Washington State’s damp climate, especially west of the Cascades, creates ideal conditions for a wide range of insects and spiders to move indoors. The most common house bugs you’ll encounter include giant house spiders, carpenter ants, stink bugs, boxelder bugs, silverfish, springtails, and various small overwintering insects that seek shelter in fall. Here’s what to look for and what to do about each one.
Giant House Spiders
If you live west of the Cascade Mountains, the giant house spider is almost certainly the large, fast-moving spider you’ve seen darting across your floor. These brown to grayish-brown spiders have bodies roughly half an inch to one inch long, but their leg span can reach up to four inches in large adults. Their legs and bodies look hairy and matte rather than shiny, and their coloring is highly variable, ranging from light tan to dark brown with markings that can be faint or bold.
In the Puget Sound region, giant house spiders far outnumber hobo spiders and are the ones people encounter most often. Males are smaller and frequently mistaken for hobo spiders, which have a similar body shape. The distinction matters less than people think: despite years of concern, there is no conclusive evidence that hobo spider venom causes tissue damage in humans. A large body of research now shows it does not. Most “spider bite” wounds that won’t heal turn out to be bacterial infections, particularly MRSA, which can enter through any small puncture. Giant house spiders are harmless and actually help control other pests.
Carpenter Ants
Carpenter ants are one of the most structurally significant pests in Washington homes. These large black ants, often a quarter inch to half an inch long, don’t eat wood the way termites do. Instead, they excavate damp or moisture-compromised lumber to carve out smooth galleries for their colonies. If you find small piles of fine wood shavings near baseboards, window frames, or under decks, carpenter ants are the likely culprit.
You can distinguish carpenter ants from termites by three features: ants have elbowed antennae (termites have straight ones), ants have a narrow pinched waist (termites are the same width all the way down), and winged ants have two pairs of wings of different lengths (termite wings are all equal length and much longer than the body). Both pests are attracted to moisture, so they tend to show up in homes with leaky roofs, poor drainage, or wood that stays wet.
Dampwood Termites
Washington’s native dampwood termites are larger than the subterranean termites common in warmer states, and they have a strong preference for wood with high moisture content. They’re most often found in crawl spaces, decaying stumps in contact with a home’s foundation, or areas where plumbing leaks have softened structural wood. Unlike carpenter ants, termites actually consume the wood and live inside the tunnels they create.
The good news is that dampwood termites rarely infest sound, dry lumber. If your home stays dry and well-ventilated, your risk drops significantly. Repairing rotten or weather-damaged wood and eliminating sources of moisture are the most effective defenses.
Brown Marmorated Stink Bugs
Originally from Asia, brown marmorated stink bugs arrived in the eastern U.S. in the 1990s and have since spread to nearly every county in Washington State. They’re shield-shaped, about the size of a dime, and mottled brown with lighter bands on their antennae. You’ll notice them most in fall, when they congregate on sunny exterior walls looking for ways inside to overwinter.
Stink bugs don’t bite, don’t sting, and don’t damage your home’s structure. They’re purely a nuisance, and the smell they release when crushed or disturbed is their main offense. Pesticides applied indoors are largely ineffective against them. The best approach is prevention: sealing cracks around windows, doors, and utility pipes before autumn keeps the bulk of them out. A vacuum cleaner works well for removing the ones that get through.
Boxelder Bugs and Fall Invaders
Boxelder bugs are flat, black insects about half an inch long with distinctive red or orange lines on their wings. They feed on boxelder and maple trees during summer and then swarm toward buildings in fall, looking for sheltered spots to hibernate. They’re especially attracted to white or light-colored houses, where they gather on south-facing walls that catch afternoon sun.
Washington is also home to several species of small gray bugs, locally called “sage bugs” or “grey bugs,” that behave similarly. These feed on grassland plants during spring and early summer, then seek layered hiding spots in and around homes for winter. They’ll tuck themselves into shingle gaps, stacked firewood, folded fabric, and newspapers. Like stink bugs, these fall invaders are harmless to your home and to you. Sealing entry points before they arrive is far more effective than spraying after they’ve settled in.
Springtails
Springtails are tiny, pale or grayish insects (often barely visible) that jump when disturbed. If you’ve ever seen dozens of minuscule hopping creatures in your kitchen sink, bathtub drain, or around the soil of a houseplant, you’ve met springtails. They enter homes through window screens, open doors, vent pipes, or potted plants brought inside.
Springtails cannot survive without moisture. They die quickly indoors unless they find a damp environment, so their continued presence is a reliable indicator of excess moisture somewhere nearby, whether that’s a slow plumbing leak, poor ventilation in a bathroom, or an overwatered houseplant. The fix is straightforward: dry out the area, run a fan to speed evaporation, fix any leaks, and let potted plant soil dry between waterings. Once the moisture is gone, the springtails disappear.
Other Common Indoor Pests
A few other bugs show up regularly in Washington homes. Silverfish are teardrop-shaped, silvery-gray insects about half an inch long that move in a distinctive fish-like wiggle. They thrive in humid environments like bathrooms, basements, and laundry rooms, and feed on starchy materials including paper, wallpaper paste, and cardboard. Cat fleas are common in homes with pets and can persist in carpets and upholstery even after the pet has been treated. Yellowjacket wasps sometimes build nests in wall voids or attic spaces, and their presence inside the home usually means there’s an entry point in the exterior wall that needs sealing.
Keeping Bugs Out of a PNW Home
Washington’s wet climate means moisture control is the single most important thing you can do to reduce indoor pests. Carpenter ants, dampwood termites, springtails, and silverfish all depend on damp conditions. A few specific steps make a big difference:
- Fix leaks promptly. Check roofs, plumbing, and gutters regularly. Even small, slow leaks create enough moisture to sustain colonies of wood-destroying insects.
- Ventilate damp spaces. Use vented fans in bathrooms and kitchens, and run a dehumidifier in basements or crawl spaces that stay humid.
- Clean gutters and manage drainage. Water pooling near your foundation attracts ants and creates conditions for wood rot.
- Seal entry points. Cover unscreened crawl space openings, gaps around pipes and wires entering walls, vent gaps, pet doors, and cracks around windows and doors. Use sheet metal, screening, wood, or caulk depending on the size of the gap.
- Create clearance around your foundation. Keep firewood at least 18 inches off the ground and plants at least 3 feet from the foundation. Remove debris and leaf litter that give insects a place to colonize near your home.
- Repair damaged wood. Rotten or weather-damaged wood is a magnet for carpenter ants and termites. Replacing it removes the habitat they need.
Most of these steps also help with fall invaders like stink bugs and boxelder bugs. Sealing cracks and gaps before September, when these insects start looking for winter shelter, prevents the large indoor gatherings that catch people off guard in October and November.