German cockroaches, unwanted by anyone but found in almost every household. As the most abundant cockroach species in the world, they thrive solely among humans, with humans possibly to blame for their spread. A genetic study published on May 20 in the Proceedings of the National Academy of Sciences of the United States of America revealed that medieval wars and colonial trade aided in the near-global proliferation of cockroaches.
The German cockroach did not originate from Europe. Image Source: SMUAY/GETTY IMAGES
Biogeographer csamar Capinha from the University of Lisbon in Portugal, not involved in the study, noted that this finding is striking, indicating that many species have intentionally or unintentionally left their habitats through travel and trade. He pointed out that a report from the United Nations in 2023 stated that humans have introduced over 37,000 species to new regions worldwide, with over 200 new species being introduced each year.
In 1776, the renowned naturalist Carl Linnaeus formally described the German cockroach based on specimens collected in Germany. However, naturalists soon realized that it likely did not originate in Europe, as similar cockroaches were not present in Europe at that time. Since then, this insect has become common on every continent except Antarctica, often considered a major pest by humans.
"The German cockroach is common, but our understanding of it is limited," said Tang Qiantang, a biologist at the Roland Institute at Harvard University. Therefore, he and his colleagues attempted to use DNA to find answers.
Ten years ago, Tang Qiantang unraveled the history of cockroaches by analyzing DNA fragments called microsatellites. However, these tiny genetic materials were not sufficient to explain how different populations of German cockroaches were related. Now, with whole-genome sequencing being easily achievable, Tang Qiantang decided to reexamine his original specimens.
By comparing mitochondrial genes, researchers found that the German cockroach is most closely related to the Asian cockroach Blattella nipponica. The nipponica cockroach was initially described in Japan and later spread to the United States. To trace the path of German cockroaches leaving Asia, scientists mapped the genetic similarities of 281 samples from 17 countries on every inhabited continent. This pattern of association suggests that cockroaches dominated the world in two migration waves, both likely facilitated by human assistance.
Around 2,100 years ago, possibly in India or Myanmar, the German cockroach differentiated from the nipponica cockroach by adapting to human food, leading to its expansion. Approximately 1,200 years ago, it migrated westward to the Middle East, coinciding with the trade and expansion of two Islamic caliphates – the Umayyad and Abbasid. Researchers believe these insects hitchhiked across deserts in breadbaskets.
About 390 years ago, the species continued to spread eastward to Southeast Asia, coinciding with the establishment of colonial trade by the Dutch and British East India Companies. Within about a century, trading ships brought them to Europe. Tang Qiantang stated that steamships and improved heating and plumbing systems in housing created warm, humid conditions favored by cockroaches, enabling these mobile insects to spread globally.
Cleo Bertelsmeier, a conservation biologist at the University of Lausanne in Switzerland, said, "Researchers have shown that adapting to human lifestyles is key to the spread of German cockroaches, which I think is truly original."
"This paper does a great job of matching patterns with actual historical events," added Capinha. However, he wonders if the genomic data aligns with historical records of cockroach arrivals in different locations.
For Tang Qiantang, he is further investigating the past by using DNA from museum specimens to better understand the evolution of cockroaches. "This genus has a history of 200 million years, but we know very little about it," he said.
"Understanding the origin and genetic diversity of cockroaches can also help us find ways to combat insecticide resistance. This is crucial when they become pests in restaurants and hotels," Tang Qiantang said. "The population of German cockroaches grows very rapidly, and if left unchecked, it could reach a huge number in just a few months."
Related Paper Information: https://doi.org/10.1073/pnas.2401185121
