the lease. SDCL 43-32-8 requires the landlord to keep the premises in reasonable repair and fit for human habitation, except when the disrepair was caused by the tenant's negligent, willful, or malicious conduct. This isn't legal advice.
Should you vacuum up mouse droppings?
No. The CDC says not to sweep or vacuum droppings. Ventilate, put on gloves, soak with disinfectant or bleach solution for 5 minutes, then wipe up with paper towels.
Is your rental sealed up before the first hard freeze?
A fall walk-around costs an afternoon. A winter of mice costs a lot more. Want to know what your rental should actually be earning? Get a free rental analysis. Let's talk. Blair Allen Property Management serves owners across South Dakota and Montana.
Chris Twiggs
President, Blair Allen Property Management
📞 605-545-1218
✉️ chris@blairallen.com