If it feels as if attempts to fix a cockroach infestation by spraying pesticides are futile, there may be a reason for that.
Researchers warn that pesticides are failing to work on the most common type of cockroach in the world, the German cockroach.
The study, published in the journal Scientific Reports, found that the insects are developing a resistance to the insecticides on which exterminators rely.
“This is a previously unrealized challenge in cockroaches,” said Michael Scharf, an author of the study and a professor at Purdue University. “Cockroaches developing resistance to multiple classes of insecticides at once will make controlling these pests almost impossible with chemicals alone.”
Over the span of six months, Scharf and his fellow researchers used pesticide combinations in Indiana buildings containing cockroaches. One treatment involved a rotation of three different insecticides. Another used a mixture of two insecticides, and the last one used insecticides to which the cockroaches seemed to not be immune.
They captured cockroaches before beginning the experiment to test which pesticides would be the most effective.
“If you have the ability to test the roaches first and pick an insecticide that has low resistance, that ups the odds,” Scharf said. “But even then, we had trouble controlling populations.”
In the first treatment, which involved a rotation of three pesticides, researchers were able to keep cockroach populations steady but could not reduce them. The second treatment method, involving a mixture of pesticides, did not work at all.
The third trial found mixed results. One of the selected insecticides worked, but another, which the insects had about a 10% starting resistance to, failed. The percentage of cockroaches that were immune produced offspring that were resistant, too. Scharf said one generation could see a fourfold or sixfold increase in resistance.
“We didn’t have a clue that something like that could happen this fast,” Scharf said.
He recommended that pest management combine insecticides with other methods, like traps and vacuums to 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,” Scharf said.