They've overtaken movie theatres, apartment complexes, clothing stores and military barracks.

Bedbugs have returned to both northeastern U.S. and Canada with a vengeance, and now experts are gathering to discuss how to stop them in their tiny little tracks.

The tiny, apple-seed-shaped bugs were once all but eradicated a few decades ago, but their populations have surged in recent years, presumably after developing resistance to the common pesticides now used against them.

To discuss how to fight the creepy crawlies, around 400 bedbug experts and pest control workers have gathered for a sold-out, two-day summit in a Chicago suburb this week.

The summit includes seminars from researchers, as well as trade show displays from pest control service vendors showing off the latest weapons and technologies.

New York City is thought to be the epicentre for the bedbug outbreak, with the bugs turning up everywhere from posh apartments to housing projects. Just this week, Nike Inc. said it was temporarily closing its Niketown store in New York City, after it found an infestation of the bloodsucking pests.

The pest control company Terminix listed New York, Philadelphia and Detroit as the three most bedbug-infested cities, based on call volume to its service centres.

Bedbugs, which feast on blood at night, all but vanished by the 1940s with the widespread use of DDT. But when DDT was banned in the early 1970s, other chemicals were used instead.

But now the bugs appear to have developed resistance to those chemicals, leaving exterminators with fewer weapons in their arsenal.

The problem was worsened in the U.S. after a 1996 law required older pesticides to be re-evaluated based on more stringent health standards. That led to a ban on the indoor use of a key bedbug-fighting insecticide, called propoxur.

In Ohio, where bedbug infestations are widespread in Cincinnati, Columbus, and other cities, authorities are pleading with federal regulators to allow them to use propoxur, even though it's been deemed a "probable carcinogen."

About 25 other states are supporting Ohio's request for an emergency exemption.

Without insecticides, the bedbug fight can become herculean. The tiny critters can live an entire year between feedings, hiding behind baseboards or between floorboards while lying in wait for their next victim.

The bug – technically named Cimex lectularius -- also can be spread easily, often catching rides to new homes in luggage, handbags and clothing.

Bedbugs cause itchy bite marks and in those sensitive to their bites, allergic skin reactions such as hives.

The one bit of good news is that bedbugs don't appear to spread disease. In a study released last year in JAMA, the Journal of the American Medical Association, a team of medical entomologists from Mississippi State University reviewed dozens of studies on bedbugs and concluded there is little evidence that disease transmission has ever occurred because of bedbugs.

"To our knowledge, no study to date has demonstrated bed bug 'vector competence' (the ability to acquire, maintain, and transmit an infectious agent)," the authors wrote.

With reports from the Associated Press