The evolutionary mystery of the German cockroach

Around ten years ago I was involved in a fascinating collaboration to examine evolution within the environments built by humans. Humans create a huge variety of environments within the structures they build. Evolutionary biologists are working to understand how organisms colonize and adapt to these novel environments: from houses and garages to the insides of HVAC systems and water heaters. These built environments have exploded in numbers and diversity during the last few thousand years, and that makes them natural laboratories for studying rapid evolution.

I became interested in the evolution of these species coadapted to human structures because of my work on recent human evolution. Many human populations have been adapting to some very extreme environments over the last few thousand years. From the high Andes Mountains to the hot Kalahari Desert, and from the high Arctic to the floating villages of southeast Asia, people have been adapting to extremes. Inventing new cultural practices is one way that people push physiological boundaries. Houses, shelters, clothing, food storage bins, water containers: all these help to moderate extremes. They also create microhabitats that other species colonize, from mice to microbes.

One of the most striking examples of adaptation to the environments built by humans is the German cockroach. This species, Blattella germanica, is one of around a half dozen cockroach species that commonly inhabit human structures. Most, like the American cockroach and Oriental cockroach, are a lot bigger in size than German cockroaches, and all of the others can still be found inhabiting natural habitats in the part of the world where they originated. But not the German cockroach. This small species only exists within and nearby human-built structures. Despite what may seem like a huge limitation, the species has been incredibly successful in spreading globally—as a 2019 review by Qian Tang and coworkers put it, from Alaska to Antarctica and everywhere in between.

But nobody has had a clear idea of where it came from. It's a great evolutionary mystery: This is a species most people have encountered somewhere in their homes, places of work, hotels, or restaurants. Yet there seemed to be no trace of its origin.

“There are many other names for it; the English call it Shiner or Steam Fly; in Russia it has been called the ‘Prussian,’ and in Prussia it was known as the ‘Russian.’”—James Rehn

Surely German cockroaches didn't start out in Germany. The species does not tolerate cold temperatures and has no wild-living relatives in Europe. Other members of the genus Blattella are found across tropical and subtropical regions of Asia and Africa. The species' closest relative is Blattella asahinai, known commonly as the Asian cockroach, which can be found in natural environments of Sri Lanka and mainland southeast Asia.

Usually the geographic range of the closest relatives of a species are strong clues about where it originated. So even though nobody has ever found German cockroaches living “wild”, researchers have hypothesized that its ancestry lies in south or southeast Asia. Tang and collaborators discussed this “out of Asia” idea in their 2019 review article. They added that other relatives of B. germanica include the so-called false German cockroach from east and southeast Asia, and the field cockroach from Pakistan and Afghanistan. These all point to an origin of German cockroaches in south or southeast Asia.

But for a couple of reasons, Tang and coworkers were not satisfied with this hypothesis. For one, German cockroaches have unique social and behavioral ecology compared to relatives like Asian cockroaches, field cockroaches, and false German cockroaches. Each of these species is invasive in some parts of the world but all of them live outdoors. The Asian cockroach itself has been a successful invader, including within Florida and other parts of the southeast U.S. Tens of thousands of Asian cockroaches may live on an acre of land where their preferred leaf litter or grass is available. While they sometimes end up indoors, especially at night when they are attracted to lights, they aren't indoor specialists. Their dispersal has been limited to environments that they tolerate.

Human structures are tough places for most insects to live. People try to keep houses dry and generally clean, don't allow organic litter to build up, and they kill most small critters on sight. German cockroaches transcended these limits, finding behavioral strategies to moderate their humidity, hide from being squished, and find food where their close relatives have not.

Another stumbling block was historical. It may seem easy to say that German cockroaches can't be German, but every early record of the species does indeed come from central or eastern Europe.

Sure, written history has many holes. But the case of the Oriental cockroach, Blattella orientalis, is one where the history gives many more clues about dispersal. This species is attested in ancient records from Egypt, Greece, and Rome, and later became ubiquitous across Europe. Wild populations of this species are found in southwest Asia and Cyprus. The historical spread of B. orientalis looks like a range expansion proceeding slowly toward the colder regions of northern Europe by exploiting human-built structures. By contrast, German cockroaches appeared suddenly in the historical record in the eighteenth century in urban northern Europe, seeming to skip the trade routes from warmer climates entirely.

“The first occurrence of the German cockroach outside Europe was from New York, United States, when the Croton Aqueduct was constructed in 1842. New Yorkers believed that the aqueduct brought the pest to the city, and so called German cockroaches ‘the Croton bugs’”—Qian Tang and coworkers 

So Tang and collaborators suggested a modified hypothesis. They accepted that German cockroaches had evolved from Asian cockroaches. But the origin of the species was not within Asia. Instead, they proposed that when Asian cockroaches were transported along the trade routes from south and southwest Asia into Europe, this species could not establish persistent populations. Without local success, they were hardly noted by anybody. Only a few survivors that found ways to live within structures could make it through the cold European winters, and over time these survivors evolved into the German cockroach. The species was therefore truly European. Just as some species evolve from small founder populations that reach an island and rapidly adapt to their new habitat, German cockroaches evolved in urban islands from a few founders. Once it adapted to urban life, B. germanica exploded onto the world stage, spreading to America, southeast Asia, and onward by European trade routes.

