When Mice Come Indoors in St. Louis (And How to Stop Them)
Most people call us about mice in November. The mice arrived in September.
By the time you hear scratching in the ceiling, you're not preventing an infestation. You're managing one that's had two months to establish and probably to breed.
The actual timeline in St. Louis
Late August. First scouts. Nights start dropping and mice begin testing structures along their normal foraging routes. Nothing you'd notice.
September. The real move-in. This is the month that decides your winter. A few cool nights in a row and mice start working their way in through whatever gap is available.
October. Population establishes. They've found the food, mapped the route and started nesting in insulation, wall voids and stored boxes.
November. You notice. Droppings behind the toaster, a bag of dog food chewed open, scratching overhead at 2am.
December through February. Breeding. A pair of mice can produce a litter roughly every three weeks, and those offspring mature in about six weeks. The math gets ugly fast.
The takeaway: September is worth ten Novembers.
How small a gap actually is
A house mouse fits through a gap about a quarter inch wide. If a pencil fits, a mouse fits. Their skull is the limiting factor and it compresses more than seems reasonable.
The gaps they actually use, in rough order of how often we find them:
- Garage door corners. The bottom seal almost never sits flush at the corners. This is the number one entry point in St. Louis homes and it's the one nobody checks.
- Utility penetrations. Where the AC line set, gas line, water line or electrical conduit enters the house. Builders foam these and foam degrades.
- Weep holes and gaps in brick veneer.
- Dryer vents with broken or missing flapper covers.
- Sill plate gaps where the frame meets the foundation.
- Roof line and soffit gaps, especially where a lower roof meets a wall. Mice climb better than people expect.
- Crawl space vents with damaged screening.
Who gets them worst
Homes backing to woods, fields or creek corridors. That's a lot of Wildwood, Eureka, Chesterfield, Ballwin and anywhere along the Meramec.
Older homes with stone or block foundations. Homes with attached garages, which is most of them. And homes where a neighbor's problem became everyone's problem, because mice don't respect property lines.
Why traps alone usually don't finish it
Snap traps work on the mice that are out and moving. They don't touch the ones nesting in a wall void, and they don't address the reason mice keep arriving.
The pattern we see constantly: someone catches four mice, the catches stop, they think it's over, and six weeks later it starts again. The four they caught were foragers. The nest kept producing.
Traps are a useful tool. They just aren't a strategy on their own.
What actually works
Exterior interception before interior control. Tamper-resistant bait stations placed around the exterior perimeter intercept mice on their approach, before they're inside your walls. That's the whole idea behind Rodent Guard: stop the pressure at the property line rather than fighting it in your kitchen.
Interior monitoring in the right places. Unfinished basement areas, utility rooms, attic access, garage perimeter. Placed where mice actually travel, which is along walls, not out in the open.
Exclusion. Sealing the gaps. Steel wool and copper mesh into penetrations, hardware cloth over vents, and fixing that garage door seal. This is the part that makes the results permanent rather than ongoing.
Timing. Stations in place by early September, before the move-in, rather than in November after it.
What we do
Rodent Guard setup includes tamper-resistant exterior bait stations placed around the structure, interior monitoring boxes in unfinished or low-access areas placed safely away from kids and pets, and a full entry-point inspection with specific recommendations for what to seal.
On every visit after that we inspect and refill stations, track where activity is showing up and adjust placement, and check the interior monitors.
It bundles with Home Shield, and bundling means you only pay one setup fee instead of two.
What you can do this weekend
Go look at your garage door corners with the door closed. If you can see daylight, that's your problem.
Walk the foundation and find every place a pipe or wire enters the house. Anything you can push a pencil into needs steel wool and sealant.
Get pet food into sealed containers, and get bird feeders more than a few feet from the house. A bird feeder is a mouse buffet with a roof over it.
Cut vegetation back off the siding. Mice use it as cover and as a ladder.
Frequently asked questions
When do mice come into houses in Missouri?
The move-in starts in late August and peaks in September and October as overnight temperatures drop. Most homeowners don't notice until November, by which point the population is established.
How small a hole can a mouse get through?
About a quarter inch, roughly the width of a pencil. Young mice get through even less.
Do I have mice or rats?
Mouse droppings are rice-grain sized with pointed ends. Rat droppings are noticeably larger, more like a raisin. Rats are also far more common near water, alleys and commercial areas. In most St. Louis residential settings, it's mice.
How much does rodent control cost in St. Louis?
Rodent Guard is $85 a quarter on its own. Bundled with Home Shield, most homes pay $49 to $58 a month for both, with one first visit instead of two. See pricing for details.
Are the bait stations safe around kids and pets?
Yes. Exterior stations are tamper-resistant and secured, and interior monitors go in unfinished or low-access areas away from where kids and pets go. We'll walk you through every placement.
Will they come back next year?
Not if the entry points get sealed and the exterior stations stay in place. Ongoing service plus exclusion is what turns it from an annual event into a non-event.
Best time to deal with mice is before you hear them. Get a free quote or call (314) 912-2929, and read more about rodent control in St. Louis.
