Their preferred food is human blood, and they eat at night while we sleep. They can leave itchy bites but are not known to spread disease. The small pests - one-fourth to three-eighths of an inch long - hide in mattresses, furniture, floorboards, electrical outlets, even alarm clocks.
Levy worries that they could become more dangerous as they spread.
"I think we have to be very concerned and vigilant about bedbugs as potential vectors of disease," he said.
The study was published Wednesday in the Journal of Medical Entomology.
Initially, Levy wondered whether the bugs had started in one city neighborhood and fanned out. "We quickly found out that the bugs are all over the city," he said. There were a trio of hot spots in South Philadelphia and a couple of others farther north, but there were bedbugs in all kinds of neighborhoods and housing.
"It's really one of those situations where everyone's dealing with it quietly and privately, but it's very, very prevalent," he said.
Residents do not have to report bedbugs to the city, and Levy suspects that the vast majority call exterminators directly.
One of those, Terminix, last year listed Philadelphia as the nation's No. 2 city for bedbugs, behind Cincinnati.
Still, Levy thought the city's numbers were valuable. The number of reports rose from 106 in 2009 to 196 in 2010. They peaked in August and bottomed out in February each year.
Levy said the bugs, which can go a year without a meal, do not die in the winter. "They certainly don't go away," he said. "They're probably just hiding out in people's houses and not being very active."
A couple of studies in the past have suggested seasonality. Google searches for bedbugs also rise in the summer, Levy said.
A survey last year by the National Pest Management Association found that about half of pest professionals said they dealt with the most infestations in the summer and the fewest in the winter.
Greg Baumann, vice president of training and technical services for Orkin, said this may be because people travel more in the summer. The bugs hitch rides in bags and suitcases.
It takes extreme temperatures to kill them, so the difference between your home's summer and winter temperatures won't do it. "Where we're comfortable, they're going to be comfortable," he said.
Levy thinks the bugs may be more active, and hence more visible, in the summer. He has a colony of 2,000 or so. He puts them in the refrigerator when he wants to slow them down. He hopes that learning more about their life cycle will help scientists figure out how to kill them more efficiently.
A Philadelphia native, Levy took a circuitous route from Amherst College to a Ph.D. in population biology, ecology, and evolution. After college, he spent some time working for a circus in Chile. While there, he read about Chagas disease, a life-threatening parasitic ailment spread by triatomine bugs, which also feed on human blood at night and live in cracks and crevices in people's houses.
"I was terrified," he said. "I really had a phobia of these triatomine bugs." He has been working for 10 years with scientists in Peru to control them. The program there, which includes active surveillance and a concerted effort from residents, is working "fairly well." Triatomine bugs are more responsive to pesticides than bedbugs are.
Levy and other researchers will go door-to-door in Philadelphia this summer looking for bedbugs to learn more. He's also interested in whether the bugs can do more harm than we think.
Any time bugs suck more than one person's blood, Levy said, there's potential for trouble. "I am not convinced that they are not able to transmit diseases," he said.