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Dung beetles lose their way as lit up skies drown out their compasses

Dung beetles, like many animals researchers have discovered, use the moon, the sun and stars to orientate themselves. Picture by Chris Collingridge.

Dung beetles, like many animals researchers have discovered, use the moon, the sun and stars to orientate themselves. Picture by Chris Collingridge.

Published Aug 15, 2021

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Johannesburg - Across parts of South Africa the sight of the Milky Way splashed across the heavens has become a thing of the past, and for some insects this is a problem.

Dung beetles use the Milky Way as a compass but light pollution means that some of these insects can no longer rely on this age-old visual cue.

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Researchers have found that they have had to opt for other navigational aids with sometimes catastrophic results.

They are one of a host of animals that are being affected by what is known as skyglow, a light glaze over the urban night sky that is drowning out most of the stars.

Dung beetles, like many animals researchers have discovered, use the moon, the sun and stars to orientate themselves.

They do this so that when they have created a poo ball at a dung pile they can roll it off in a straight line. After travelling a distance, the beetle will bury it and lay eggs in the dung.

If the insect doesn’t use a visual cue like the Milky Way, experiments have shown that they can’t maintain a straight line and often will end up back at the dung pile, where their ball is stolen by other beetles.

But light pollution, it was recently discovered, throws out the insects’ navigational system.

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On the roof of the biology building at Wits University, scientists were able to assess how the insect would cope in an environment where the city lights fade out the night sky.

“If we block them from being able to view the surrounding buildings, then they completely roll in circles,” says Dr James Foster, Research Fellow, Julius Maximilian University of Würzburg, in Germany.

Even in a rural setting, far away from the skyglow of the big city, light pollution is affecting dung beetles, as Foster discovered, while conducting further experiments.

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These experiments involved bright security lights that are now often found on farms.

“If you turn on a security light all of the beetles will roll towards it then if you turn it off again, then they disperse in different directions as they normally do, which allows them to avoid each other,” explains Forster.

“So I think that's going to be a problem for them to create unnecessary fights and it is going to take them towards the last place they don’t want to be, an urban centre where there is nowhere for them to dig into the ground.”

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In these experiments dung beetles rolling to the security light have ended up on concrete and tar where they can’t bury their balls.

Overall South Africa doesn’t have the light pollution that is affecting Europe and the US, but it is becoming a problem in some places.

“Johannesburg, Durban, Cape Town are at international light pollution intensities. But they are not as extensive. If you look at Europe and North America they are sort of blanketed under lighting,” explains Dr Bernard Coetzee, of the department of zoology and entomology at the University of Pretoria.

But continued electrification means that light pollution is a growing concern in South Africa.

Its spread can be tracked by satellite images.

Recently Coetzee was puzzled by the strange splodges of light that could be seen off the coast of Cape Town.

What it was found to be were hundreds of chokka fishing boats that were lit up for night fishing.

The effect of light on insects could have another serious impact on humans, in that it might assist in spreading disease.

Coetzee has been working on how lighting influences the behaviour of mosquitoes that carry malaria.

Some laboratory work has shown how mosquitoes bite more when exposed to LED lighting, even in the daytime.

And it is not just insects that appear to be experiencing behaviour changes because of artificial light.

“There are problems with migratory birds flying into brightly lit buildings, light houses and skyscrapers and we wonder if they are sort of blinded by the bright light,” says Foster. Fish behaviour is known to change under light and there is research into how skyglow is affecting migratory bats.

The next step, Coetzee says, is to consolidate the research.

“So we have these sort of piecemeal examples of how it is affecting a bat or a dung beetle and that is really useful but what you want to know is what does this mean for an ecosystem?”

South Africa may not be where other countries are when it comes to light pollution and this Foster believes is an advantage.

“The fantastic thing is that other pollution is going to be there for decades, but with light pollution we switch off lights and it is gone. So really it is a question of better lighting solutions and switching off lights that don’t need to be on.

The Saturday Star

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