Cockroaches are known to be tenacious little pests that taunt humans with their resiliency. According to a new study, these vile bugs may been even more resilient than previously thought.

Researchers from Purdue University found German cockroaches — the most common species worldwide — are becoming increasingly resistant to almost all insecticides. German cockroaches live exclusively in human environments, according to the study published in the journal Scientific Reports. They produce asthma-triggering allergens and can carry pathogens like Salmonella, Enterococcus, E. coli, and other antibiotic-resistant microbes.

Insecticides are often used to combat these unwelcome house guests, but after testing multiple kinds, scientists realized cockroaches could have developed a resistance to their one-time foe, the exterminator.

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Many insects can can develop resistance to at least one kind of insecticide. Cockroaches, which live only about 100 days, can evolve very quickly. Those that develop the resistance can pass the gene down to new generations.

To test the roaches' resistance, researchers tried three different insecticides on three different cockroach colonies over six months. All the products were store-bought from retailers and were E.P.A. registered. They were applied by licensed pest management professionals to the cockroach colonies in two low-rise housing developments in Indiana and Illinois.

Researchers tested common insecticides and found cockroaches have developed high levels of resistance. istockphoto

One colony got treated with all three insecticides, one at a time. Another colony was hit with a mixture of the insecticides. The third colony was hit with just one chemical that the roaches had low resistance to.

Regardless of which treatment was used, most of the cockroach colonies did not decline in size. Even when the researchers used multiple insecticides at once, a technique exterminators often employ, the population size did not drop. Not only that, but resistance levels actually increased in most cases, the researchers found. The results suggest cockroaches are quickly evolving resistance to the three chemicals tested.

However, the researchers also found one chemical insecticide, abamectin gel bait, could wipe out at least a portion of a cockroach colony — if the roaches had a low level of resistance to begin with. Further genetic testing will be needed to determine why.

The researchers' grim conclusion is that roaches are becoming virtually impossible to get rid of. "Overall, the unexpectedly poor performance of a majority of treatments in the field study suggested significant levels of starting resistance and/or selection for higher-level resistance in 4–6 months," they write.

In other words, in the battle between cockroaches and exterminators, it looks like the roaches are winning.