What happens in late September
Rodent calls in Snohomish County climb sharply from the last week of September through November, and the trigger is the first sustained cold rain. Norway rats that spent the summer perfectly happy in a blackberry hedgerow, a compost pile or a creek bank start looking for somewhere warm and dry. The warmest, driest structure in the yard is the house.
Nothing invaded. The population was always there. The weather simply changed the calculation, and your crawl-space vent screen has been rusted out for a decade.
Seal it before it rains
- Crawl-space vents: replace rusted screen with quarter-inch galvanized hardware cloth, fastened, not wedged.
- Foundation and utility penetrations: copper mesh packed tight, then sealant or mortar. Expanding foam alone is a snack.
- Garage door: a rubber sweep that actually touches concrete across the full width, corners included.
- Gable and roof vents: screened. Chimney: capped.
- Roof-to-wall junctions, fascia and soffit gaps: repaired, because that is the roof rat door.
- Dryer vents and hose bibs: sealed at the sleeve.
- A rat gets through a hole the size of a quarter. A mouse gets through a hole the size of a dime.
Cut the harborage
Blackberry is the rodent nursery of the Puget lowlands. So is ivy on a wall, English laurel against the siding, a woodpile on bare dirt, a pile of construction material behind the shed, and an unused hot tub. Aim for ten feet of clear, open, dry ground between anything dense and the foundation.
Then cut the food. Bird feeders are rat feeders. Fallen apples and plums are rat food. Pet bowls left out overnight are rat food. Compost without a sealed base is a rat colony. Garbage cans need lids that latch.
Finally, cut the bridges. Trim tree limbs back at least six feet from the roofline. Roof rats do not climb siding, they walk in off a cedar branch or a utility line.
If they are already inside
Trap before you seal. Sealing a structure with an active population inside gives you dead rats in a wall void and a smell that lasts a month. Trapping goes on the runways, identified by droppings and greasy rub marks, and it runs for two to four weeks of servicing.
Be careful about rodenticide near a living space, and be careful about it outdoors too. Secondary poisoning of barn owls, hawks, coyotes and neighborhood cats is well documented, and those animals are doing the same job for free.
And a serious note on mice: deer mice are the hantavirus reservoir in the Pacific Northwest. Never dry-sweep or vacuum mouse droppings or nesting material in a crawl space, shed or barn. Wet the material down with a disinfectant first, and get help for a heavy contamination.
The order that works
Inspect. Trap. Seal. Clean up. Correct the yard. Then, and only then, is a maintenance program worth paying for. Doing it in any other order buys a subscription rather than a solution.
If you are hearing running in the attic after dark or finding rice-grain droppings behind a cabinet, the population is established and the sealing conversation needs to happen with someone under the house.
Further reading: Snohomish County Health Department.
If you would rather hand this to somebody, see Rodent Control in Marysville, WA or call 360-233-2008.