The Glowing Twinkle of Glow-worms May Diminish with Brighter Nights

The bright lights of big cities are a wonder of the modern world. They aim to help us function, stay safe, and enjoy the world around us long after the sun goes down. While artificial light has been great for increasing human productivity, some nocturnal animals, and even people, pay a price for this lighting. From increasing the amount of time predators are active to disrupting migration, light pollution affects many animals; But how do animals that use their luminescence to attract food or attract friends fare against this new, brighter background?

Female common glowworms (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. For this reason, Estelle Mubarak, Sofia Fernandez, Alan Stewart and Jeremy Niven of the University of Sussex, UK, wondered how difficult it would be for males of common glowworms to find mates in a brighter environment.

They published their discovery explaining that white light makes it more difficult for male glow worms to find glowing females, which could lead to catastrophic consequences for global glow worm populations, in Journal of Experimental Biology.

After collecting glowworms at night from the UK’s South Downs, Mubarak brought them back to the lab, before beginning the challenging task of moving the male insects into a Y-shaped “maze” without exposing them to artificial light. The team placed male glowworms at the bottom of a Y, and a green LED, simulating a female’s glow, at the top of an arm, toward which the male had to walk.

Then they recorded if the males could find the fake female, and how long it took. Next, the team turned on a white light over the maze, ranging from 25 lux (25 times brighter than moonlight) to 145 lux (equivalent to the light under a street lamp). While all glowworms found the LED in the dark, only 70% found the LED in the lowest levels of white light, and only 21% of the insects found their potential partner in the brightest light.

Not only did the white light affect the glowworm’s ability to find a female, but it also made it take longer to reach the LED. In the dark, the worms took about 48 seconds to reach the female-mimicking LED; However, it took the same glowworms about 60 seconds to reach the LED in the lowest levels of white light.

Lighting the maze also causes the male glowworms to spend more time at the bottom of the maze without moving toward the female. In the dark, the insects spent only about 32 seconds at the bottom of the Y, while they spent about 81 seconds at the bottom of the maze in the brightest conditions.

Mubarak notes that male glow worms were unable to move toward females when dazzled by white light because they covered their compound eyes with a hood that acted like a pair of sunglasses, reducing the amount of bright light they could see. In fact, when white light illuminated the area with the fake female LED, the glow-worms shaded their eyes for approximately 25% of the trial, compared to only about 0.5% of the time when the maze was in the dark.

“Keeping their eyes under their head shield shows that the male glowworms are trying to avoid exposure to white light, which suggests that they really dislike it,” says Niven.

So, while our bright, nocturnal world helped give rise to our modern society, it also had a significant impact on male glowworms and their ability to find mates. If this trend were correct, the meadows and vegetation across Europe and Asia that had been lit up with the twinkling of glowworms for millions of years would still be darkened.