Roaches rule as they become harder to eliminate

n A German cockroach feeds on an insecticide in a laboratory at Purdue University. PICTURE: John Obermeyer

COCKROACHES are increasingly becoming invincible with cross-resistance to exterminators’ best insecticides.

Purdue University researcher Michael Scharf, professor in the Department of Entomology, says cockroaches are becoming more difficult to eliminate as they develop cross-resistance to exterminators’ best insecticides.

“This is a previously unrealised challenge in cockroaches,” Scharf says.

“Cockroaches developing resistance to multiple classes of insecticides at once will make controlling these pests almost impossible with chemicals alone.”

Cockroaches carry dozens of types of bacteria, such as E. coli and salmonella.

Each class of insecticide works in a different way. Exterminators often use a mixture of classes or change classes from treatment to treatment.

The hope is even if a small percentage of cockroaches is resistant to one class, insecticides from other classes will eliminate them.

Scharf tested these methods over six months. In one treatment, three insecticides from different classes were rotated into use each month for three months and then repeated.

In the second, he used a mixture of two insecticides from different classes for six months. In the third, he chose an insecticide to which cockroaches had low-level starting resistance and used it the entire time.

Cockroaches were captured before the study and lab-tested to determine the most effective insecticides for each treatment, setting up for the best possible outcomes.

“If you have the ability to test the roaches first and pick an insecticide that has low resistance, that ups the odds,” Scharf says. “But even then, we had trouble controlling populations.”

Rotating three insecticides, he was able to keep cockroach populations flat over a six-month period, but could not reduce them. The two-insecticide mixture did not work, and populations flourished.

With a single-insecticide, Scharf found there was little starting resistance to the chosen insecticide, and he was able to all but eliminate the cockroach population. In the other, there was about 10-per-cent starting resistance and populations grew.

In lab tests of the remaining cockroaches, Scharf found cross-resistance likely played a significant role. A certain percentage of cockroaches would be resistant to a particular class of pesticide.

Those surviving a treatment and their offspring were essentially immune to that insecticide. But they also gained resistance to other classes of insecticide, even if they hadn’t been exposed to them and had not had previous resistance.


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