Cockroaches Are Becoming Resistant to Bug Sprays, Controlling Them Will Become 'Almost Impossible'

Cockroaches are developing a resistance to insecticides used in exterminators' bug spray and may soon be "almost impossible" to control with chemicals alone, scientists warn.

New research recently published by Purdue University in West Lafayette, Indiana, has suggested that a strain of German cockroach—Blattella germanica L.—will only become more difficult to eliminate as future generations are becoming increasingly immune to human efforts of population control.

Such control is very necessary, scientists said, because the pests are often a threat to human health. They can spread bacteria and the feces they produce trigger allergies and asthma in adults and children.

"This is a previously unrealized challenge in cockroaches," commented Michael Scharf, a professor in the Department of Entomology, who led the six-month-long study and whose findings have now been published in the journal Scientific Reports. "Cockroaches developing resistance to multiple classes of insecticides at once will make controlling these pests almost impossible with chemicals alone."

The University of Florida's Institute of Food and Agricultural Sciences describes the type of bug being tested as "the species that gives all other cockroaches a bad name." It says they are "unable to survive in locations away from humans or human activity" and thrive in warm indoor areas with access to food and water.

According to Purdue researchers, insecticides used to control their spread come in different classes and each works differently to kill cockroaches. Bug sprays often contain a mixture of multiple classes to make sure that at least one will have an impact on the tough insects. New experiments aimed to test their effectiveness.

View of a giant cockroach (Blaberus giganteus) during an exibition at the Explora Park in Medellin, Antioquia department, Colombia on February 5, 2013. New research suggests some roaches are becoming increasingly resistant to bug spray. RAUL ARBOLEDA/AFP/Getty

German cockroach strains were collected from apartments in housing sites from Indianapolis and Danville. The formulated chemicals were purchased from Univar Solutions, the research paper said.

In the first test, three types of insecticide were rotated for three months, before the process was repeated. In the second, a mixture of two insecticides was used for the six months. And in the third, scientists used a single insecticide on roaches found to have a low starting resistance, also for the entire six months.

In the first rotation experiment, researchers determined that they could keep the population flat, but failed to reduce it. In the mixture test, nothing worked and the cockroach population soon flourished.

But it was what happened next that was extraordinary.

In one single-insecticide test, scientists found they were able to essentially eliminate the roach population because of the low starting resistance to the chosen insecticide. However, it wasn't all good news. In another single-insecticide experiment, a population with only 10 percent starting resistance actually grew.

According to Scharf, not only would roaches that survived a treatment be "essentially immune" to that type of insecticide class in the future—their offspring would be too. Crucially, they also gained resistance to other classes "even if they hadn't been exposed to them and had not had previous resistance.

"We would see resistance increase four or six-fold in just one generation," Scharf said. "We didn't have a clue that something like that could happen this fast."

Scharf said female roaches have a three-month reproductive cycle, which results in about 50 offspring. This means even a small number of insecticide-resistant cockroaches could quickly spell trouble. A population wiped out by a single working bug spray could spring back in mere months, research suggested.

According to Scharf, humans now need to combine chemical treatments with traps and improved sanitation, and he warned areas of low-income or federally subsidized housing are likely to be worst hit by the issues.

The research paper is titled "Rapid evolutionary responses to insecticide resistance management interventions by the German cockroach." Authors also included Mahsa Fardisi, Ameya Gondhalekar and Aaron Ashbrook.

A German cockroach feeds on an insecticide in the laboratory portion of a Purdue University study that determined the insects are gaining cross-resistance to multiple insecticides at one time. John Obermeyer/Purdue Entomology