Media release

From: The Company of Biologists

Brighter nights risk extinguishing glow-worm twinkle

Brief summary: Glow-worms are a sign of summer, but dark night skies are being drowned out by light pollution. What effect might this have on glow-worm populations? Jeremy Niven and colleagues from the University of Sussex, UK, have discovered that female glow-worms cannot outshine light pollution to attract males, which could lead to the eradication of glow-worm populations across the planet. They publish this discovery in Journal of Experimental Biology.

Press release: The bright lights of big cities are wonders of the modern world; intended to help us work, stay safe and enjoy the world around us long after the sun has set. While artificial light has been great for increasing human productivity, some nocturnal animals, and even people, pay a price for this illumination. From increasing the amount of time that predators are active to disrupting migrations, light pollution affects many animals; but how do animals that use their own luminescence to lure food or attract mates fair against this new, brighter background? Female common glow-worms (Lampyris noctiluca) emit a green glow from their abdomen to attract flying males, but they are unable to fly themselves to new locations to escape light pollution. Because of this, Estelle Moubarak, Sofia Fernandes, Alan Stewart and Jeremy Niven of the University of Sussex, UK, wondered how hard it is for male common glow-worms to find mates in an ever-brighter environment. They publish their discovery that white light makes it harder for male glow-worms to find glowing females with potentially disastrous consequences for global glow-worm populations in Journal of Experimental Biology at https://journals.biologists.com/jeb.

After collecting glow-worms at night from the South Downs, UK, Moubarak transported them back to the lab, before beginning the tricky task of transferring the male insects to a Y-shaped ‘maze’ without exposing them to artificial light. The team placed the male glow-worms at the bottom of the Y and a green LED, which mimicked a female’s glow, at the top of one of the arms, which the male had to walk toward. They then recorded if and how long it took the males to find the fake female. Then, the team switched on a white light above the maze, ranging from 25 Lux (25 times brighter than moonlight) to 145 Lux (equivalent to the light beneath a streetlamp). While all of the glow-worms found the LED in the dark, only 70% found the LED at the dimmest levels of white light, and just 21% of the insects found their potential mate in the brightest light.

A female glow worm in the dark (Right) and with light from a camera flash (Left). Photo credit: Jeremy Niven)
Not only did the white light affect the glow-worm’s ability to find a female, but it also caused them to take longer to reach the LED. In the dark, the worms took ~48 s to reach the female-mimicking LED, however, it took the same glow-worms ~60 s to reach the LED in the lowest white light levels. Illuminating the maze also caused the male glow-worms to spend more time in the bottom part of the maze without moving towards a female. In the dark, the insects only spent ~32 s in the bottom of the Y, while they spent ~81 s in the bottom of the maze in the brightest conditions.

Moubarak suggests that male glow-worms were unable move towards the females when dazzled by white light because they cover their compound eyes with a head shield, which acts like a pair of sunglasses, reducing the amount of bright light they see. In fact, when the white light illuminated the area with the fake female LED, the glow-worms shaded their eyes for ~25% of the trial compared to only ~0.5% of the time when the maze was in the dark. ‘Keeping their eyes beneath their head shield shows male glow-worms trying to avoid exposure to the white light which suggests that they strongly dislike it,’ says Niven. So, while our bright night-time world has helped give rise to our modern society, it has had a drastic impact on male glow-worms and their ability to find mates. If this trend holds true, meadows and heaths across Europe and Asia that have lit up with the twinkling of the female glow-worms for millions of years will fall dark.