The OPM, or egeprocessionsspinder in Danish, has already closed paths and a preschool in Odense, and so far this year it has also been spotted in Kerteminde, Nyborg, Morsø and in the capital, Copenhagen.
In Germany, the OPM becomes a seasonal problem from late spring onwards, affecting parks, schoolyards and residential areas.
Entire sports facilities in Berlin have been temporarily closed as a precaution due to infestations, highlighting the practical disruption caused to daily life.
While the situation is not yet as bad in Denmark as it is in Germany, authorities warn that contact with the caterpillars’ tiny hairs can cause skin irritation, allergic reactions and breathing difficulties.
So what exactly are these creatures, and how concerned should you be?
What exactly is an oak processionary moth?
Despite the alarming headlines, the egeprocessionsspinder itself is a relatively inconspicuous species of moth. The real issue lies with its caterpillars, which hatch in spring and feed almost exclusively on oak trees.
These caterpillars live and move in groups, often forming long head-to-tail “processions” as they climb trees to feed at night. They build distinctive white-grey, web-like nests on tree trunks or branches, which can range in size from a tennis ball to a football.
Biologically, their impact is twofold. Left unchecked they can strip oak trees of their foliage, sometimes completely defoliating them.
But they’re also equipped with a defence mechanism in the form of hundreds of thousands of microscopic stinging hairs. A fully grown caterpillar can have around half a million of these hairs, and each one contains a toxin called thaumetopoein.
These hairs are extremely light and can be carried by the wind over distances of up to 200 metres.
How dangerous are they really?
The primary risk posed by oak processionary moth caterpillars is to human health. Coming into contact with their stinging hairs or breathing them in can lead to a range of symptoms, including itchy skin rashes, redness and swelling, as well as irritation of the eyes and mucous membranes.
In most cases the symptoms are mild but some people may experience coughing, bronchitis or asthma-like symptoms.
More general symptoms such as fever, dizziness or fatigue have also been reported and, in rare cases, highly sensitive individuals can potentially suffer anaphylactic shock.
One other factor to bear in mind is that the danger doesn’t disappear once the caterpillars are gone. The hairs can remain active in nests and surrounding vegetation for several years, and can cling to clothing or shoes, potentially triggering delayed reactions.
At the same time, the caterpillars pose a longer-term ecological issue: by stripping trees of their leaves they can weaken or even kill oaks, especially during repeated infestations.
What residents in affected areas need to know
Encounters with oak processionary moths are most likely in areas with oak trees – particularly in urban green spaces, parks, schoolyards and along roadsides. Trees in open, sunny locations are especially prone to infestation.
Odense is, currently, the worst affected region, but it's not impossible that outbreaks elsewhere could become a regular feature of Danish summers – but one that requires awareness rather than alarm, providing sensible precautions are taken.
As a result, you may notice cordoned-off areas, warning signs or even temporary closures of public facilities during peak periods.
Beyond the warning signs, there are several tell-tales to watch out for, including partially eaten oak leaves and dense white-grey webbing on trunks or branches.
Experts advise avoiding any contact with the caterpillars or their nests and keeping a safe distance from infested trees.
Children in particular should be warned not to touch or climb affected trees. If contact does occur, it is recommended to shower, wash clothing and finally to seek medical advice if symptoms develop.
Adapted from an article in The Local Germany by Tom Pugh.
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