Light pollution disturbs glow and reproduction of glow-worms

University of Helsinki

Life on Earth has adapted to a bright day always being followed by a darker night. Due to light pollution, the natural dark is nevertheless a diminishing natural resource. What happens to the species that depend on darkness if the night no longer arrives?

Glow-worms are beetles, whose females produce a green glow to attract flying males. (Image: Christina Elgert)

Nightly artificial light introduces light into environments where it would not naturally occur. Humans benefit from artificial light in many ways, but when more light is used than intended or needed, light pollution is created. Because many organisms utilise the natural fluctuations in light to time their activities, light pollution may disturb for example animal behaviour and communication.

Nocturnal animals are especially sensitive to light pollution. Examples of such vulnerable animals include fireflies and glow-worms, as they use bioluminescence, i.e. light they produce themselves, in their mate attraction.

In her doctoral dissertation, to be publicly examined on the 27th of January, MSc Christina Elgert investigated through field and laboratory experiments, how light pollution affects the glow and mate attraction of the common glow-worm (Lampyris noctiluca). Glow-worms are beetles, whose females produce a green glow to attract flying males. The species spends several years as a larva, but the short adult phase spans only a few weeks. As the species is dependent on darkness and has limited time for finding a mate, light pollution is a potential threat to glow-worms.

In her study, Elgert found that light pollution affects the ability of glow-worms to find a mate, as males struggle to locate females glowing under light. This problem would be solved by females moving away from light towards darker areas, but instead they react by hiding more and glowing less. This is likely due to light pollution causing the females to believe dawn has arrived.

“The results of my research indicate that adapting to light pollution is demanding for glow-worms, as adaptive (i.e. problem-solving) behaviour was not found. If the reproductive success is lowered, this may cause a decline in the number of glow-worms in the long term”, Elgert states.

Even though it appears that glow-worms do not cope well with light pollution, there is still hope. The study also found that the duration, colour and intensity of light markedly affected how the glow-worms reacted to light. Shorter bouts of light, dimmer lights and longer wavelengths (yellow, red) notably reduced the effects of light pollution, in some cases even completely.

“By using dimmable lights, longer wavelengths meaning yellow and red light, and by using lights only when they are really needed, the effects of light pollution can be markedly reduced. This is beneficial not only for the glow-worms, but also for the other species dependent on darkness. By studying light pollution we gain an understanding of how organisms react to nightly light, and acquire more information for the needs of lighting and city planning”, Elgert summarises.

Christina Elgert, FM will defend the doctoral dissertation entitled “Can you see me? Sexual signalling in an artificially lit world” in the Faculty of Biological and Environmental Sciences, University of Helsinki, on 27 January 2023 at 13:15. The public examination will take place at the following address: Biocenter 3, room 2402, Viikinkaari 1.

Professor Leena Lindström, from University of Jyväskylä, will serve as the opponent, and Professor Johanna Mappes from University of Helsinki as the custos.

The dissertation is also available in electronic form in Helda.

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