Do street lights interfere with glow-worms?

Entomologist Richard Jones discusses the impact of human lighting on courting glow-worms.

Female glow worm. © Ian Redding/Getty

Yes. Male glow-worms find the
non-flying females by their green
glow. This cold light is produced 
by the chemical metabolism of a complex energy-storing molecule luciferin, by a special enzyme luciferase, and is the opposite of photosynthesis, releasing light photons into the night sky.

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Like moths, male glow-worms are attracted to artificial lights, which appear to confuse their navigation. The worry is that excessive lighting will distract them, preventing them from finding the females in the rough, grassy banks where they breed.

Experiments in a small Swiss town found that when road lighting was on, simple LED traps only received males in dark areas far from lamp posts, but when the lights were off, males occurred in all areas.

The current thinking is that street lighting cuts ‘cheese holes’ in the beetles’ mating landscape.

Does every stage of a glow-worm glow?

Yes. It is in fact a bit of an oddity that, as well as the wingless female, the larva and the winged male of the glow-worm also glow. However, only the female glows brightly, and these are the ‘grass stars’ sometimes spotted in short turf on warm summer evenings.

It is generally accepted that the female glows to allow a male to find her for mating (female moths similarly release pheromone scents to attract partners in the dark). Why the male glows is still not known for certain – it could be an evolutionary hangover.

There is also a theory that glowing evolved to serve as a warning to predators that these insects contain noxious chemicals and will therefore taste foul. This is analogous to the bright warning colours of wasps, in which case it would be advantageous for males and larvae to glow too.


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Main image: Female glow-worm. © Ian Redding/Getty