Why shop-bought sprays sometimes make it worse
The problem is not that the can from the shop is weak. It is that most of them repel rather than eliminate, which pushes the colony deeper and makes the eventual treatment harder and more expensive.
It always starts sensibly. Insects appear, a can gets bought, the kitchen gets sprayed. They vanish for two days and come back more widespread, in rooms they were never in before. That is not bad luck. It is the predictable outcome.
Repellent versus slow-acting
Most consumer sprays are repellent. They kill on direct contact and leave a residue the insect avoids. The colony is not eliminated, it is scattered. Cockroaches that were all behind the fridge distribute themselves into deeper cracks and adjoining rooms.
One reachable harbourage has just become five that nobody can locate.
Professional treatment deliberately avoids this, using non-repellent products or baits that the insect carries back to the nest and shares. Slow is the feature here, because it reaches what you cannot see.
Three mistakes that make it worse
- Spraying open surfaces. Insects do not live on the floor, they cross it. Spraying there spends the product in the wrong place and exposes people to it.
- Spraying over bait. If bait has been placed and repellent spray goes down around it, the insect avoids the bait entirely and it stops working.
- Repeated light applications. This favours the individuals least affected by it and pushes the problem toward ones that are harder to treat.
What is genuinely worth doing yourself
Not everything needs a technician. These give a real result and do not spoil anything that comes after:
- Fix water leaks. The single highest-value thing you can do, because moisture is a survival condition for most pests.
- Seal the gaps around pipework with a suitable filler.
- Store food in sealed containers and take rubbish out at night.
- Clean under and behind appliances on a schedule, not only when a problem shows up.
Four actions that scatter nothing and invalidate no later treatment. They make it work better.
When to stop trying
If you have sprayed twice and they came back, the problem is not the quantity of product. You are treating what you can see while the source sits somewhere you have not reached. Every further attempt scatters the colony a little more and raises the cost of doing it properly later.