I had to get the cockroach exterminators out a few years ago. They sprayed drains etc. and recommended a spray every spring.... but I didn't....
Cockroaches can fly (some of them) and therefore nowhere is potentially safe. Elimination is almost instantaneous, but with it being strong chemicals, I was reluctant to over pollute. If they occurred again, I'd get sprayed again. They cause tourists no end of shocks, as they're rarely seen in northern Europe.... IMO
Cockroaches and the Council
Re: Cockroaches and the Council
Sorry for bumping this, but I didn't know that Permethrin helps with cockroaches. I heard it is used as an insecticide for agricultural pests and even as a lice treatment. But I wouldn't recommend using it indoors due to its high toxicity (especially for children and pets), cockroaches also quickly develop immunity to it. I could also recommend checking this article for both treatment and prevention tips because every next infestation is more unpleasant to deal with 

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