As far back as 1913, the US Department of Agriculture was warning the populace about an insect called cimex lectularius, which, they said, was quickly becoming a problem in newly expanding urban areas such as New York, Chicago, Boston, etc. The USDA also said that the problem was mitigated by the fact that these pests — better known as ‘bed bugs’— lacked wings, “otherwise there would be no safety from this pest, even for the most careful and thorough housekeeper.”
That was well over a hundred years ago, but scientists are once again telling us that bed bugs are a problem in the USA. Bed bugs, they say “are back — and with a vengeance”. They noted that:
“Fifty years ago, the blood-sucking pests were nearly eradicated in the United
States thanks in part to the use of pesticides like DDT. Today, they are creeping
over sheets — and tormenting hapless sleepers — across the country.”
So, what happened? Why did a bug that everyone — experts included — thought was eradicated make such a dramatic come-back?
The answer is, of course, evolution!
Theories of Bed Bug Evolution
As one writer so neatly put it:
“Bedbugs have returned . . . partly because they have evolved resistance to pesticides,
and scientists are struggling to learn more about these pests. It’s a much bigger challenge than examining, say, monarch butterflies.”
But, before we take a closer look at this theory, we should make mention of another notion which, if not quite a competing theory, does deserve mention — something I call the ‘Exoskeleton Defense’. The term is my own, and naysayers notwithstanding, the principle is sound:
“The bed bugs of today . . . have thicker, waxier exoskeletons (to shield them
from insecticides).”
However, while this is undoubtedly true, this bug-related development represents merely one evolutionary modification, which does not satisfactorily account for the world-wide explosion of the bed bug population. Other factors, such as increased international travel and the enormous expansion of the human population, probably play a part too.
A more plausible explanation for the enormous growth in the bed bug population is not that today’s pesticides are weaker than they once were, but that bed bugs have developed a resistance to the most widely used chemicals. As pest management professionals (PMPs) will tell you, the products used to control bed bugs contain either chrysanthemum-derived pyrethrins, or pyrethroids, which are artificially made versions of the same substance. Fortunately, these chemicals have little to no effect on humans — which is why they are safe to use in homes and offices — but the same is not true for bugs.
How Chemicals Affect Bed Bugs
The mechanism by which these chemicals work is fairly straightforward: They attack the nervous system of the insect. Bugs, like humans, possess minuscule pores on the surface of their nerve cells that can be ‘unlocked’ to allow sodium to inundate the cells, which in turn generate unwanted nerve impulses. The aforementioned pyrethrins and pyrethroids then bind to the sodium pores and ‘bolt’ them open. As a result, the cell cannot prevent sodium from streaming in continuously, which triggers the nerve to fire uncontrollably. The outcome is cellular paralysis. In other words, death!
And yet, bed bugs survive and prosper. How?
It turns out that one, single mutation is to blame. Scientists have determined that a change in only two of the 2,000 amino acids that comprise a sodium pore allows an insect to develop its resistance levels (to pyrethroid) by as much as 250%. In practical bed bug terms this means that bed bugs have a much higher chance of surviving exposure to pesticides using pyrethroid.
Bed Bug Mutations
But here we come to the evolutionary heart of the matter: How such mutations arise and how they persist in an organism (presumably to its long-term benefit).
Creationists (and their ilk) will probably disagree with this explanation, but as the evolutionary biologist Theodosius Dobzhansky put it in an essay from 1973, "nothing in biology makes sense except in the light of evolution.” But, if we really need to drive home this point, I quote the words not of Charles Darwin or Stephen Jay Gould, but the Jesuit priest Pierre Teilhard de Chardin, who wrote:
“(Evolution) general condition to which all theories, all hypotheses, all systems
must bow and which they must satisfy henceforward if they are to be thinkable
and true. Evolution is a light which illuminates all facts, a curve that all lines
must follow.”
Evolutionary adaptations arise when a random change, or mutation, within a population of organisms happens to be of benefit to that group. In the case of insects, if that mutation helps them to better resist the effects of, say, pyrethroid, then there is a good chance that successive generations will pass along that mutation. The result is that such insects will become much, much harder to kill.
How Bed Bugs Manage to Survive
The skeptics among you might counter (quite rightly) that it would be an extremely fortunate bunch of bed bugs to somehow develop, then carry, the correct mutations that would permit them to stay alive in the face of a serious pesticide assault. Unfortunately for our sceptics, luck has nothing to do with it! Large populations of organisms possess gene pools that manifest themselves in specific kinds of physical structures; and those structures will behave in certain predetermined ways. Whales don’t walk on dry land and, failing some unprecedented natural catastrophe, probably never will. But keep in mind that it was sea dwelling creatures that first ventured onto land and began an evolutionary phase that resulted in the dominance first, of dinosaurs, then the mammals we know today.
Finally, one contrarian example for the sceptics who cast doubt on the authenticity of evolution: the existence of Flying Squirrels (known to scientists as Pteromyini or Petauristini). Wikipedia explains their importance better than I can:
“They are not capable of flight in the same way as birds or bats but are able
to glide from one tree to another with the aid of a patagium, a furry,
parachute-like membrane that stretches from wrist to ankle . . . [and they]
are able to steer and exert control over their glide path with their limbs and
tail.”
At some point in the past a random mutation (or series of mutations) gave rise to the development of this squirrel’s patagium, which furnished it with an ingenious means of escaping predators and giving it ready access to food growing in the canopies of woodlands. The mutation was of immediate benefit and was thus handed down to successive generations.
Bed Bugs and DDT
The next, not-so-obvious question is “what, in the collective past of the bed bug, triggered a mutation, or mutations, that allowed it to better resist pesticides?”
The answer actually lies in the wording of the question. Over time, bed bugs have been ‘prepped’ to develop precisely the sort of mutations needed to resist the effects of the pesticides to which they are so often exposed. There is a high probability, for example, that their encounters with such insecticides as DDT triggered a mutation that successfully neutralized its effects.
“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.”
Of course, DDT is no longer used, having been banned in the US since 1972 and worldwide since 2004. However, the mutations it likely caused in the global bed bug population have persisted and helped this pest to survive, despite our best efforts to eradicate it.
So, where does that leave us humans? Poorly situated, it would seem. According to Dr. Coby Shal, Professor of Urban Entomology at North Carolina State University:
“The magnitude and frequency of bed bugs’ resistance to insecticides is huge,
no new insecticides are due to hit the marketplace soon, and no cost-effective
strategies for bed bug control have been developed. So, we really don’t have a
good way to control bed bugs right now.”
This could change, of course, especially in light of the work now being done in genetic engineering, but an answer to the current bed bug invasion seems far off. In the meantime, best to call those redoubtable pest management professionals. They’re our only hope — at least for the time being!
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