Pesky roaches known as German cockroaches are practically invincible because of a quickly evolving resistance to pesticides, in some cases as fast as one generation.
That's the finding of a new study in Scientific Reports by Purdue University researchers, who looked at a combination of insecticides traditionally used to annihilate the bugs.
Exterminators often use a mixture of insecticides from different classes in hopes that if one class becomes ineffective, insecticides from the other classes will eliminate the pests. After conducting tests in multi-unit buildings in Indiana and Illinois over a six-month period using various combinations of insecticides, they were unable to completely eradicate cockroach populations.
"This is a previously unrealized challenge in cockroaches," lead study author Michael Scharf said.
Scientists found some roaches were resistant to a particular class of pesticides and that if they survived a treatment, then their offspring were essentially immune to the insecticide. They also discovered the bugs became immune to other classes of insecticides, even if they were not exposed to them or had no previous resistance.
"Cockroaches developing resistance to multiple classes of insecticides at once will make controlling these pests almost impossible with chemicals alone," said Scharf, a professor in Purdue's Department of Entomology.
He said the study findings were quite surprising.
“We would see resistance increase four or sixfold in just one generation,” Scharf said. “We didn’t have a clue that something like that could happen this fast.”
Female cockroaches can have up to 50 offspring during their three-month reproductive cycle. Scharf said even if a small percentage of cockroaches is resistant to an insecticide, the population can explode all over again within months.
Cockroach infestations are most prevalent in urban areas and low-income housing where a lack of resources makes it harder to fight the problem.
The bugs can pose a serious health threat because they carry dozens of types of bacteria, including E. coli and salmonella.
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