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.

A street lamp buffet

Increasing light pollution may make many vital factors go awry. If insects that regulate their life according to temperature continue to behave as usual, while the rhythm of species accustomed to varying intensities of light breaks down, these different types of species no longer come across each other as before.

Tears start appearing in the food web: one species loses a vital food source, while another is exposed to excessively severe hunting.

“The Nathusius’ pipistrelle bat and Daubenton’s bat are afraid of being eaten, which is why they avoid light. At the same time, a braver competitor enjoys a hearty insect buffet in the glow of a street lamp,” says Christina Elgert, a biologist who is writing her doctoral thesis on glow-worms and light pollution.

Furthermore, light can blind animals at the moment of danger, or diminish the effect of their protective or signal colouring. When a pattern that evolution has made visible in the dark no longer stands out, a potential mate may go unnoticed.

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.”

Related to fireflies

Glow-worms are the most eye-catching of all nocturnal species. Overall, there are roughly two thousand species of glow-worms, including the tropical ones often known as fireflies. People have a positive attitude towards them, but the number of glow-worms seems to be decreasing across the globe.

In addition to light pollution, problems are caused by habitat destruction, climate change and insecticides.

“So far, there is not very accurate information available on glow-worm numbers and distribution, making the demand for citizen science efforts great. Don’t hesitate to report any glow-worm observations to the service or the iNaturalist app!”

Insects that glow in the dark are a natural fit for illustrating changes in the lightscape and the ecological consequences of that change. Elgert, however, is interested in the broader effect that light has on animal behaviour.

“I wrote my bachelor’s thesis on the effects of light pollution on birds. Light pollution affects not only the lives of nocturnal species, since the rest of us also need darkness.”

Remember to use the switch

Light pollution is a topic of lively discussion. However, Elgert believes that decision-makers have not yet received information packages on research findings that would be comprehensive enough. “We have to make that happen.”

Solving the problem of excessive light would be relatively easy compared to many other environmental issues. “Light is an easy type of pollution in the sense that it’s easy to get rid of. If you turn off the lights in the evening, there is no build-up in the environment.”

Giving up light altogether is not necessary: for example, motion detectors are a practical way of reducing unnecessary use of light. A big obstacle in the way of reducing light pollution is people's tendency to associate brightness with safety. “Even though lights create the problem that we can't see anything in the shadow outside our bubble of light,” Elgert points out.

Light pollution is a problem that is not limited to cities. Researchers classify nocturnal illumination into two distinct phenomena. Firstly, the sky above us has lost its blackness to the degree that the Milky Way and shooting stars are increasingly difficult to distinguish, and even the moon is getting dimmer.

The second form of light pollution is one closer to home: brighter, spot-like lights, such as glass buildings that are also lit up in the night time, and the lights found in vehicles as well as on streets, runways and billboards.

Human beings perceive light from a direction and distance different to many other species. Our perception is also shaped by the human ability to see dim light and colours.

Artificial females

Elgert is employing the research design at Tvärminne to determine how different wavelengths of artificial light affect the breeding of glow-worms. The traps have been laid down for males, with a tiny lamp glowing in the middle serving as an artificial female whose wavelength emulates that of genuine glow-worms, roughly 560 nanometres.

Above these green electric females, disruptive lights mimicking various sources of light pollution are installed – alternating between red, yellow, white and blue light sources, as well as switched-off lamps for comparison. Underneath the contraption, a funnel-shaped plastic jar is waiting for male glow-worms captured in their pursuit of the fake female.

Elgert, who assembled the traps, is utilising the accumulated data together with Linnea Kivelä, who is writing her master’s thesis in biology.

“We tinkered for a couple of weeks before the actual four-week research period started,” Elgert says.

The night shift

Once the equipment was complete and the summer had reached conditions amenable to the courtship flights of the glow-worms, Elgert and Kivelä started setting the traps in night time. They were installed by eleven o’clock in the evening, a little before actual female glow-worms start turning their lights on around midnight.

Glow-worm specialists work the night shift: a few hours later the catch had to be collected and put in jars to be processed the next day.

“The lunch offered at the research station was breakfast for us. Right after the meal, we counted and measured the males caught during the previous night.”

Once identifying dots had been painted on the captured glow-worms' pronotum, or plate-like shield, using acrylic paint, it was time to release them back to the location where they were caught. With the help of the coloured dots, the researchers were able to observe how many of the males were caught in the traps again and how far away from the release location they were found when captured for the second time.

Observing the beetle hunt in Hankoniemi after midsummer, the catch for the season was 508 individual males. Of these, roughly two hundred were encountered twice or more.

Male glow-worms can fly long distances, but that skill is of no use in establishing new populations. For that, they would need a female and its eggs. Females, however, have poor mobility and are unable to react sufficiently if their territory is spoiled by light.

“At most, the female climbs up a stalk to shine their light for the night and comes back down to shelter at its root for the day,” Elgert says.

A night of wasted glowing eats up the reservoir of eggs.

“Yellow and red light appear to be less disruptive to female success compared to white light. Many insects don’t perceive red light at all, and our hypothesis is that it is also the least disruptive colour for glow-worms.”

In excessive light, females put off switching on their lights, or may leave them off altogether. The last one turns off the light?

Christina Elgert is writing a doctoral thesis in the Doctoral Programme in Wildlife Biology (LUOVA) at the University of Helsinki. On 15 July, her article on the effect of light pollution on glow-worms was published in the Proceedings of the Royal Society B: Biological Sciences series.

The article has been published in Finnish in the 7/2020 issue of the Yliopisto magazine.

What is a glow-worm?

 Lampyris noctiluca belongs to the Lampyridae beetle family of glow-worms, which comprises approximately 2,000 species.

– The species lives as larvae for two to three years, hibernating in winter.

– The favourite food of the larvae are copse snails.

– Adult glow-worms eat nothing and only live for a short time to mate.

– The female attracts males, which are able to fly, by glowing in the night time.

– Glow-worms are most active around midsummer.

– They thrive best on shores. Partially covered spots, such as the edges of paths and the walls of buildings, are the best suited to glow-worm watching.

– Due to their bad taste, glow-worms are not favoured by predators, which makes the number of their natural enemies low – apart from human beings.

– When aggravated, males and larvae too can glow dimly in the dark: bioluminescent light has probably originally served to warn others of the unpleasant taste of the insect, or in the case of certain species, even their toxicity.

Researchers are seeking glow-worm observations for both the online service and the iNaturalist mobile application.