Tracing the cause of the bedbug epidemic

First off, I don't want to give you the impression that I will be providing you with the exact explanation for the recent resurgence of bedbugs. If that's what you are looking for, then the quick answer is, no one knows for sure. Many factors have been suggested as the causes of this bedbug epidemic, such as an increase in immigration and international travel, the banning of many powerful pesticides, changes in pest control practices, little awareness and knowledge of and limited experience in controlling this pest, and pesticide resistance. But none of these has been scientifically proven. Although I don't have an exact answer myself, I will be trying to discuss some of these factors in some depth and hopefully that will help you reach your own conclusion. A few web sites also suggest that terrorists are behind this epidemic since it started roughly at the same time when 9.11 happened, but such speculation won't get us anywhere without any hard evidence, so I am going to skip that.

To identify the cause of this epidemic, the first thing to do is to find the statistics on the number of infestations and the time when they occurred. Unfortunately, they are very scarce. The most well-known figures are the number of complaints received by the city of New York and the city of Toronto, and the 70% increase in bedbug reports from 2000 to 2005, according to the National Pest Management Association (NPMA). But the figures given by the NPMA seem to be self-conflicting to me. According to this report in USA Today, "the National Pest Management Association says bedbug reports increased 71% from 2000 to 2005, with member companies that had received one or two calls a year now logging 10 to 50 a week." An increase from one or two calls a year to 10 to 50 a week is obviously much greater than 71%, no wonder some other reports suggested a 50-fold increase based on the same figures given by the NPMA. Nevertheless, I have also found some more data supplied by a few pest control companies. Based on all these numbers, I would guess that, the number of infestations started to pick up around 1999, then sometime between 2001 and 2003, it started to grow exponentially.

Although it is almost beyond doubt that some bedbugs have been brought into the United States from overseas, it does not explain the sudden explosion in bedbug populations. Throughout the entire American history, immigrants have been coming to this country and have been a major source of population growth, why are they suddenly causing this bedbug epidemic now? Nobugsonme, who runs the Bedbugger blog, also wrote an article on this subject with a focus on New York City, and asked the same question.

According to this news article on National Geographic's web site, no single tropical bedbug (Cimex hemipterus) was found in England based on surveys carried out by biologist Clive Boase of the Pest Management Consultancy in UK. Boase suggested that if international travel were the origin of the problem, he would "expect tropical bedbugs to be turning up alongside the better known temperate bedbugs." As far as I know, the bedbugs that have been found in Australia, Canada, and U.S. are also overwhelmingly temperate bedbugs (Cimex lectularius), but I'm not going to make any conclusion here since I don't have the data to back me up. Just for your information, the common bedbug is normally found in the northern temperate climates of North America, Europe, and Central Asia, whereas the tropical bedbug is limited to tropical and semi-tropical regions including Florida. According to the University of Florida's Department of Entomology and Nematology web site, these two species of bedbugs can be distinguished by looking at the first segment of the thorax. "The prothorax of the common bed bug is more expanded laterally and the extreme margins are more flattened than that of the tropical bed bug".

Statistics on immigration and international travel also do not support the claim that they are the causes of this bedbug epidemic. Table 12.1 shows the number of persons that obtained permanent resident status between 1996 and 2005. The only significant increase occurred in the period 2000-2001 and the period 2004-2005, but only after several consecutive years of decline. If you compare the numbers from 1996 and 2001, the increase was only 15% in six years, or 3% annually. Similarly, the increase from 2001 to 2005 was merely 6%, or 1.5% annually. Overall, except for the 11% increase in the 1950s and the decline in the first three decades, annualized immigration growth rates were mostly between 2% and 5% throughout the last century. For the period 2000-2005, the average number of immigrants per year actually dropped slightly to 957 thousand, from 977 thousand for the period 1990-1999. Since some people may argue that bedbugs mainly existed in the developing countries before this epidemic, I refined the calculation by subtracting the numbers from Western Europe, Canada, Australia, and New Zealand from the total. The growth rates did go up by a few percentage points from the 1940s to the 1980s, but did not change since the 1990s, and the overall pattern remains the same.

year	1996	1997	1998	1999	2000
# of immigrants	915,560	797,847	653,206	644,787	841,002
% increase	n/a	-13%	-18%	-1%	30%
year	2001	2002	2003	2004	2005
# of immigrants	1,058,902	1,059,356	703,542	957,883	1,122,373
% increase	26%	0%	-34%	36%	17%

Table 12.1 persons obtaining legal permanent resident status (1996 - 2005)
source: 2005 Yearbook of Immigration Statistics

Besides immigrants, the United States also takes in refugees every year, but the number is much smaller (less than 100 thousand a year on average), and thus was not taken into account.

