Bed Bugs in Apartments: How They Move Between Units

A bed bug infestation in an apartment is a building problem being reported one unit at a time. That is the single most useful thing to understand about it, and it explains almost every frustrating outcome in multi-family pest control: the treatment that worked for six weeks, the neighbour who will not let anyone in, the second infestation that arrives from a direction nobody inspected.

Bed bugs move through structures along routes that were built for something else entirely. They do not need doors and they do not respect lease boundaries.

The four routes between units

Shared wall cavities. The void behind a shared wall is continuous. It is dark, undisturbed, temperature-stable and close to two sleeping populations at once, which makes it near-ideal harbourage as well as a corridor.

Outlet and switch boxes. These are the specific access points into that cavity, and they matter more than the wall itself. A standard outlet box is an unsealed opening into the void, on both sides of the wall. When people ask how bed bugs got into a unit with no obvious source, this is usually the answer.

Vertical chases. Plumbing and wiring runs connect stacked units. This is why the unit directly above and directly below matter as much as the ones either side, and why an infestation on the third floor can genuinely originate on the first.

Door thresholds and corridors. Bed bugs will cross a hallway. They do it more when a unit has just been treated, which is the part that makes single-unit treatment actively counterproductive rather than merely insufficient.

There is a fifth route that is not structural: shared laundry. A common machine room moves infestations between floors and between buildings on the same site, carried in the basket rather than through the wall.

Why single-unit treatment fails so reliably

Treating one apartment and leaving the neighbours uninspected is the most expensive decision available in multi-family pest control, and it fails in a specific, repeatable way.

A residual chemical treatment makes the treated unit hostile. It does not make it hostile instantly or completely — it kills the insects that contact the deposit, over days. Meanwhile the population responds to a degrading environment the way it responds to any disturbance: it disperses. Some of that dispersal goes into the wall void, and from the wall void into the unit next door, which has not been treated and is not hostile at all.

Six weeks later the residual has degraded, the treated unit is habitable again, and the population that left comes back. The tenant reports a “reinfestation” and everyone concludes the treatment failed. It did not fail; it was scoped wrong. The insects were never all inside the lease boundary.

This is also why the cheap quote costs more. A quote covering one unit will always undercut a quote covering five, and the five-unit scope is the one with a chance of holding. Every dollar saved on inspection scope comes back multiplied, usually across more units than the first time.

What a correct scope looks like

The working minimum in any connected structure is the reported unit plus the units above, below and on either side. Inspected — not necessarily treated. Inspection determines the treatment scope; the complaint does not.

Canine detection is what makes that economically viable at building scale. A certified team can sweep a forty-unit building in a morning, where a visual inspection of the same building would take days and still miss early activity. For a landlord facing a single complaint, sweeping the surrounding block is a modest cost against the risk of discovering in October that the problem has been on three floors since August.

Treatment then needs to happen as a coordinated block rather than a rolling schedule. Treating units one at a time, in the order tenants complain, lets the population stay permanently one unit ahead of the work. Everything with confirmed activity gets treated on the same schedule, with the surrounding buffer included.

For occupied buildings, chemical programmes usually suit better than heat, because residual products keep working after the technician leaves and units do not need to be emptied for a full day. In multi-family housing, where reinfestation from an untreated neighbour is the main risk, that persistence is genuinely decisive. The trade-off is the two-to-three visits per unit, timed around the six-to-ten-day egg hatch.

What to do if you rent

Report it in writing, immediately. Not by text, not in the hallway. Email creates its own timestamp and its own copy. State the date you first noticed evidence, describe what you actually found rather than what you conclude from it, attach dated photographs, and ask for a written response with an inspection date.

Keep sleeping in your own bed. This is the advice everyone resists and it is correct. Moving to the sofa or a spare room relocates the feeding site, and the population follows the carbon dioxide and body heat — so you establish a second harbourage in a clean room and make the eventual treatment larger. Encase the mattress, put interceptors under the bed legs, and stay put.

Do not buy a spray, and above all do not use a fogger. Retail products are overwhelmingly pyrethroid-based, and pyrethroid resistance is widespread. What survives is the repellent effect: the insecticidal result is weak while the insects scatter into wall voids and, in an apartment building, into your neighbours’ units. Foggers are the worst version, treating open air and reaching no harbourage at all. In multi-family housing this is not just ineffective — it converts your problem into the building’s problem, and it is the fastest way to end up genuinely at fault for something you did not cause.

Do launder. Hot wash plus thirty minutes in a dryer on high kills every life stage in bedding and clothing. Bag it afterwards and keep it sealed.

Ask about the neighbours in writing. Ask the property manager, in writing, whether the adjoining units have been inspected. This is a reasonable question, the answer matters to whether your treatment holds, and the written record is useful if the treatment fails and the question of why comes up.

Who pays, under Washington law

In most Washington rental housing, RCW 59.18.060 places a duty on the landlord to keep the premises fit for habitation, and controlling infestation sits within that duty. That is the general position for apartments and multi-unit buildings.

The exception that national articles almost never mention concerns single-family residences. In that housing type, the landlord’s infestation duty applies at the commencement of the tenancy rather than continuously through it. If you rent a house rather than a unit in a building, the answer to “must my landlord deal with this” may be genuinely different — and establishing which category you are in is the first step in any dispute, before anyone argues about the bill.

RCW 59.18.130 sets out tenant duties, and it is the provision landlords rely on when arguing that a tenant caused an infestation. In practice causation is hard to establish. Bed bugs arrive by hitchhiking, they move between units through the structure, and demonstrating that a particular tenant introduced them requires more than an assertion. If you receive a bill, ask in writing for the evidence of causation and for the inspection results of the adjoining units before paying anything.

Retaliation against a tenant for asserting rights under the Residential Landlord-Tenant Act is restricted in Washington, and reporting an infestation in writing is an ordinary exercise of those rights.

General information, not legal advice. Tenancy outcomes depend on facts specific to your lease and building. For advice on your situation, contact the Tenants Union of Washington State, Washington LawHelp, or a landlord-tenant attorney. Nothing here creates a legal relationship or substitutes for that advice.

If you manage the building

The compliance framing is worth taking seriously, because response speed is what a good-faith response is judged on. Log the complaint with a date, acknowledge it in writing, inspect wider than the report, treat as a block, and keep the documentation — the dated inspection record naming every unit and its findings, the treatment record, and the follow-up result.

Prepare tenants properly, and resource it. Failed preparation is the leading cause of a treatment that has to be repeated at the landlord’s cost, and prep instructions handed to a resident who is elderly, working two jobs or does not read English fluently are not a plan. Interceptors in treated and adjacent units give an objective read on whether anything survived, rather than waiting for the next complaint to serve as the monitoring system.

Method choice is also different once you share a wall — heat, chemical, steam or fumigation covers why single-unit heat is a reinfestation risk in a connected building.

The apartments and multi-family service page covers the operational side in more detail, and the commercial hub covers the other property types where the same building physics apply.

Call us and we will get an exterminator out to the unit. If you need the treatment sequence itself, bed bug removal sets out what a complete job involves.