PHILADELPHIA, PENNSYLVANIA—Two new studies shed light on the amazing rise of bedbugs in the United States. The research, presented here yesterday at the annual meeting of the American Society of Tropical Medicine and Hygiene, suggests that the annoying critters were imported from overseas, not just once but on many occasions. It also shows that bedbugs are highly incestuous. As a result, a single mated female can set off an infestation in an apartment, or even an entire building.
Bedbugs have become a big problem in U.S. hotels and homes over the past decade, in part because they are resistant to a widely used class of insecticides called pyrethroids. But where the bedbugs come from is still unclear. Some researchers suspect the insects originate within the United States—most likely spreading from poultry farms, where they have been known to reside for decades. Indeed, the only genetic study of U.S. bedbug populations so far suggested the pests are homegrown.
But entomologist Coby Schal of North Carolina State University in Raleigh thinks a local origin is unlikely, in part because pyrethroids are still effective against bedbugs on chicken farms. If those bugs had expanded into human habitats, he would expect at least some of them to still be susceptible to the insecticide.
At the meeting, Schal presented an as-yet-unpublished study that suggests a different scenario. His team sampled 22 bedbug populations from nine states along the U.S. East Coast and examined nine so-called microsatellites, highly variable pieces of DNA that are particularly helpful in understanding genetic differences between populations.
The researchers discovered that bedbugs from different places were genetically very distinct, which suggests very diverse origins. And there was no clear geographic pattern to the diversity: Two populations living close together could be as different from each other as from one hundreds of kilometers away. If bedbugs were homegrown or had been introduced into the United States just once or twice, there should have been a strong geographical pattern, with nearby populations more similar than distant ones and with less diversity overall than what was observed, Schal says. Instead, he says, the data suggest the bugs were introduced from many different sources overseas.
Those origins might also explain why virtually every population is now resistant to pyrethroids. U.S. homeowners have long used pyrethroids to kill cockroaches in and around the kitchen but not in the bedroom, so it has been hard to explain why bedbugs are resistant. But over the past decade, many tropical countries have drastically scaled up mosquito control by spraying indoors and providing bed nets impregnated with pyrethroids. That has put tremendous pressure on the bedbugs to become resistant, Schal says. Global travel and trade could have taken the already-resistant bugs to the United States.
Schal would like to test bedbugs in other parts of the world to confirm this hypothesis, and he called on entomologists attending the meeting here to help him by collecting bugs abroad.
But while there was huge diversity among populations, "it was astoundingly different when we looked within populations," Schal says: Bugs within one infestation were remarkably similar genetically, and their degree of "relatedness" was unusually high—even higher than for some social insects. In fact, a single female—assuming she has mated—brought in on a mattress or a couch can likely set off an infestation because her offspring mate among themselves, Schal says. Many other species, including cockroaches, can flourish despite high levels of inbreeding, he adds.
The lack of genetic diversity within populations was confirmed in a second study presented by Schal that homed in on two big apartment buildings that were heavily hit with infestations, one in Raleigh and the other in Jersey City, New Jersey. In both, researchers found that insects within a population were strikingly similar genetically and were very closely related to each other. The analysis showed that the Raleigh infestation, which started 1 year ago, came from a single source. In the one in Jersey City, which has plagued residents for 6 years, there had been two separate introductions.
There's a glimmer of light in that, Schal says. The Jersey building is in an urban, low-income area, where one might expect many introductions of bedbugs; yet invasions apparently do not happen all that often. Once they're settled, however, the bugs seem to hop easily from one apartment to the next.
The studies offer a "very compelling" hypothesis of how the bedbug problem arose and how the bugs are spreading, says Rajeev Vaidyanathan, who studies bedbug control at SRI International, a nonprofit research organization in Harrisonburg, Virginia. He points out that if resistant bedbugs came to the United States on several occasions, the same may be true of other pest species, such as fleas and lice. "In 2011, we're talking about bedbugs," Vaidyanathan says. "Ten years from now, we'll be talking about something else."