The Daily Mail refers to it as a “slimewave”. The Sun refers to them as “an army.” Both papers reported that 500 Billion Slugs were set to invade British Gardens after a mild Winter created Perfect breeding Conditions.

Is the UK about to be overrun by slimy slugs? No, the answer is simple. But there could be worse.

The headline numbers are not always something that should make you scream. The “500 billion” number is estimated based on the maximum number of slugs per area. Slug populations can fluctuate dramatically over time and space. Even dramatic increases in slug population are not necessarily a cause for alarm. The increase is usually temporary and local, like waves on a beach. Usually, slugs will return to normal after the disruption.

The problem is the gradual, sustained, and perhaps less spectacular increase in numbers that, like a tsunami, continues for a much longer time and spreads throughout the countryside. This is the real invasion of slugs in Britain. What can we do?

In isolation, the trigger appears innocuous: a few nonnative slugs have been accidentally introduced from continental Europe. These species are so closely related in the UK that only specialists could tell them apart. They can also interbreed. Many animals can produce hybrids, and they are not a danger. But what makes these hybrids dangerous is the deviant sexual behavior of slugs.

This hybrid is between the UK’s black slug and a Spanish stealth slug. Les Noble, the Author, provided

Slugs are hermaphrodites. This means that the same individual can exist as both sexes. They first develop as males before going through a hermaphrodite stage to become female. They can, therefore, dispense with the normal mating requirement. This is where the “La Difference” between British species and continental species becomes significant.

Why British slugs differ

Slugs found a UK island that had been recently covered in ice sheets and lacked the same biological diversity as continental Europe. In this situation, self-fertilization was an evolutionary strategy that ensured reproduction, even when the slug population was decimated by harsh weather.

The downside of this continued inbreeding is that genetic diversity is rapidly lost. Some British slug species ended up consisting of nearly identical individuals. They were, therefore, more susceptible to parasites, pathogens, and other organisms that can quickly evolve to overcome their defenses.

In continental Europe, meanwhile, slugs became more diverse as the weather warmed up, making parasites and disease more prevalent than the search for a mate. The slugs were highly genetically variable and did not self-fertilize. At least some were more resistant to parasite attacks, a chance that the inbred British Slugs did not have.

The echoes of past environments are still present in modern species. British slugs have adapted to the variable climates and lack of mates. They’ve fallen into the “No sex, please, we are British” cliche, producing larger eggs by self-fertilization later in life. Continental slugs have adapted their defenses to combat rapidly evolving enemies. They produce smaller eggs early in life to maximize genetic diversity and to compensate for the loss of many individuals due to infection.

The Spanish slug was one of Britain’s main invaders. Tviolet

Humans disturbed the natural order when they moved slugs around as stowaways on commercial produce. This has led to widespread breeding between British species and continental ones. These hybrid “super-slugs,” which are extremely fertile and have genetically diverse offspring, are adapted for the British climate as well as parasites and diseases, many of which are still found in continental Europe.

Fighting the slug invaders?

The EU has banned the commercial use of Molluscicides, which are pelleted chemicals that poison slugs and cause collateral damage to wildlife. The focus is instead on natural enemies, such as nematode worms. However, these are generally not effective when it comes to the larger hybrids.

The increased biomass of slugs could host important parasites or pathogens in agriculture and veterinary medicine, which can spread more diseases to plants and animals. We are still surprisingly ignorant about the biology of slugs despite the fact that they are so common in gardens. Recent research has shown an increase of over a fifth of British species.

Where are we heading with this phenomenon, then? Invasive slugs, snails, and worms have been found to destabilize ecosystems in North America as well as Scandinavia. In the UK, we are seeing a similar situation. Our research shows that population sizes eventually start to decrease after 30-40 years. The ecosystem may eventually recover from the slug invasion. However, it remains to see how long this will take and whether it will have any lasting effects on the spread of disease, ecosystem services, or British biodiversity.