Toxic to humans – spreading across Europe
Large numbers of the toxic moth caterpillar have been found in Denmark and there is a chance it could spread further north, says researcher.
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Anders Endrestøl believes the oak processionary moth could eventually make its way to Norway.
He is a researcher at the Norwegian Institute for Nature Research. Endrestøl is also working on developing Norway's alien species list, and he leads the committee that studies terrestrial insects.
"It depends on what happens in Denmark in the coming years," he says.
The caterpillars can cluster in groups of up to 100,000. Originally found further south in Europe, the species has steadily expanded its range and has already been detected several times in Denmark.
The caterpillars are covered in microscopic toxic hairs that can cause rashes, breathing difficulties, and eye problems in both humans and animals. It's not the large, visible hairs that are toxic.
The species thrives in oak forests. If it were to appear in Norway, the southern coastline would be the most likely location, Endrestøl explains, as there are plenty of oak forests, favourable conditions, and mild winters.
So far, however, Endrestøl confirms that no oak processionary moth colonies have been detected in Norway.
Found in Denmark
Earlier this summer, a large colony was uncovered near a kindergarten in East Jutland, according to Danish TV2 (link in Danish).
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In Odense municipality on Funen, more than a thousand colonies of this species have been found. The species is so toxic and abundant that the municipality is actively working to combat the caterpillars.
You should never touch this caterpillar, whether dead or alive, because of its toxic hairs.
With so many colonies, efforts are focused on those located near schools and heavily frequented public areas.
Toxic hairs and dead caterpillars
Over the years, this caterpillar has steadily spread to new parts of Europe.
It was first discovered in London in 2006, and it's now spread throughout the city and into southeast England. Outbreaks beyond these areas are actively controlled, according to the Forestry Commission in the UK.
The oak processionary moth is known for creating striking, large-scale gatherings of caterpillars when they are on the move. They also live in huge colonies with thousands of caterpillars bound together by silk threads.
"It's a form of deterrence. It makes them appear larger than they really are, which may help protect them from predators," says Endrestøl.
True to its name, the phenomenon is called a procession. Hundreds of caterpillars wind their way up an oak trunk like a thick, moving snake.
In London, people are advised to keep children away from these processions, since they are so intriguing that kids may be tempted to watch and touch them.
The danger lies in the toxic hairs. During large outbreaks, such as the one in the Netherlands in 2019, there were so many caterpillars that countless hairs were carried by the wind. These caused skin problems, as well as eye problems when people got them in their eyes. All of this is described in a Dutch study.
"I can easily picture them affecting large numbers of people in a park setting. The hairs are also found on molted caterpillar skins, which dry up and can come loose in vast quantities," Endrestøl tells Science Norway, noting that he is not an expert on this specific moth.
This species goes through several developmental stages. Insect expert Hans Peter Ravn explains that the first three stages are harmless, but in the final three the caterpillars develop these toxic hairs, according to videnskab.dk.
That's why it's also important to avoid contact with molted caterpillar skins.
An alien species?
In 2023, Norway updated its alien species list, which includes species that could pose a threat to Norwegian nature.
Endrestøl explains that the assessment looks at two things: how likely a species is to invade and establish itself, and the potential damage it could cause.
In 2023, the oak processionary moth was not considered a risk in either respect.
"But if it establishes itself in Denmark, that may change. In that case, it will probably be included in the next evaluation," he says.
A key question is whether it should be considered an alien species in Denmark. Eggs from the oak processionary moth may arrive in Denmark through imported oak, according to videnskab.dk. If so, it would officially be classified as an alien species.
"If it's considered alien in Denmark, then it would also be classified as alien in Norway if it spreads from there," he says.
Still, the moth is native to Europe, and its northward expansion may simply be the result of climate change creating more favourable living conditions.
"If it spreads naturally due to a warming climate, then it's technically not an alien species," says Endrestøl.
Regardless of classification, it could still require control measures if it becomes a nuisance to people or nature.
Endrestøl adds that he doubts the moth would pose a serious threat to Norwegian oak forests, even if it were to establish itself here.
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Translated by Alette Bjordal Gjellesvik
Read the Norwegian version of this article on forskning.no
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