Bed bugs bite back thanks to evolution

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Bed bugs bite back thanks to evolution September 2010

Where's the evolution? What's to be done if you wind up the unhappy bunkmate to a nest of these pests? In the past, the answer was simply to spray with a pesticide. Unfortunately, that response is less effective than it used to be not because the pesticides used today are weak but because bed bugs have evolved resistance to the most commonly used chemicals. The top choice for bed bug infestations are two related groups of chemicals that are both toxic and repellent to the bugs: pyrethrins, which are extracted from chrysanthemum plants, and pyrethroids, the synthetic versions of those chemicals. The evolution of pyrethrin in plants in the first place probably resulted from natural selection for plants better able to avoid being eaten by insects. We humans have simply co-opted the plants' chemicals defenses to deal with our own insect problems. Pyrethrins and pyrethroids are especially useful to us because they generally have a stronger effect on bugs than on mammals, making them relatively safe for use in homes.

These compounds work by attacking the nervous system. Insects (and humans) have tiny pores in the membranes of their nerve cells that can be opened to allow sodium into the cells, triggering a nerve impulse. Pyrethrins and pyrethroids muck up the nervous system by binding to the sodium pores, locking them in the open position. This allows sodium to pour into the cell continuously, causing the nerve to fire repeatedly and eventually leading to paralysis. Mammals and insects inherited similar nerve cells from our common ancestor meaning that the human nervous system is also vulnerable to these pesticides. However, the compounds are relatively safe for us because, in comparison to insects, our bodies have more effective ways to break the compounds down before they can do major damage.

So, how do resistant bed bugs survive pyrethroid spraying? Biologists have actually figured out exactly which mutations are responsible for many cases of resistance. For example, changing just two of the 2000 amino acids that make up part of the sodium pore is enough to make an insect 250 times more resistant to a commonly used pyrethroid. These mutations may change the pore so that the insecticide can no longer bind to it effectively and/or may change the way the pore responds when the insecticide binds.

Such mutations arise randomly and are favored when a population of organisms winds up in an environment in which the mutations happen to be useful in this case a bed sprayed with a pyrethroid. In that situation, if some (or even just one) of the insects carry the resistance mutations, those insects will be better able to survive and reproduce and will wind up passing the mutation on to their offspring. As this process continues through several generations, the population may evolve such that every individual carries the resistance mutations an outcome which is great for the bugs but immensely frustrating for the human occupants of the bed!

The key to this process of natural selection is having the right genetic variation in the insect population. If the population doesn't happen to carry any of the advantageous resistance mutations, the pyrethroid treatment will wipe out the bed bug population. It might seem then, that resistant populations should be rare after all, how many bed bug populations are likely to be lucky enough to carry just the right mutations to survive pyrethroid spraying? A lot, it turns out. Here's why. Bed bug populations have been primed with the right sort of genetic variation by their evolutionary history a history which includes extensive exposure to a different insecticide, DDT. Like pyrethroids, DDT kills insects by acting on the sodium pores in their nerve cells and it just so happens that many of the same mutations that protect an insect against DDT also happen to protect it from pyrethroids. When DDT was first introduced, such mutations were probably extremely rare. However, with the widespread use of DDT in the 1950s and 60s, such mutations became much more common among bed bugs through the process of natural selection. Though DDT is rarely used today because of its environmental effects, these mutations have stuck around and are still present in modern bed bug populations. Because of the action of natural selection in the past (favoring resistance to DDT), many bed bug populations today are primed with the right sort of genetic variation to evolve resistance to pyrethroids rapidly.

And evolve rapidly they have! In the last decade, resistance to pyrethroids among bed bugs has become a major problem in the U.S. and may help explain why the pests are crawling into bed next to more and more of us. The map below shows how prevalent just two of the mutations conferring resistance have become. The pace at which widespread resistance has evolved suggests that relying on chemicals alone to control bed bug infestations is not enough and may even encourage the evolution of more resistant populations. Instead, the CDC and the EPA recommend a more integrated approach, one that incorporates pesticides, along with other techniques to which resistance is unlikely to evolve: heat treatment (temperatures between 113 and 120F can kill the bugs), vacuuming, removing clutter, and sealing cracks and crevices. The rapid evolution of insecticide resistance in these pests has made it harder but not impossible to kick them out of bed for good!

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Bed bugs bite back thanks to evolution