How does light pollution affect glow-worms?

Electric light confuses the lives of nocturnal insects and entire ecosystems. Researchers use artificial female glow-worms to learn more about organisms accustomed to twilight.

Already as children, many of us have watched a lamp suck in and capture twilight insects in its sphere of light, as if by magic. The scene is both horrific and mesmerising.

Headlights, neon lights and street lamps throw the lives of entire populations into disarray. When the moon and stars are overpowered by artificial light, nocturnal insects may mistake the sky for the ground. They no longer know which way is up or down, the direction of north and south, or that of open water and the shore. Many of them are lost, living their lives in unfavourable and stressful circumstances.

However, light is not only a map and a means for spatial orientation for insects. It also serves as a measure of the passage of time. Many species regulate their periodic activities on the basis of the variation in the level of illumination. Food acquisition, sheltering from predators, egg-laying, resting periods and other activities are carried out according to an exact daily, monthly or annual rhythm.

For example, African burrowing mayflies (Povilla adusta), whose adulthood only lasts for some hours, time their courtship, mating and egg-laying to occur during a bright moon. The new generation emerges on the second night after the full moon.

Lampyris noctiluca

A street lamp buffet

Moni elintärkeä asia voi siis mennä sijoiltaan, kun valosaaste lisääntyy. Jos lämpötilan mukaan elämäänsä säätävät hyönteiset jatkavat totuttuun malliin, mutta valon vaihteluihin kiinnittäytyneiden rytmit rikkoutuvat, eri lajit eivät enää kohtaa entiseen tapaan.

Ravintoverkko repeilee: yksi laji menettää elintärkeän ravinnonlähteen ja toinen altistuu liian ankaralle metsästykselle.

– Pikkulepakko ja vesisiippa pelkäävät tulevansa syödyksi ja karttelevat siksi valoa. Joku niiden rohkeampi kilpailija taas napsii katulampun valopiiristä kunnon buffaillallisen hyönteisiä, selittää kiiltomadoista ja valosaasteesta väitöstutkimusta tekevä biologi Christina Elgert.

Lisäksi valo voi sokaista eläimen vaaran hetkellä tai heikentää suoja- ja huomiovärien tehoa. Mahdollinen kumppani jää ehkä tunnistamatta, kun pimeässä erottuvaksi hioutunut kuviointi ei enää näykään.

White glow

There is chilling magic in the sight of a moth being drawn to a flame. However, the disruption of the life of insects that are active in the twilight and dark is not merely a peculiar feature of the biosphere. Nearly 30% of all vertebrates and 60% of invertebrates are nocturnal. This easily goes unnoticed by us humans, as we are mainly active in the daytime.

Countless species are currently witnessing first-hand how quickly light pollution is both increasing and changing. Lamps shining a classical yellow hue of light are being replaced by LED lights and other powerful light sources. White and blue light are increasingly prevalent, but next to nothing is known about the effect of this change on the environment.

In addition to wavelengths, the impact of light pollution depends at least on the intensity of light and the duration of disruptions caused by light.

“The increase of light pollution is a problem, as it reduces natural darkness and makes the nocturnal environment fragmented,” Elgert explains.

“Glow-worms have a particular dependence on darkness, since the female attracts flying males by glowing in the darkest summer night hours.”

If there is the possibility to choose, the male prefers the female that glows the brightest. And for a good reason: fertility.

“Bright females carry as much as ten times the number of eggs compared to others that glow faintly.”

Bright spots along the path

Christina Elgert has observed the life of glow-worms at the University of Helsinki’s Tvärminne Zoological Station in Hankoniemi for three summers.

“Here they are at their most active for a few weeks around midsummer, in June and July, provided the weather is warm enough and there’s not too much rain or wind. Still, you get observations from across the country even in August.”

Elgert says that anyone can look for and find glow-worms, with success most likely achieved by focusing on areas relatively close to seashores, lakesides and riverbanks.

“Wild strawberries may be a sign of an environment favoured by glow-worms.”

According to Elgert, particular attention should be paid to partially covered spots, such as the sides of paths or the walls of buildings.

A non-worm worm

This summer, a total of 15 worm traps were installed along the sides of a small road leading away from the Tvärminne station.

“Of course, glow-worms are not worms, but beetles,” Elgert points out, even though some of the features of the wingless female resemble those of larvae.

With its green-glowing behind, the female glow-worm is a rarity in the animal kingdom. Usually, ornaments used for attracting mates are the exclusive right of males, such as the peacock’s tail feathers or the lion’s mane.

“The female glow-worm has to invest a large share of its energy to emit light, whereas the male concentrates on growing the wing muscles needed for flight.”

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