Marcus Arnerup is the on-call biologist at the Swedish Museum of Natural History
The scene along the Nordanå riverbank could be straight out of an American zombie movie, according to one passerby's startled observation. But fear not, it's just the annual return of the bird cherry ermine moth.
These moths visit every year, but their numbers fluctuate. Every few years, the larvae erupt in such vast quantities that they completely transform the landscape.
– It's a natural phenomenon, and nothing to be alarmed about, reassures Marcus Arnerup, an on-call biologist at the Swedish Museum of Natural History. He speculates that the recent heatwave in Skellefteå (mid-May) might have contributed to the moth's rapid population growth this year.
Astrid Löfgren, a park worker for Skellefteå municipality, experiences the moth's presence differently depending on her job.
– While they don't cause issues in the city parks, they're a nuisance at my other job – the summer theater in Medlefors. The larvae get everywhere, even on our costumes! It's not ideal, but it's part of nature's cycle.
The moth larvae feed on and lay their eggs on bird cherry trees. Their population depends on weather conditions and the previous year's numbers. Thankfully, a natural control mechanism exists.
– Certain wasps prey on them. They lay their eggs on the larvae, essentially eating them from the inside out. Additionally, small birds find them quite tasty, says Arnerup.
The intricate webs spun by the larvae serve a dual purpose: protection from predators and the elements. Birds and insects have a harder time reaching the larvae when they're cocooned in this silky shield. It also offers some defense against wind and harsh weather.
The sheer number of larvae is what creates these impressive webs.
– Each larva spins a tiny thread, explains Arnerup.
– But with hundreds of thousands on a single tree, the combined effect is remarkable.
The abundance of bird cherry trees along the riverbank, rather than a specific preference for the environment, is the reason the ermine moth thrives there.
Despite the inconvenience they can cause humans, these moths are generally not harmful to the trees themselves.
–There's only a risk if they infest the same trees year after year, reassures Arnerup.
– Bird cherries are resilient.
For those with gardens besieged by the spinning larvae, Arnerup suggests patience.
– Personally, I wouldn't bother trying to fight them. A simple spray with water might suffice if you're truly bothered.
Bland annat området vid älven nedanför Nordanå är angripet av häggspinnmalen.
The bird cherry ermine is just one of many species of ermine moths that weave their webs around the trees.
"They are ghost trees," says Kaisa Edström.
According to the biologist, it won't be long until the larvae transition to the next phase of life, and the trees are no longer covered in larvae and webs.
– The moths will likely start flying soon. Maybe in early July.
He also explains that a common misconception is that bird-cherry ermine moths spread to different types of trees.
– The bird-cherry ermine moth usually doesn't move to other trees or plants. However, there are many other ermine moths that can cause confusion. So, if your apple tree is covered in webs, it's probably the apple ermine moth.
Kaisa Edström took a stroll along the river, unfazed by the trees draped in webs. She views it simply as a facet of nature's ever-turning cycle.
– They're just 'ghost trees' for a little while, she says.
– While I wouldn't necessarily choose to stand directly under them, it's a reminder that nature marches to its own beat, whether we think it's beautiful or not.
Kaisa Edström remembers a significant number of ermine moths in Skellefteå just a few years back, and isn't surprised by the current abundance.