PHILADELPHIA—When you have bedbugs (Cimex lectularius), less interesting is the question of how they got there than the conundrum of how best to get them out. Ridding homes and businesses of these pests has become a multimillion dollar industry in many cities in the U.S. and throughout the world.

A few scientists, however, are now asking just how these populations have been spreading from town to town and from headboard to headboard. Answering that question might lead to better ways of controlling their spread into the future.

Researchers are using genetics to try to trace bedbug lineages in the U.S. as well as those in individual apartment buildings. "It's actually kind of a forensics question," Coby Schal, an entomologist at North Carolina State University in Raleigh, said at the American Society of Tropical Medicine and Hygiene's annual meeting in Philadelphia on Tuesday.

Schal and others have analyzed bugs collected up and down the U.S. east coast—and across the country to see whether the vermin have emerged from a few foreign entries, from local animal populations or whether they are repeatedly imported from abroad and then spread throughout the country. In more finite searches of regional apartment buildings, the researchers also found some dark secrets about bedbug pairing practices that help to explain how they can infest an area so quickly.

Crying fowl
Bedbugs have a cozy history with humans; there is evidence they plagued ancient Egyptians some 3,500 years ago. And now, with so many people living in cities across the globe, "we've created the perfect habitat," Rajeev Vaidyanathan, associate director of vector biology and zoonotic disease at the research institute SRI International, said here Tuesday.

But humans are not the only animals on whose blood bedbugs feast—they seem to have a taste for chickens' as well, Vaidyanathan noted. And we have conveniently created massive poultry operations in which the bugs can thrive. In fact, "you can collect them by the bucket off the walls of a poultry house," he says. Because of this, some scientists have argued that these reservoirs of bugs might be to blame for their reemergence.

In a poultry house, however, a bedbug would not be exposed to the same insecticides as one living among humans. Further, the insects rapidly develop resistance to common pesticides, so the prevalence of insecticide-resistant bedbugs in homes suggests that the bugs have not simply come crawling in from the henhouse, Schal pointed out. The populations are more likely to have come from farther afield.

Global hitchhikers
Even before bedbugs made their resurgence in the U.S., they were already biting many more people in Australia and Canada. And they have been a persistent problem in other, less developed countries.

If the bugs arrived from just a few travelers several years ago and had mostly been spreading in the U.S. since then, containment efforts might have been more effective than if they were recurrently entering the country.

A genetic survey across the U.S. found that among towns, bedbugs have great genetic variation. That diversity suggests that, rather than having been brought into the country only a few times, the parasites crossed the borders many times—and are likely continuing to do so. These founder bugs go on to create larger populations. Even in well-connected and highly populated cities on the Interstate 95 corridor along the east coast, bedbug populations remained relatively distinct from one another.

Big families
Within each domestic population, however, the genetic distribution is a much different story. In a single home or apartment building, for example, the bugs show "an extremely high rate of inbreeding," Schal said. His team found that an entire population within an apartment building—such as were the cases in infested Jersey City, N.J., and Raleigh, N.C. buildings—was so genetically similar that it might have been founded by a single fertilized female. Bugs in the next generation then would have mated with one another, and so on, in a seemingly indefinite incestuous line.

Just how these and some other bugs can sustain this level of inbreeding and continue a healthy, bourgeoning population base remains unclear. In most species, such tight mating circles lead to genetic bottlenecks and mutations, which weaken a population. But in some insects, such as bedbugs and cockroaches, inbreeding seems to have become a perfectly safe form of reproduction.

In one Jersey City apartment building studied, the group even found a clue that the bugs prefer this arrangement. In this multi-unit building, two genetically distinct populations had infested several apartments each. But on one floor, one apartment had bedbugs from one genetic group, whereas the neighboring apartment had bugs from the other group. And the two populations showed no evidence of mixing, which suggests they might prefer their own gene pools, or even be incompatible with other groups.

Bedbugs can also cover a lot of ground—and quickly. In lab experiments, they have been shown to wander more than 2.5 meters in five minutes, noted Ken Haynes, of the University of Kentucky.

And once they have moved in, bedbugs are tough to stop. Females can live up to a year without imbibing so much as a drop of blood. So wrapping your belongings in plastic for a few weeks or going on vacation will hardly put a dent in their numbers. "They'll win," Vaidyanathan said. "They can hold out longer than we can."

The best strategy to head off a full-blown inbred infestation is still early detection, when they are easiest to excise.