Bed bugs have fed off human blood for more than 3,000 years and the parasites have only grown more pervasive in recent years.

Despite a dip in bed bug populations in the 1950s due to heavy use of DDT and other pesticides, infestations are now reported in all 50 states. According to survey results released earlier this year by the National Pest Management Association, 99.6 percent of pest management professionals across the country reported a bed bug infestation over the previous 12 months. And more than three-quarters of respondents said that bed bugs were the toughest pests they came up against.

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As bed bugs strike dread and produce itchy welts in a growing number of people, scientists are racing to better understand what makes the insects tick with the goal of ultimately finding better ways to control them.

But even as refined guidelines emerge for how to avoid and treat bed bugs -- including a new study that details the frigid temperatures needed to freeze bed bugs to death -- it’s becoming clear that the blood-suckers won’t go down without a fight.

"Most bed bugs have multiple types of resistance to pesticides," said Joshua Benoit, an entomologist at the University of Cincinnati. "Until someone develops a new chemical that serves as a very good pesticide that can also be safe enough to treat with inside houses, it’s going to be a while before we’re bed-bug free."

Bed bugs are tiny, flat, reddish-brown insects that require warm blood meals to survive. They are hearty creatures, able to survive up to eight months between eating by settling into a quiescent state. And they reproduce quickly, making them extremely difficult to get rid of.

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Two species of bed bugs are responsible for most infestations that affect people, according to the Centers for Disease Control and Prevention. And while the insects don’t carry diseases, they can cause sores, expensive property damage and major headaches for people who are unlucky enough to host colonies in their mattresses. Public health concerns include allergic reactions, secondary infections, depression, sleep-deprivation and anxiety.

In an attempt to get the upper hand, scientists have been looking for weaknesses in bed bug biology. One theoretical target is a type of symbiotic bacteria that lives inside the insects and produces B vitamins for their hosts. When researchers removed the bacteria from bed bugs, they reported in Proceedings of the National Academy of Sciences in 2010, the insects grew more slowly and became sterile.

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Scientists have yet to use that discovery to develop any new weapons against bed bugs but creative solutions may become necessary as studies show that bed bugs are becoming increasingly resistant to the pesticides most commonly used to fight them.

During the winter of 2011 and 2012, a team of scientists from the University of Kentucky collected 21 populations of bed bugs from homes in four Midwestern cities. Genetic analyses revealed 14 mutations involved in resistance to pyrethroids, which are the standard insecticides used against bed bugs.

The genes worked in different ways but many reduced the ability of the toxin to penetrate the skin and reach the insects’ nerve cells, the researchers reported earlier this year in Scientific Reports. Every single population had at least two forms of resistance to the insecticide. More than 70 percent of pesticide-resistance populations used at least five mechanisms to evade the poison.

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"It’s not just one little thing," Benoit said. "As people have been applying more and more pesticides, bed bugs have become more and more resistant in order to survive."

When chemicals fail, freezing the bugs out may be a viable option, suggests a new study. Based on preliminary results and anecdotal evidence that cold can kill the parasites, researchers from the University of Minnesota embarked on a systematic attempt to determine just how cold is cold enough.

Bed bugs don’t have antifreeze proteins or other strategies for tolerating frosty temperatures, the researchers reported this month in the Journal of Economic Entomology. But they can resist below-freezing conditions for weeks.

To kill bed bugs at all life stages, said University of Minnesota entomologist Joelle Olson, temperatures needed to drop below -13 degrees Celsius (8.6 F), but death did not come quickly. It took 80 continuous hours -- or more than three days -- at -16 degrees Celsius (3.2 F). Below -20 C (-4 F), death came after about 48 hours.

Kitchen freezers tend to fluctuate in temperature, Olson said, which means it might take longer than expected to freeze bed bugs to death at home. Using a thermometer can ensure you're achieving cold enough temperatures for long enough. The study also suggested bagging infested items to limit condensation and to prevent the bugs from escaping into the freezer.

As useful as freezing can be in combination with pesticides, reduction of clutter and discarding of infested items, Olson added, it will never work as the only strategy for killing bed bugs.

"Freezing is another option to kill bed bugs, but it is just one part of many steps that should occur," she said. "I want to make sure people understand they should contact pest management professionals. It is not a treatment protocol in itself."