What Rats Are Doing in October, by City.

If rodent control had a single month circled on the calendar, it would be October. Across Oregon, this is when the year's accumulated population — bred all summer in fields, gardens, and woodpiles — collides with the first sustained cold and rain and starts looking, hard, for somewhere warm and dry to spend the winter. That somewhere is frequently your house. But Oregon is not one climate, and "October pressure" means something different in a Portland basement than it does in a Bend garage. Here is the month, read city by city.

October is when a whole summer's worth of rodents goes looking for a door at once. Where that door is depends on where you live.

Portland and the inner Willamette Valley

In Portland, Salem, and the valley floor, October is the classic indoor migration. The Norway rat and the house mouse dominate, and the trigger is the shift from dry September to wet, cooling October nights. Outdoor harborage that was comfortable all summer — the ivy bank, the woodpile, the compost — turns cold and saturated, and the animals move toward the warm, dry envelope of heated buildings. This is the month the rim joist, the garage corner, and the old sewer lateral all start earning their reputations. The first scratching in the wall, the first droppings under the sink, the first chewed bag of dog food: in the valley, they cluster in the back half of October with remarkable consistency.

Bend, Redmond, and the high desert

East of the Cascades the story is colder and faster. Bend's October can deliver hard freezes and even early snow, and the dominant animal is the deer mouse (Peromyscus maniculatus), a native field mouse that does not wait around. As the high desert's nighttime temperatures crash, deer mice push into garages, sheds, cabins, crawl spaces, and the walls of homes at the wildland edge. The high-desert October migration is sharper and earlier than the valley's, and it carries an added concern the valley largely lacks: deer mice are the reservoir for hantavirus, which makes their move indoors a genuine health matter, not just a nuisance. Out here, October is when seasonal cabins and shop buildings get colonized while no one is watching.

The southern Oregon valleys: Medford and the Rogue

In the Rogue Valley, October has a second character on top of the usual fall pressure: it is harvest. Southern Oregon's pears, apples, and backyard fruit are dropping and fermenting, and the region's growing roof-rat population is feasting and breeding on that windfall before the trees go bare. As the fruit runs out late in the month and the nights cool, those well-fed climbers turn toward attics and outbuildings. Medford's October is therefore a transition — from an outdoor fruit economy to an indoor search for shelter — and the roof rat, a strong climber, heads up rather than down.

The coast: Astoria, Newport, and the rainforest edge

The Oregon coast plays October differently because it never had a dry summer to end. The coastal strip's mild, wet, maritime climate keeps rodent pressure high year-round, so October is less a sudden migration than a steady continuation, intensified by the season's bigger storms. What October does bring on the coast is weather that drives animals to cover and storms that open new entries — lifted flashing, blown debris, a knocked-loose vent. Astoria's old hillside housing stock, already covered in our pre-1950 piece, meets the wet season with a lot of doors ajar.

Eugene, Corvallis, and the south valley

The southern Willamette Valley combines the valley's indoor-migration pattern with a strong agricultural edge. Eugene and Corvallis sit in the middle of the grass-seed capital of the world, and by October the fields have long since been harvested, which means the small-mammal populations that lived in them have already been displaced and are looking for the next habitat — often the residential edge. University towns add their own wrinkle: thousands of rental units, many of them older and poorly sealed, fill with students in September, and October is when those houses register the consequences.

What to do, wherever you are

1. Treat late September as the deadline, not October. The goal is to have the house sealed before the migration starts, not during it. If you only do one thing, walk the perimeter and close the ground-level gaps — rim joist, garage corners, vents.
2. Cut the outdoor harborage now. Move the woodpile off the foundation, cut back the ivy and dense plantings against the house, pick up fallen fruit, and secure the compost and the pet food.
3. Watch the early signs. Droppings, gnaw marks, and night sounds in October are the leading edge of a population that will breed indoors all winter if left alone. Early action in October prevents a February colony.
4. In high-desert and rural homes, take the health step seriously. Where deer mice are the animal, follow ventilated cleanup precautions rather than sweeping droppings — see our hantavirus and cleanup pieces.

When to call

October is the month operators are busiest, and for good reason — it is the highest-leverage time of year to intervene. If you are seeing fresh sign now, the value of acting is at its peak: one animal sealed out and removed in October is a litter, or several, that never happens over the winter. Whether you are in a valley basement, a Bend shop, or a Medford attic, the principle is the same and the timing is the whole point. The rats are doing the same thing everywhere this month — moving toward shelter — and the only variable that matters is whether your house is ready before they arrive.