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Surface sprays miss the nest and the eggs
Most retail sprays are contact products. They knock down the insects walking across the treated area at that moment, and little else. The pests you see on the wall are a small fraction of the colony.
The rest sit out of reach in the harbourage: wall cavities, under the fridge motor, behind the dishwasher, above the ceiling and in subfloors. Cockroach egg cases are especially stubborn. A single German cockroach ootheca can carry dozens of young, and the casing shields them from surface products entirely. Once those hatch, you are back to square one within a few weeks, which is why the problem seems to keep returning no matter how much you spray.
Wrong product, wrong dose, wrong place
Pest control is not one product for everything. Ants, cockroaches, spiders and silverfish respond to different actives, formulations and placements, and getting the match wrong wastes the treatment.
Dose is the other trap. Too little does nothing useful. Too much is not stronger, it just costs more, lingers where it should not, and raises the safety risk to your family and pets. Retail labels are deliberately general, so most people either under-treat the real harbourage or over-treat the visible living areas. A licensed technician reads the species and the conditions first, then chooses the product, the concentration and the exact placement to suit.
Repeated spraying can make it worse
Spraying the same repellent product over and over can actually scatter a colony rather than kill it. Cockroaches and ants sense the treated zone and simply relocate, splitting a single problem into several new ones deeper in the home.
Resistance is the bigger long-term issue. German cockroaches in particular are known to build tolerance to commonly overused actives. Every partial DIY hit that kills the weak insects but leaves the survivors to breed selects for a hardier population. Professionals rotate actives and lean on non-repellent products and targeted baits precisely to avoid breeding a resistant colony.
The exterior and entry points get ignored
Pests come from outside. If the treatment stops at the kitchen skirting, the pipeline of new arrivals never closes.
A proper general pest treatment works the exterior perimeter, the weep holes and window frames, the eaves, the subfloor and the entry points around pipes and doors, as well as the internal hot spots. That barrier approach is most of what keeps a Brisbane home clear through the warm months, and it is the part DIY almost always skips. Sealing and treating where pests enter matters more than chasing the ones already inside.
When DIY is fine, and when it is not
DIY has a genuine place. If you have light, occasional activity, keep good hygiene, seal food and take out the rubbish promptly, then a well-placed bait station and basic prevention will often hold the line. There is no need to call anyone for a stray ant trail after rain.
It is worth a licensed technician once the activity is established or keeps coming back: a persistent cockroach population in the kitchen, ants you have treated more than once, or any sign of a nest you cannot locate. General pest treatments start from $180, and a professional treats the source, the harbourage and the perimeter rather than just the surface, so the result actually lasts. If you have sprayed twice and it keeps returning, that is the signal to stop spending on cans and get it done once, properly.