After sketching out this scenario, Tang and coworkers then went to work testing the hypothesis by sequencing genetic data from German cockroaches in many parts of the world, as well as Asian cockroaches and other relatives. Last month they published the results of this work.

The new data showed something a little different from the urban Europe hypothesis. B. germanica didn't originate in Germany or elsewhere in Europe as a new species and later spread throughout other parts of the world. Island southeast Asia, Ethiopia, and China all had diversity of B. germanica that was as old as in Europe. Every one of these populations came from common ancestors that shared a history of drift and adaptation around 2100 years long. Before this, those ancestors diverged from the Asian cockroach, B. asahinai.

After the species first evolved, a multipronged series of dispersals began. Starting around 1200 years ago, the species was carried westward into India, Pakistan, and Iran. After 1600 C.E. some founder groups were carried into Malaysia and Indonesia. The first European dispersal was indeed in the mid-eighteenth century—just as the historical record suggests. Around the same time the species was carried to Ethiopia. Dispersals into the Americas and China began only in the nineteenth century.

The German cockroach deserves to be widely known as an example of rapid evolution of a new species. Not only does it inhabit environments where few insects survive, its environments didn't even exist before a few thousand years ago. No populations of this species have yet been identified outside human structures. Its closest living relatives, including the presumed ancestral species B. asahinai, have very different behaviors and habitats. There's no question that Blattella germanica is new.

Sometimes people who are hesitant to accept evolutionary biology claim that nobody has ever seen a new species evolve. That idea reflects a misconception about the process of biological evolution. New species evolve gradually from ancestral species, and this process may take hundreds or even thousands of generations. As Darwin emphasized, it may be only the extinction of intermediate populations that makes it clear to us that two species are different. Even so, there are many fascinating examples of species that first evolved within the span of human history.

This example is not only a historically interesting case, it is a hugely important in terms of its effects on humanity. The economic impact of pest control in the U.S. alone is more than 16 billion dollars, and around 15% of all pest control expenses are for cockroaches. Not only a new species, German cockroaches continue to evolve, building up impressive resistance to an enormous battery of pesticides that kill other cockroach species.

The proportion of buildings impacted by German cockroaches compared to other species, like the American and Oriental cockroaches, has been growing worldwide for more than 100 years. Meanwhile their footprint in different kinds of structures continues to grow. By some estimates more than 80% of commercial airliners are home to German cockroach populations, and a similar proportion of passenger ships and trains. Anywhere that people sleep or eat, B. germanica is not far behind. One thing is for certain: They aren't going away anytime soon.

Notes: I am not an entomologist. When choosing images of German cockroaches, I have two problems. First, many images are just gross. Nobody wants to see a whole clutch of cockroach nymphs with all their sticky poop hiding out behind a cabinet. Second, there are not so many without usage restrictions. So while I've chosen some “beauty shots” that are creative commons images for illustration, and I've checked against a range of sources, I make no warranty that the people who shared these images have correctly identified the insects.

Figures and evidence on the economic importance of German cockroaches are drawn from the chapter by Lee and Wang.

References

Lee, Chow-Yang & Wang, Changlu. (2021). German cockroach infestations in the world and their social and economic impacts. In C. Wang, C.-Y. Lee, & M. K. Rust (Eds.), Biology and Management of the German Cockroach (pp. 1–16). CSIRO Publishing.

Martin, L. J., Adams, R. I., Bateman, A., Bik, H. M., Hawks, J., Hird, S. M., Hughes, D., Kembel, S. W., Kinney, K., Kolokotronis, S.-O., Levy, G., McClain, C., Meadow, J. F., Medina, R. F., Mhuireach, G., Moreau, C. S., Munshi-South, J., Nichols, L. M., Palmer, C., … Dunn, R. R. (2015). Evolution of the indoor biome. Trends in Ecology & Evolution, 30(4), 223–232. https://doi.org/10.1016/j.tree.2015.02.001

Rehn, J. A. G. (1945). Man’s Uninvited Fellow Traveler—The Cockroach. The Scientific Monthly, 61(4), 265–276.

Tang, Q., Bourguignon, T., Willenmse, L., De Coninck, E., & Evans, T. (2019). Global spread of the German cockroach, Blattella germanica. Biological Invasions, 21(3), 693–707. https://doi.org/10.1007/s10530-018-1865-2

Tang, Q., Vargo, E. L., Ahmad, I., Jiang, H., Varadínová, Z. K., Dovih, P., Kim, D., Bourguignon, T., Booth, W., Schal, C., Mukha, D. V., Rheindt, F. E., & Evans, T. A. (2024). Solving the 250-year-old mystery of the origin and global spread of the German cockroach, Blattella germanica. Proceedings of the National Academy of Sciences, 121(22), e2401185121. https://doi.org/10.1073/pnas.2401185121