Bright white lights are a buzzkill for glow-worm romance

Males take longer to find their mates thanks to increasingly light night skies.
A glow-worm on a leaf.
Glow-worms are common insects in parts of Europe and Asia. Deposit Photos

Like the luminous lighting bugs and fireflies that light up the night sky across the United States, glow-worms and their green glow shine across parts of Europe and Asia. These beetles are in the Lampyridae family along with fireflies and more than 2,000 other cousins..

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However, as the night skies continue to brighten, animals like the glow-worm are paying the price for the bright lights in big cities. A study published June 13 in the Journal of Experimental Biology finds that white light makes it harder for male glow-worms to find the green glow of females, which has potentially disastrous consequences for the global glow-worm population.

The team from the University of Sussex in England collected glow-worms at night and brought them back to the lab. There, team member Estelle Moubarak carefully transferred the male insects into a makeshift maze shaped like the letter Y. They placed the male glow-worms at the bottom of the Y and put a green LED light mimicking a female’s glow at the top of one of the Y’s arms. The male needed to walk towards the green LED and they recorded how long it took for the male glow-worm to walk towards the fake female. 

The team turned on a white light above the maze that ranged from 25 times brighter than moonlight (25 Lux) to the equivalent to the light beneath a streetlamp (145 Lux). 

All of the glow-worms could find the LED light in the dark. Only 70 percent of the males found the fake female at the dimmest levels of white light, and just 21 percent could find the potential mate at the brightest light the team tested. 

In addition to making it much harder to find a female, the white light caused the male glow-worms to take a longer time to scamper towards the green LED light. In total darkness, it took the insects roughly 48 seconds to reach the fake female. It took those same glow-worms about 60 seconds to find the LED at the lowest levels of white light. 

Illuminating the maze also caused the males to spend more time towards the bottom of the maze without moving towards the green light. In the dark, the glow-worms only spent roughly 32 seconds in the bottom of the Y, while they spent approximately 81 seconds in the bottom of the maze in the brightest conditions.

[Related: Light pollution is messing with coral reproduction.]

The team suggests that the male glow-worms were unable to move towards their potential mates when they were under the dazzling white light, since their compound eyes couldn’t be covered with a head shield. This acts like a pair of sunglasses, which protects their eyes reduces the amount of bright light the insects see. 

The glow-worms shaded their eyes for approximately 25 percent of the trial when the white light lit up the area with the fake female LED. They did this roughly 0.5 percent of the time when the maze was 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,” study co-author and zoologist Jeremy Niven said in a statement. 

If our skies continue to brighten and this trend stays holds, the twinkling lights of female glow-worms could fall dark.  Some individual methods to reduce light pollution include removing nighttime lighting that is not necessarily needed for public safety, removing all unnecessary light even if it is just one in a backyard, and switching away from white lights to more muted red lights.