The fundamental rule: If you trap or poison mice without sealing entry points, you are managing a revolving door. New mice will enter as fast as you remove existing ones. Exclusion must come first or happen simultaneously with trapping.
Find the Entry Points
Inspect the exterior of the foundation at dusk with a flashlight. Look for gaps around utility penetrations (gas lines, electrical conduit, HVAC pipes), weep holes in brick veneer, deteriorated crawlspace vent screens, gaps where siding meets the foundation, and door threshold gaps that show daylight. Mice enter through gaps as small as 1/4 inch — the diameter of a dime. Note every gap you find before sealing anything.
Seal All Entry Points
Use steel wool packed into gaps, followed by caulk or expanding foam over the steel wool. Steel wool alone compresses and shifts; foam or caulk alone can be chewed through. The combination holds. For larger gaps, hardware cloth (1/4-inch mesh) secured with screws or staples before caulking. Replace damaged crawlspace vent screens with hardware cloth. Address door threshold gaps with replacement thresholds or door sweeps.
Set Snap Traps at Active Locations
Place snap traps perpendicular to the wall with the trigger end toward the wall — mice run along wall edges and will trip perpendicular traps as they pass. Peanut butter is the most reliable bait. Place traps in every area where you've seen droppings: behind appliances, in cabinet corners, along basement walls, in crawlspace perimeter areas. Check and reset every 24–48 hours. Density matters: 6–12 traps in a single room is not excessive for an active infestation.
When to Call a Professional
If trap counts stay high after a week, or if you're finding mice in areas you can't access for sealing, professional exclusion service is the efficient path. D&D Pest Control provides rodent exclusion and baiting programs for Franklin County and rural Missouri — visit ddpestcontrolmo.com.