How the German cockroach spread around the world

German cockroaches flourish inside buildings worldwide. They rank among the most widespread cockroach species and create problems for people both in the UK and abroad. Yet they are absent from natural environments.

Until recently, no one knew precisely how this city-dwelling pest evolved and came to occupy our homes.

By sequencing DNA, we investigated the German cockroach (Blattella germanica) and traced its origins to eastern India and Bangladesh.

It is a compelling account of the way people helped one of our most disliked pests to evolve and spread.

A German enigma

Our story’s central character first drew attention in eastern Europe after being seen in military food stores during the Seven Years' War (1756–63). Both sides gave the cockroach the other’s name: Russians referred to it as the "Prussian cockroach", whereas British and Prussian troops called it the "Russian cockroach".

In 1767, Swedish biologist Carl Linnaeus formally classified and named the species (Blatta germanica). Blatta comes from the Latin for "avoids the light", while germanica reflected the fact that the specimens he studied had been collected in Germany. (The genus was subsequently changed to Blattella to bring together smaller cockroach varieties.)

Scientists later identified closely related species with comparable anatomy in Africa and Asia. They proposed, in different cases, that the German cockroach had evolved first in Africa or Asia before subsequently conquering the world.

However, they lacked the means to test these ideas. We do not.

Enter gene sequencing

We collected DNA samples from 281 cockroaches across 17 countries worldwide.

We then compared DNA sequences from a particular genetic region known as CO1, an approach called "DNA barcoding".

Comparing the German cockroach with similar Asian species revealed a match. Its sequence was nearly identical to that of Blattella asahinai, found around the Bay of Bengal.

More than 80 percent of the German cockroach samples matched perfectly, while the other 20 percent showed only minimal differences.

This indicates that the two species split from each other only 2,100 years ago - an instant in evolutionary time.

From the Bay of Bengal to the world

We believe B. asahinai began adapting to life alongside humans after farmers cleared its natural habitat, as has happened with other species.

The ancestors of B. asahinai therefore left Indian fields for buildings and became reliant on people. The question was how they subsequently reached the rest of the world.

To investigate this, we examined a further set of DNA sequences within the cockroach genome.

On this occasion, we looked at sequences called SNPs (single nucleotide polymorphisms). Samples from 17 countries on six continents enabled us to establish how the German cockroach travelled from its native range across the globe.

The initial migration wave left the Bay of Bengal roughly 1,200 years ago, moving west. The cockroaches probably travelled alongside traders and armies belonging to the growing Islamic Umayyad and Abbasid Caliphates.

A second wave headed east into Indonesia about 390 years ago. They may have been carried by European trading firms, including the British East India Company and the Dutch East India Company. From the start of the 17th century, a number of these companies traded throughout South East Asia and returned to Europe.

Our findings indicate that German cockroaches reached Europe around 270 years ago, in line with the historical accounts from the Seven Years' War.

Around 120 years ago, the German cockroach spread from Europe throughout the rest of the world. This worldwide expansion accords with historical records documenting the species in different countries.

We think global trade drove this distribution because more closely related populations occur in countries connected culturally, rather than in countries that are merely geographically nearby. Consistent with this pattern, we identified another Asian expansion - north and east into China and Korea - around 170 years ago.

When steam-powered vessels replaced sailing ships, these hitchhikers could travel faster. Reduced voyage times increased their chances of arriving alive and establishing themselves in new countries.

Later, housing advances, including plumbing and indoor heating, provided conditions that allowed them to survive and prosper in buildings worldwide.

The German cockroach pest-control arms race

Naturally, people dislike cockroaches, meaning the invaders depended on remaining out of sight to survive.

The German cockroach adapted by becoming nocturnal, as its name implies, and by avoiding exposed areas. Although it retained wings, it ceased to fly.

These cockroaches are well known for rapidly developing resistance to many insecticides applied as surface sprays. Such resistance can emerge within only a few years. Finding new active ingredients is therefore difficult, particularly because discovery, safety testing and registration are expensive.

Cockroach baits were inexpensive and effective when they appeared in the 1980s. Yet they quickly lost effectiveness against German cockroaches because the baits relied on sugars to attract them. Cockroaches with a "sweet tooth" died, whereas those favouring other flavours survived and reproduced.

As we create new methods to control German cockroaches, we must account for the ways they may evolve to avoid intervention. Understanding how resistance develops can help us devise stronger countermeasures. It can also reveal vulnerabilities to exploit.

After all, the German cockroach will keep evolving and adapting in order to survive, so the arms race between humans and cockroaches will continue for many years.

Theo Evans, Associate Professor of Applied Entomology, The University of Western Australia and Qian Tang, Research Associate in Evolutionary Biology, Harvard University

This article is republished from The Conversation under a Creative Commons licence. Read the original article.