Why Cockroaches Spread So Fast in Apartments — And How to Stop It

One cockroach sighting in an apartment building is rarely just one cockroach. By the time a tenant reports the problem, a colony is almost always already distributed across multiple units — which is why building managers who act immediately on the first complaint protect themselves from a far larger and costlier problem weeks later.

How Cockroaches Move Through Buildings

Cockroaches travel through shared infrastructure: plumbing chases, electrical conduits, HVAC pathways, gaps behind baseboard heaters, and hollow walls. A German cockroach — the most common species in Ontario apartment buildings — doesn't need much. A gap of 1.6 mm (about the thickness of a dime) is enough.

They're also nocturnal and photophobic, so a daytime sighting almost always means the harborage areas are overcrowded — the colony is larger than it first appears.

The German Cockroach Problem

German cockroaches (*Blattella germanica*) are the dominant species in London, Ontario's multi-unit residential buildings. They differ from outdoor species in critical ways:

• They don't come from outside. They are typically introduced through infested furniture, grocery boxes, or second-hand appliances.
• They breed faster than any other common cockroach species. A single female produces an egg case (ootheca) containing 30–40 eggs roughly every 3–4 weeks.
• They develop resistance to pesticides quickly when the same products are rotated carelessly.

This is why over-the-counter foggers and sprays are counterproductive: they scatter cockroaches into wall voids and adjacent units without eliminating the population, and they contribute to resistance.

The Right Treatment Protocol

Effective cockroach control in multi-unit buildings requires:

1. Unit-by-unit inspection to map distribution and identify harborage zones

2. Gel bait application in cracks, crevices, and under appliances — not sprays that scatter colonies

3. Insect growth regulators (IGRs) to interrupt the breeding cycle

4. Void treatments for wall and ceiling voids where colonies establish

5. Follow-up monitoring with bait stations to track re-population and confirm elimination

For buildings with active infestations, treating adjacent units — not just the reported unit — is standard practice. Treating one unit while leaving a connected colony untouched guarantees re-infestation within weeks.

What Property Managers Can Do Right Now

• Respond to the first complaint within 24 hours
• Seal gaps around pipes under sinks and behind toilets in all units
• Replace any failed caulking around plumbing penetrations
• Establish a written pest management protocol with a licensed exterminator

We work directly with London-area property managers to implement building-wide cockroach control programs. Our reporting keeps you informed at every stage — which matters for your compliance obligations under the Residential Tenancies Act.

View our property manager services or book a commercial inspection.