Cockroach populations are rapidly evolving to become "almost impossible" to kill with chemicals alone, a team of US scientists said.
When cockroaches survive an insecticide treatment, they and their offspring quickly become "essentially immune" to it, a Purdue University study published in Scientific Reports found.
But crucially, they also develop immunity to a range of other insecticides, even if they were never exposed to them – something the scientists call "cross-resistance".
"We didn't have a clue that something like that could happen this fast," Michael Scharf, professor of entomology – the study of insects and their relationship to humans – said in a statement.
"We would see resistance increase four- or six-fold in just one generation."
Female cockroaches can produce up to 50 offspring every three months, passing their immunity on to them. So even if just a fraction of a cockroach population survives an insecticide treatment and becomes cross-resistant, a decimated population could skyrocket again.
The scientists said in the statement insecticides should still be an important component in controlling cockroach populations.
However, they said that when the insecticide alone could completely eliminate cockroaches, the most effective way to deal with infestations was to combine pesticides with other methods – including traps, improved sanitation and vacuums.
"Some of these methods are more expensive than using only insecticides, but if those insecticides aren't going to control or eliminate a population, you're just throwing money away," Scharf said. "Combining several methods will be the most effective way to eliminate cockroaches."