The number of immigrants is just a drop in the bucket compared with the number of international travelers. According to my calculation based on the statistics for the last ten years, on average, the number of international travelers (inbound and outbound combined) was over 100 times as many as the number of immigrants, with the outbound travelers outnumbering inbound travelers slightly. Table 12.2 shows the number of international travelers for the period 1998-2005 (earlier data are not available). Again, there was no significant increase in the number of international travelers during this whole 18-year period. On the contrary, the annualized growth rates were 3.2% and 1.5% for the periods 1988-1996 and 1997-2005, respectively, indicating a decline in growth during the time when this epidemic started.

year	1988	1989	1990	1991	1992	1993	1994	1995	1996
# of vistors	74,611	77,503	83,986	84,240	91,159	90,190	91,203	94,081	98,800
% increase	n/a	4%	8%	0%	8%	-1%	1%	3%	5%
year	1997	1998	1999	2000	2001	2002	2003	2004	2005
# of vistors	101,104	102,075	105,731	112,565	106,369	101,647	97,468	107,894	112,711
% increase	2%	1%	4%	6%	-6%	-4%	-4%	11%	4%

Table 12.2 international travelers (1988 - 2005)
numbers in thousands, inbound and outbound combined
source: Office of Travel & Tourism Industries

It also depends on how you look at it. If you believe that all the bedbugs in this country have been brought in from overseas, then you would expect to see a spike in the number of immigrants and international travelers, but the data clearly indicate otherwise. In particular, both tables show a significant decline in immigration and international travel after 9.11, while the number of bedbug infestations surged during the same period. But if you believe that a few bedbugs have been brought into this country and have multiplied, then you should know that, contrary to common belief, bedbugs have never been completely eradicated in the U.S., and that based on the rate at which they are spreading right now, all other conditions being equal, this epidemic should have started many years ago. It should also be noted that on average, outbound visitors actually outnumber inbound visitors, meaning that while many people come to the United State, U.S. residents also go abroad for various reasons. Furthermore, don't forget the vast amount of U.S. troops deployed overseas. This number has been over half a million on average since the end of World War II (a total of 27.3 million billets during 1950-2000), and the troops often operate in areas with serious insect-transmitted diseases, let alone some bedbugs. Isolation of a bed is no easy task, isolation of a country is simply not feasible.

A more convincing explanation is the change in pest control techniques. We know that bedbugs were largely eliminated by the use of DDT. But bedbugs weren't the target pests, other pests such as cockroaches were. However, although bedbugs were largely eliminated by DDT, cockroaches, or German cockroaches to be specific, developed strong resistance to DDT within a decade and survived (does this suggest that cockroaches are more hardy and resilient than bedbugs?). Even though DDT was banned, baseboard treatment with residual pesticides such as organophosphates, carbamates and pyrethroids became a standard procedure for cockroach control. Because of this practice, it's believed that the bedbugs that had been brought into someone's home accidentally often got killed before they could establish an infestation. In the meantime, the roaches developed resistance to every insecticide used, until the early 1980s, when hydramethylnon bait was introduced to fight these hard-beaten bugs. The bait achieved much greater results than expected, largely due to its secondary killing effect. Once a roach feeds on a bait and dies, the lethal doses would remain in the feces and carcass, which would be fed upon by other roaches in the harborage (Interestingly, many common cockroach species including German cockroaches have all been found to eat one another). Because of the high effectiveness of the baits, according to Dr. Phil Koehler, an entomologist at the University of Florida, the pest control industry began to switch from baseboard treatment to the use of baits and IGRs in the early 1990s, and sprays have been practically eliminated by 1995, a few years before this bedbug epidemic started. Although the German cockroaches have yet developed resistance to hydramethylnon, a recent research from the University Of Florida indicated that about 60% of the German cockroaches refused to east most commercial baits. This just shows how adaptive these pests are.

Another reasonable explanation is the cancellation and restriction of many powerful pesticides. We all know that DDT was banned on Jan 1, 1973. But many more pesticides have been banned or heavily restricted during the past few decades. Some of them are believed to be more effective than the pesticides available today.

The first powerful but also the most notorious class of pesticides is the organochlorines, which include DDT, aldrin, chlordane, endrin, endosulfan, dicofol, etc. The modes of action of most of these pesticides are not clearly understood. They are generally broad spectrum, relatively low in toxicity to mammals, but persist in the environment long after application and in organisms long after exposure, which is the main reason why most of them have been banned. A few of them are still being used today. For example, lindane was recently banned by the EPA for all agricultural uses, but is still being used for the treatment of headlice and scabies.

