Why Cockroaches Are So Hard to Get Rid Of

One

The Egg Case Is Sealed Against You

Cockroaches do not lay loose eggs. They produce an ootheca — a hard purse-shaped capsule holding anything from around fifteen to forty eggs depending on the species — and that capsule is substantially protected against insecticide.

The consequence is the pattern everyone reports. A treatment goes in, the visible population disappears within a day or two, and everything is fine for a few weeks. Then the oothecae hatch and there are cockroaches again, and it looks as though the treatment failed.

It did not fail. It killed the adults and nymphs it could reach and could not reach eggs inside a capsule. That is precisely why a follow-up visit, timed against the hatch, is part of the job rather than an admission that something went wrong.

The German cockroach makes this worse: the female carries her ootheca attached to her body until it is nearly ready to hatch, keeping it in the same protected places she is.

Two

Resistance, and the Species That Has It

The German cockroach is one of the most insecticide-resistant insects there is. Populations have developed resistance to multiple classes of insecticide, and because they breed quickly in heated buildings, resistance can build within a single infestation that is being treated badly.

There is also cross-resistance, where selection for resistance to one compound confers some resistance to others the population has never met. This is the technical reason professional work rotates actives and leans on gel baits with different modes of action rather than repeatedly hitting the same population with the same thing.

It is also why the shop-bought route is particularly poor value here. A pyrethroid spray against a resistant German cockroach population mostly succeeds in scattering it.

Three

They Live in the Building, Not in the Room

Cockroaches are thigmotactic — they want contact on both sides of the body — so they spend the day wedged into tight gaps, not sitting in the open. In practice that means the motor housings of the fridge and the dishwasher, the cavity behind a kitchen unit, the void round a waste pipe, a hollow door frame, the underside of a counter, a warm electrical enclosure.

Three things follow. First, what you see at night in a kitchen is a fraction of what is there. Second, a surface spray in the open misses almost the whole population. Third, in a block of flats or a parade of shops they move between units through shared voids and service runs, which is why treating one unit while the neighbours do nothing does not hold.

German cockroaches favour warmth and are found high as well as low — behind and above appliances. Oriental cockroaches prefer cool and damp and are found low: drains, basements, ducts, under floors. Where you find them narrows which one you are dealing with, and that changes where the treatment goes.

What works

Gel Bait, Patience and Two Visits

The approach that actually works is baiting rather than spraying, placed into the harbourages rather than on open surfaces, combined with monitoring, and followed up on a schedule that assumes the oothecae will hatch.

Gel bait works for cockroaches the way it does for ants: it is carried, shared and passed on through the population, including to individuals that never touched the bait themselves. Which is exactly why a repellent spray applied alongside it ruins the treatment — it drives them away from the bait.

Your part is real: clear the harbourages you can, pull appliances out so they can be reached, deal with food and grease behind and beneath units, and do not spray anything yourself in the meantime. Cockroach treatments here are guaranteed, and a follow-up visit may be recommended as part of the course.

In a food business the priority is different again — records, scheduled visits, and the ability to show what was found and what was done.

Short answers

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