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Volker Steger/Science Source

Cockroaches may soon be unstoppable—thanks to fast-evolving insecticide resistance

The day that squeamish humans—and exterminators—have long feared may have come at last: Cockroaches are becoming invincible. Or at least German cockroaches (Blattella germanica) are, according to a new study. Researchers have found that these creatures, which have long been a prevalent urban pest, are becoming increasingly resistant to almost every kind of chemical insecticide.

Not all insecticides are created equal. Some degrade the nervous system, whereas others attack the exoskeleton; they also have to be left out for varying amounts of time. But many insects, including cockroaches, have evolved resistance to at least one of the most commonly-used insecticides. And because cockroaches live only for about 100 days, that resistance can evolve quickly, with genes from the most resistant cockroaches being passed to the next generation.

To test resistance in German cockroaches, researchers treated three different colonies in multiple apartment buildings in Indiana and Illinois over the course of 6 months. The populations were tested for their level of resistance to three different insecticides: abamectin, boric acid, and thiamethoxam. One treatment used all three pesticides, one after another, for 3 months before repeating the cycle. In another treatment, researchers used a mixture of insecticides over the full 6 months. A final treatment scenario used just one chemical that the selected roach population had a low resistance to for the entire time.

Regardless of the different treatments, the size of most of the cockroach populations didn’t drop over time, the researchers wrote last month in Scientific Reports. That was true even when the researchers used multiple insecticides at once—a standard practice among exterminators. That suggests cockroaches are quickly evolving resistance to all three of the chemicals that were tested. On the upside, the researchers found that one kind of treatment—abamectin gel bait—could wipe out a portion of the colony—if the roaches had a low-level resistance.

Just how the cockroaches are evolving is unclear without further genetic testing. But if the findings hold, this widespread resistance could make it impossible to treat cockroach infestations with chemical insecticides alone. Instead, the researchers say, people will have to use what’s known as “integrated pest management,” which involves setting traps, cleaning debris off surfaces, and even vacuuming up the tiny suckers, in addition to chemical treatments. Talk about a nightmare—at least for the roaches.