Researchers warn cockroaches are evolving to be "almost impossible to kill" with chemicals, bringing them "closer to invincibility" and posing "serious threats to human health".
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A new study published in the journal Scientific Reports has found that German cockroaches, the most common cockroach species in the world, quickly developed an immunity to several different insecticides.
"This is a previously unrealized challenge in cockroaches," says Purdue University professor Michael Scharf.
"Cockroaches developing resistance to multiple classes of insecticides at once will make controlling these pests almost impossible with chemicals alone."
Cockroaches are "serious threats to human health", carrying dozens of types of bacteria including E. coli and salmonella. Their saliva, faeces and body parts can also trigger allergies and asthma and sicken children.
The researchers tested three different insecticides on roach populations over six months, then surveyed the survivors. To their horror, they found all the insecticides were "mostly ineffective", with cockroach populations remaining stable or flourishing as resistance was passed down through the generations.
"We would see resistance increase four- or six-fold in just one generation," Scharf say. "We didn't have a clue that something like that could happen this fast."
Their results have dramatic implications for pest controllers, who often combine different insecticides to make sure they're killing bugs that might have developed an immunity to one kind.
Scharf recoments pest controllers combine chemical treatments with traps, improved sanitation and vacuums that can remove cockroaches.
"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," he adds.
"Combining several methods will be the most effective way to eliminate cockroaches."