Light pollution could snuff out glow-worm twinkle

Light pollution from human activities is making it harder for glow-worms to find one another, with potentially disastrous results for global glow-worm mating according to a study published in the Journal of Experimental Biology.

An experiment by zoologists from the University of Sussex in the UK, shows the presence of artificial white light dazzles male glow-worms and obscures their ability to find females.

Study co-author Professor Jeremy Niven from the University of Sussex tells Cosmos light is essential for the survival of nocturnal Common glow-worms (Lampyris noctiluca). The species relies on a bioluminescent signalling system involving males finding females via a tiny green glow within the darkness of meadows and heaths.

Niven says female glow-worms are wingless, and attract males by producing a bright green glow. Male glow-worms fly towards the glow, landing nearby and then walking towards the female.

“So females and males can only mate if the males can detect the female glow,” Niven says. “The males and the juveniles (larvae) also have a glow, but this is probably a warning signal to predators.”

In the experiment, the researchers collected glow-worms from the South Downs in the UK and transported them back to the lab. They then transferred the male glow-worms into a y-shaped maze without exposing them to artificial light. 

The researchers added a green LED at the end of the maze designed to mimick a female glow-worm. They then recorded how long it took the males to find the fake female under no light and a range of artificial white light settings. The light levels ranged from 25 Lux (25 times brighter than moonlight) to 145 Lux (the equivalent of light beneath a streetlamp).

While all of the glow-worms found the fake female (green LED) in the dark, only 70% found the LED at the dimmest levels of white light. Just 21% of the insects found their potential mate under the brightest light settings.

The glow-worms also took longer to complete the maze under the presence of artificial light. In the dark, the glow-worms completed the maze in 48 seconds, but around 60 seconds at 25 Lux. 

The males also spent more time at the bottom of the maze not moving when there was artificial light. In the dark conditions, the glow-worms spent 32 seconds without moving, compared to 81 seconds in the brightest conditions. 

The researchers say the likely reason the glow-worms were unable to move is that they were dazzled by the presence of artificial light and shielding their eyes in response.

“When males see a female they extend their head and walk towards her. White light causes the males to be slower […] Part of this is that the bright light causes the males to pull back their head beneath a small shield that covers the eyes. For the males, we think it’s a bit like putting on a pair of sunglasses – the head shield makes the light dimmer but seems to stop them searching out a female,” Niven says.

Common glow-worms are found are found in chalk grasslands, hedgerows and heaths in the UK, Ireland, Europe and across Asia.

Niven says glow-worms spend about two years as larvae feeding on snails, before becoming adults that live for just a few weeks. The males also have nocturnal colour vision, which is rather unusual for any animal. 

“Glow-worms are toxic and they don’t themselves seem to have any obvious predators, which is probably useful when you advertise where you are with a glow,” he says.

The study results could help explain why glow-worms are in decline and disappearing from sites they’ve been present at historically.

“Several recent studies have suggested that glow worms are in decline […] Nobody knows why but there are several possibilities including that humans are now producing so much light at night (street lamps, stadium lights, even domestic garden lights) that males can no longer reach females, interrupting their mating and causing population declines. 

“Other possibilities for glow worm decline include climate heating and land use change,” he says.