Why cockroaches appear in Maltese homes every July

Every year, from the last week of June, the calls start. Large, dark, glossy cockroaches in the bathroom at night. On the garage floor. Around the pool skimmer. Almost always at ground level, almost always after dark.

That insect is the Oriental cockroach, and it is not a hygiene problem. It is a drainage problem.

What changes in July

Oriental cockroaches live in drains, soakaways, wells and manholes all year round. They need moisture and they avoid light. For most of the year the drain run gives them everything they need and you never see one.

Then two things happen at once:

• The water table drops and standing water in soakaways dries out
• Ground temperatures climb past the point where the drain run is comfortable

The colony moves. It follows the pipework upwards, and the first dry, cool opening it finds is a floor gully, a shower waste or a gap around a soil pipe, inside your building.

Seeing one large cockroach indoors at night almost always means there is a population in the drainage nearby. Seeing several means it is already established.

The three things that actually work

1. Seal the route in, not the insect

Oriental cockroaches are poor climbers. They arrive at floor level, through gaps you can find in ten minutes with a torch:

• Floor gullies and shower wastes without a working trap
• The gap where a soil pipe passes through a wall or slab
• Manhole covers with a perished or missing seal
• Unused drain connections that were never capped

Fitting a sealed gully grate and filling pipe penetrations removes the route. This is the single highest-value thing you can do, and it lasts.

2. Keep the traps wet

Every waste in the building has a U-bend holding a plug of water. That water is the barrier. In a bathroom or laundry that is not used for a few weeks in summer, it evaporates and the barrier is gone.

Run every tap and shower for thirty seconds once a week. In a property that stands empty, pour a cup of water down each waste before you lock up.

3. Treat the source, not the sighting

Spraying the cockroach you can see does nothing about the hundred you cannot. Professional treatment for this species means treating the drain run and the external harbourage (the manholes, the soakaway, the shaded courtyard) with a residual product, and placing monitors to confirm the population has actually gone rather than merely gone quiet.

When to call someone

Call if you have seen more than one indoors, if you are seeing them in daylight, or if you are running a food business of any kind. In a commercial kitchen a single sighting during an inspection is a finding, and findings need a documented response.

If you have found one insect, once, after a hot night, and your gully is properly sealed, keep the traps wet and watch for a fortnight.