The second class is the organophosphates (OPs), which include parathion, malathion, diazinon, dichlorvos, chlorpyrifos, etc. They bind to an enzyme in the synapse called acetylcholinesterase that is responsible for breaking down acetylcholine after it has carried its message across the synapse, which results in repetitive stimulation of the nerve. The over-stimulation of the nervous system eventually causes the insect to die (DDT and pyrethroids also cause over-stimulation of the nervous system, but through different mechanisms). Although OPs degrade much faster than organochlorines, they have much greater acute toxicity, mainly because humans also use acetylcholine as a neurotransmitter and acetylcholinesterase to break it down. When the Food Quality Protection Act was first signed into law in 1996, 49 OP pesticides were registered for use in pest control. By the end of the EPA's 10-year review period, seventeen of them including the popular household pesticides diazinon and chlorpyrifos had been cancelled or heavily restricted. For diazinon, retail sales for indoor household use stopped by the end of 2002. For chlorpyrifos, all sales to consumers stopped by the end of 2001.

The third class is the carbamates, which includes carbofuran, aldicarb, carbaryl, bendiocarb, propoxur, etc. Like organophosphates, carbamates are also acetylcholine esterase inhibitors. The difference is that organophosphates are irreversible inhibitors, meaning that they do not release the bound cholinesterase and recovery depends on the new cholinesterase produced by the body, whereas carbamates are reversible inhibitors. Carbamates generally degrade faster and are less toxic than organochlorines, although some of them can be just as lethal. The EPA has already proposed the ban on carbofuran, and is finishing the re-assessment on aldicarb.

My own search for insecticides registered and intended for bedbug control has turned up these results:
- Organochlolines: no product has been found
- Organophosphates: there are 341 products, but most have been cancelled, only 24 products are available with the active ingredient being either malathion or DDVP (dichlorvos).
- Carbamates: there are 266 products, only 76 are still available with the active ingredient being carbaryl.

According to this article on CDC web site, apparently two carbamates - bendiocarb and propoxur, have also been used to treat bedbug infestations in Toronto. Although the EPA registration of bendiocarb has been voluntarily cancelled by the registrants, the registration status of propoxur is still active, but I failed to find any products intended for bedbug control. I cannot guarantee the accuracy of these search results, but it should give you an idea how many pesticides we have lost in the past few decades.

With most of the pesticides in these three classes phased out, what's left is a class of pesticides that is supposedly almost perfect, i.e., pyrethroids, due to its quick acting, relatively low toxic to mammals, and less persistent in the environment. However, experts believe that pyrethroids are less effective than many organophosphates and carbamates, and are even repellent to bedbugs (although a recent research indicated otherwise). More importantly, whenever a pesticide or a class of pesticides is being heavily relied upon, one thing is bound to happen - pesticide resistance. In my opinion, if pyrethroid resistance is not already the biggest obstacle we are facing now, it will soon be.

Some people believe that before this bedbug epidemic, while bedbugs were non-existent in this country, they were common in the developing countries. This is simply another myth and misconception. As nobugsonme questioned in her another article, "If DDT more or less eliminated bed bugs in the USA, but they still flourished overseas, why has it taken about 30 years for them to come back?" One thing is almost certain, with the affordability and accessibility of many powerful pesticides, just like malaria, bedbugs were largely eliminated not only from this country, but also from many other regions of the world including many developing countries. Given that bedbugs are such good hitchhikers and that so many people are visiting or returning to U.S. everyday (currently, there are over 100 million international travelers per year, or over a quarter of a million per day), if bedbugs were common in other parts of the world, this epidemic would have happened many years ago and would have been much worse than it is now. Under the pressure from U.S. and E.U., many developing countries have also banned many powerful pesticides, particularly the organochlorines in the past. In 2001, more than 90 countries signed the Stockholm Convention on POPs, and agreed to reduce or eliminate the production, use, and/or release of 12 key Persistent Orgainc Pollutants (POPs), which included nine organochlorine pesticides. Could the banning of these pesticides have also resulted in a resurgence of bedbug infestations in these countries, which in turn brought more bedbugs into this country? If it has happened here, I don't see why it can't happen elsewhere.

Many people also believe that bedbugs were completely eradicated in the U.S. and other developed countries by DDT. This is not true either. Most experts agree that, although rare, bedbug infestations have always existed in this country. Dr. George Rotramel, a pest management consultant and a professor at the University of Illinois-Chicago took one step further and suggested something different in his article "What Caused the Explosion in Bed Bug Populations?"

"Bed bugs were immediately investigated as potential vectors when Acquired Immune Deficiency Syndrome (AIDS) first appeared in the early 1980s. These studies would not have been conducted unless the Centers for Disease Control and Prevention (CDC) and other health experts knew that bed bugs were common in parts of San Francisco and other major cities."

I don't know if there are statistics that support Dr. Rotramel's theory, but given the secretive behavior of bedbugs and the fact that many people do not react to the bites, this is not entirely impossible.

Due to the fact that so many factors may be involved and that so many changes have taken place, amazingly, all within the last five to ten years, it is almost impossible to know what exactly has caused this bedbug epidemic. "The bottom line is it may be a convergence of all those factors, but none of that really explains the rapid increase in recent years," said Michael Potter, a professor and urban entomologist at the University of Kentucky.