Bedbugs not averse to inbreeding

PHILADELPHIA — Bedbugs that infest a room and spread within a building are often one big extended family, the offspring of a single female that begot sons and daughters that then interbred with impunity, researchers reported December 6 at a meeting of the American Society of Tropical Medicine and Hygiene.

Other scientists at the meeting reported on mechanisms that allow bedbugs to escape death by rapidly evolving to detoxify insecticides thrown their way. In that study, the researchers identified enzymes that the insects need in this detoxification process.  

Cimex lectularius, the bedbug, has become a scourge of slum tenements and upscale hotels alike in the past 10 years, staging an impressive comeback after being knocked back to insignificance with insecticides in the 1950s and 1960s. But even before that there were hints of future problems, said entomologist Kenneth Haynes of the University of Kentucky. The first reports of the insecticide DDT failing to kill bedbugs surfaced in 1948, he noted.

To get a reading on the current level of resistance, Haynes and his colleagues tested 108 bedbug populations and found that 88 percent had one or two genetic mutations associated with resistance to either DDT or pyrethroids, a widely used family of pesticides. “Pyrethroid resistance has facilitated in part the resurgence and/or spread of bedbugs,” Haynes said. The group traced this resistance to genetic changes that enabled the bugs to produce enzymes that detoxify insecticides. To establish this, the researchers shut down the enzymes’ production, which rendered the bugs vulnerable to deltamethrin, a synthetic pyrethroid.

But bedbugs do more than survive — they reproduce like crazy, in isolation. Entomologist Coby Schal of North Carolina State University and his team investigated apartment buildings that had reported infestations and found bugs’ genetic material typically showed stunning similarities, suggesting a single mother. “This is an extremely high level of inbreeding,” Schal said, and it apparently didn’t harm the insects’ survival.

When the team studied dozens of infestations from Maine to Florida, they found that diversity does exist in the bedbug world — among infestations that are cities apart. The difference is great enough to indicate that the current bedbug crisis couldn’t have arisen from a single introduction of bedbugs that then spread like wildfire. “The genetic signature doesn’t show that,” Schal said. Rather, the bugs’ clear diversity between cities suggests many arrivals from outside the United States, he said, pointing to a downside of global interconnectedness.

While the inbreeding helps bedbugs populate a given space, the multiple arrivals of bugs from abroad might have delivered insecticide-resistant bugs that now make eradication difficult, Schal said. For example, many people in the tropics use mosquito nets treated with deltamethrin. Bedbugs that evolved to resist its effects probably found their way to the United States and reproduced, he said.

While genetic resistance and global travel appear to have conspired to bring on the bedbugs, humans are assisting in other ways, says Rajeev Vaidyanathan of SRI International in Harrisonburg, Va. “The resurgence of these bugs didn’t happen overnight,” says Vaidyanathan. “For the first time in the history of our species, we are concentrated in cities. We’ve created the perfect habitat for this ectoparasite that lives in mammals’ nests.”