An interesting story came out last month from the National Evolutionary Synthesis Center at U.C. Berkeley. The story does a nice job of explaining the connection between DDT and pyrethroid resistance, and why pyrethroid resistance has appeared to develop so quickly in the short time that bed bugs have re-emerged as an important North American pest (The one bone I have to pick with the story is its confusion of the relationship and terminology between pyrethrins and pyrethroids--there is no such term as "pyrethrums").
The answer is that when bed bugs reemerged this decade as a major pest they had already been "pre-selected" for resistance to pyrethroids by their previous exposure to DDT. Because DDT acts at a similar site in the nervous system as pyrethroids, researchers theorize that the same mutations that conferred resistance to DDT bestowed protection upon the bearers of those mutations from pyrethroids.
I remember John Osmun, one of my professors at Purdue University, recounting with excitement one of his early army adventures with bed bugs. As an entomologist with the military in the early 1940s he had been assigned to use a secret insecticide to treat bed bug infested army barracks. Almost miraculously, the long-persisting infestation was eradicated. The insecticide was, of course, DDT. Since WWII, DDT went on to become a widely used tool to manage many insect populations, including bed bugs. Unfortunately bed bugs have used their long experience with this insecticide to fight back against more advanced pesticides.
A recent study by Zhu et al (2010) looked at bed bugs collected from 97 locations in 17 states and found resistance to deltamethrin in 88% of the sites. Because of the scattered geographical sampling conducted in this study, the status of bed bug resistance in Texas is still tentative. The only sample from Texas collected and analyzed in this study was from Beaumont, and showed resistance based on one genetic mutation. Many of the samples revealed populations with another genetic mutation, or even two mutations. Relatively few populations sampled were classified as fully susceptible to deltamethrin.
This does not mean that resistant bed bugs cannot be killed with deltamethrin or similar products; but it does mean that the dose needed to kill will be greatly increased.
The research, especially the resistance maps of the U.S., should be considered tentative; but does give us a better idea why bed bugs are so difficult to control with standard pyrethroid insecticides. Even though the study focused on deltamethrin only, the results should be mostly applicable to all pyrethroids. Chlorfenapyr is the only other liquid residual insecticide that is widely used at the present time that is not a pyrethroid insecticide and to which bed bugs have no known resistance.