Pest pressure is seasonal, and the seasons are more predictable than the pests. Every autumn, the first run of nights below about 50°F pushes rodents, cluster flies, stink bugs and overwintering spiders toward the warmest structure available. Every spring, ants that have been dormant in a wall void start foraging again and find the same crumbs they found last year.

The useful thing about that predictability is that it gives you a window. Two weekends of work in early autumn does more than a year of reactive treatment.

Start outside, not inside

Almost everyone begins a pest-proofing exercise in the kitchen. The kitchen is where you notice the problem; it is rarely where the problem starts.

Walk the perimeter of the building slowly with a flashlight, even in daylight — the beam picks up gaps that ambient light hides. You are looking for:

• Gaps where utility lines, gas pipes and cables enter the wall.
• Failed mortar at the base of the siding or at foundation corners.
• Vents with damaged or missing screens.
• The gap under a garage door, especially at the two bottom corners.
• Anywhere a downpipe discharges against the wall instead of away from it.

The quarter-inch rule

A house mouse fits through a quarter-inch gap; a young rat needs half an inch. If a pencil goes in, so does a mouse. This sounds like a party fact until you run a pencil along your own foundation line and find eleven of them.

Seal properly, once

Expanding foam is not a seal. Mice chew straight through it, and they will, because a foam plug is softer than the mortar around it. What works:

1. Copper mesh or stainless wool, packed tight into the void.
2. Mortar, cement or metal flashing over the top of it.
3. Door sweeps with a rigid backing on every external door.
4. Hardware cloth — quarter-inch, not the garden mesh — over vents.

Every professional exclusion job is the same three materials. The skill is in finding all the gaps, not in the sealing.

Then deal with the invitations

Sealing removes the way in. The next job is removing the reason to come.

• Move firewood at least twenty feet from the building, and off the ground.
• Cut back any branch touching the roof — that is a bridge, not a tree.
• Clear leaf litter from against the foundation; it holds moisture and everything that likes moisture.
• Empty and dry the pet bowl overnight.
• Take the trash out of the garage, not into it.

What each season actually needs

Spring is ants and wasps. Bait for ants before the colony is large; check the eaves and the shed roof for the first small nests, which take five minutes to deal with and are impossible to ignore by August.

Summer is mosquitoes and flies. Both are water problems. Tip out anything holding standing water once a week — a saucer under a plant pot is enough for a generation.

Autumn is the important one: rodents and overwintering insects. This is when the sealing work pays for itself.

Winter is quiet outside and active inside. If you hear scratching in a ceiling in January, something came in during October.

When to call somebody

Call if you see droppings inside a living space, if you hear movement in a wall or ceiling, if you find mud tubes on a foundation wall, or if a wasp nest is larger than a tennis ball or anywhere near a doorway. Everything else on this list is a Saturday and a caulk gun.