This is the most common call we get: somebody sprayed, the kitchen was clear for a fortnight, and now it is worse than before. The spray was not faulty. It simply did a different job from the one that was needed.
What you see is a fraction of what is there
Cockroaches spend most of their lives in harbourage, the warm, tight, dark gaps they can touch on both sides at once. Behind the fridge motor, inside the gap between a cabinet and the wall, under a sink, in the cavity round a waste pipe. What you meet at night on the counter is the overflow, and it is a small share of the population. A contact spray reaches that share and almost nothing else.
And the eggs are not affected at all
A female carries her eggs in a hardened case called an ootheca, and in several species she glues it into a crevice and leaves it. The case is designed to survive: it is essentially waterproof, and a surface spray does not penetrate it. Depending on species and temperature it hatches in roughly three to eight weeks. That is your three-week return, arriving exactly on schedule.
What actually works
- Gel bait in the harbourage rather than spray on the surfaces. It is carried back and shared, so it reaches insects that never come out.
- A second visit timed to the hatch, not to the calendar. Treating once and hoping is how the cycle restarts.
- Closing the gaps. Silicone round pipe entries and a strip under a badly fitting door removes the harbourage permanently.
- Dealing with water before food. A dripping tap or a damp cupboard base sustains a population that has almost nothing to eat.
- Emptying the bin at night rather than in the morning.
The part nobody enjoys hearing
In flats and terraces, cockroaches move between units through shared service ducts and waste stacks. You can do everything right and still be reinfested from next door. If you are in a building where several households have the same problem, treating together is far cheaper and works far better than each treating alone, and it is worth raising with the management committee before anybody spends money.