As It Happens

Cockroaches have become nearly impossible to kill with chemicals, study finds

The roach populations not only survived the chemical assaults, but ultimately grew stronger. In some cases, they even saw their populations grow.

The disease-carrying pests are rapidly developing immunity to pesticides and passing it on to their young

Cockroaches are extremely resistant to pesticides, new research has found. (Joel Sartore/Getty Images )
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Cockroaches are tough as heck and getting tougher. 

The apartment-plaguing, bacteria-carrying pests have grown so resistant to pesticides, they are almost impossible to eradicate with chemicals alone, according to a new study published in Scientific Reports.

Not only did the cockroaches in the study survive being blasted by various combinations of powerful pesticides — in some cases, they grew even stronger and saw their populations grow.

"Resistance can build really quickly in populations — much quicker than we had anticipated," Michael Scharf, an entomologist at Indiana's Purdue University, told As It Happens guest host Susan Bonner.

3 extermination approaches 

The study, led by Scharf and his colleague O.W. Rollins, tested three chemical-based approaches on cockroach-infested apartment complexes in Indiana and Illinois over six months.

When they doused the bugs with a combination of two different pesticides, it was complete failure and the roach population boomed. 

When they rotated three different pesticides, one after the other, the scientists were able to keep the roach population flat, but not reduce it.

Michael Scharf is a professor of entomology at Purdue University. (Submitted by Michael Scharf)

Finally, when they used a single pesticide for the full six months, they almost got rid of the cockroaches entirely.

But the only reason this worked, cautioned Scharf, is that the scientists had already studied the cockroaches ahead of time and picked a chemical they showed no resistance to. 

When they tried the same approach with a different pesticide, the roach population grew. 

"We ... somehow have to figure out a way to screen cockroaches before pest control treatments begin so we can choose insecticides that have the best chance of being effective," Scharf said. 

They always come back 

But no matter how many roaches the scientists were able to kill, the results were short-lived. 

The surviving roaches not only developed resistance to the pesticide used on their brethren, they even became immune to other types of pesticides — a phenomenon known as "cross-resistance."

"It's explainable kind of at the molecular level," Scharf said. "You can have a type of physiological mechanism that just gives resistance to lots of different types of insecticides."

Even if only a few roaches survive being doused by a pesticide, they can pass on their chemical resistance to their many, many offspring. (Anne-Christine Poujoulat/AFP/Getty Images)

The surviving roaches then pass that immunity on to their offspring.

And since female cockroaches can produce 50 offspring every three months, their populations can bounce back entirely within a few months.

Scharf says while it may be nearly impossible to eliminate cockroach infestations altogether, people can still keep the unwelcome guests at bay the old fashioned way.

That means things like keeping your apartment clutter-free, taking out the garbage every day and not leaving pet food out.

"We've known for a long time that that can go a long way towards keeping cockroach numbers in check," he said. 

"If they don't have food and places to hide, their populations will shrink no matter what. They're not going to become resistant to that."

Written by Sheena Goodyear. Produced by Katie Geleff. 

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