The cockroaches that plague our homes are even more indestructible than we thought, according to a recent study by researchers at Purdue University in Indiana. The bug scientists used a variety of strategies and different insecticides to root out real-life infestations of the German cockroach (Blattella germanica), but found that the roaches were able to survive nearly all of their efforts.
What’s worse, their research suggests that these roaches can quickly develop resistance to more than one insecticide at the same time—a feat we didn’t know they were capable of.
German cockroaches are one of many insects that have steadily learned to fend off the chemical weapons we use against them. That’s forced exterminators and entomologists alike to scramble for new strategies to control home infestations. These have included periodically rotating the classes of insecticides for a pest treatment, combining multiple insecticides, and relying more on gel baits rather than sprays.
The team behind this study, published in June in Scientific Reports, decided to compare three of these strategies in the real world. At three roach-infested apartment complexes in Illinois and Indiana, they either used a rotation of three professional-grade insecticides, rotating every month for six months; sprayed two insecticides at the same time monthly; or laid down monthly gel baits with an insecticide that the roaches were previously shown to be susceptible to. The roaches in these homes had earlier been found to have at least some resistance to nearly all classes of insecticides.
The only method that put a significant dent in the roach population, they found, was the gel bait. The roaches laughed off the combination insecticides and even started to spread to new homes, while their numbers were unaffected by the rotation method. And the gel bait trick only worked because the roaches were especially vulnerable to it; in areas where only 10 percent of the population was resistant to the specific chemical used, the population again flourished.
Roach resistance is nothing new, but the researchers were surprised by their ability to foster as much “cross-resistance” as they did. Cross resistance is when surviving roaches exposed to one insecticide are then able to better resist a different insecticide. But while scientists have documented cross resistance in German cockroaches in the lab, these roaches were able to cross-resist a variety of insecticides across different classes to a degree not documented before, according to the paper. And that’s really bad news for the future of roach eradication.
“This is a previously unrealized challenge in cockroaches,” lead author Michael Scharf said in a release from Purdue University. “Cockroaches developing resistance to multiple classes of insecticides at once will make controlling these pests almost impossible with chemicals alone.”
Not all hope is lost, though. Scharf and his team were able to successfully combat the infestations left behind after the study was over with a rotation method using gel baits. And if people can first nail down the exact chemicals roaches in a particular infestation are resistant to, then it’s possible to find the right mix of insecticides to manage it. Non-chemical methods, like keeping food secure, repairing cracks where roaches get in, and vacuuming up large infestations, can also increase the chances of success.
“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.”
Unfortunately, being able to afford these treatments is a major hurdle for the disadvantaged people most affected by household pests like roaches and bed bugs. It’s likely humans will be plagued by these insects for centuries to come.