Bedbugs are a growing public health issue in the United States and around the world, but their resurgence in recent years may have been aided by humans who unwittingly helped the pests evolve numerous ways of thwarting a common insecticide, scientists say.

In a new study published online today in Scientific Reports, researchers examined the genes of bedbugs from different U.S. cities and found that several of the populations had multiple means of resisting a class of insecticides called pyrethroids.

Pyrethroid insecticides are commonly used in bedbug control because of their relative safety for humans and pets, effectiveness, and low cost, but their use has also led to widespread development of resistance in the pests. (Learn about bedbugs and how to tell if you have them.)

To investigate how some bedbugs were defending themselves, scientists compared the genes of 20 pyrethroid-resistant populations of the insects from around the country against a susceptible colony from Los Angeles, California.

The team identified 14 genes that coded for proteins that were expressed at higher levels in the resistant insects, compared with the nonresistant Los Angeles population.

Insecticide Armor

Further investigation revealed that all of these "overexpressed" genes were active in the tough outer shell of resistant bedbugs, and either helped neutralize pyrethroid insecticides before they could take effect or prevented them from entering the insects' bodies in the first place.

"Many mechanisms of resistance seem to be in play in these various populations simultaneously," said study co-author Kenneth Haynes, an entomologist at the University of Kentucky.

The study also revealed that despite sometimes being separated by hundreds of miles, resistant bedbug colonies utilized many of the same genes to protect themselves against pyrethroid insecticides.

Bedbugs by themselves aren't very mobile, but Haynes suspects the bugs are catching rides with humans—or rather, their furniture.

"The transcontinental transport of furniture is likely a major source of [bedbug] movement," he said.

Long-Distance Relations

Human involvement would also explain another puzzling pattern, recently discovered by scientists at North Carolina State University, in which bedbugs from different cities were sometimes more closely related to one another than to other populations from the same city.

"We see isolated pockets where bedbugs across town may not be closely related to each other, but populations in [North Carolina] might be closely related to one out of Michigan," Haynes said, indicating that the bedbugs hitched a ride with a human moving to another part of the country.

Haynes cautioned that his idea is "highly speculative" and still unproven, but entomologist Zachary Adelman of Virginia Tech said the hypothesis makes sense.

"If local populations evolved their own [insecticide resistance] strategy and then mixed [with other populations], the result would be quite similar to what we see now," said Adelman, who was not involved in the study.

An intriguing question raised by the new findings, researchers say, is just when did bedbugs evolve their resistance?

"[Was it] in the past ten years? The past 50? Or are we looking at the past several hundred years or more?" Adelman said.

Figuring out the resistance time line, Adelman added, could provide insights about how fast evolution is working in bedbugs and could have implications for the future of bedbug control.

A New Prescription

Changlu Wang, an entomologist at Rutgers University in New Jersey, said the new findings are already providing valuable clues about how to better handle bedbugs.

For example, knowing that the cuticle, or protective outer layer, plays a key role in insecticide resistance in bedbugs "implies that better formulations can be designed to penetrate the cuticle more effectively and thus provide better control," said Wang, who was not involved in the research.