Falling numbers of one of England’s threatened insects, the enigmatic glow worm, seem to have been halted in Purbeck in Dorset, as steps are being taken to protect their natural habitat.
Now volunteers from Worth Matravers to Durlston are making late night surveys in July and August to see whether an encouraging recovery of glow worm numbers last year is holding firm for 2025.
Volunteers in Worth Matravers, Mike Wenham, Kathy Hollidge and Kathy Jones are counting glow worm numbers this summer
Cause for cautious optimism
Glow worm numbers have declined across the whole of Britain this century, and the beetle is officially categorised as Near Threatened as its habitat is under attack from pesticides and light pollution.
While wartime children may tell tales of entire fields glowing green with the insects, today they are more likely to be found in single figures, spread widely down rural lanes.
Now, wildlife groups in Worth Matravers and Swanage, in conjunction with the National Trust and Purbeck Natural History Forum, are carrying out summer surveys to find out exactly how healthy the local glow worm population is.
Volunteers are walking out after dark to carry out glow worm counts along 12 routes known as transects between the Durlston Learning Centre and Winspit, on dates in July and August 2025.
It is the third year that the survey has been carried out and although it is too early to tell how the hot spring and summer has affected Purbeck’s glow worm population, results from the previous two years have given cause for cautious optimism.
The insects may take a little while to spot, but once you see the glow there is no mistaking them
“July should be peak season”
Mike Wenham, of Sustainable Worth, said:
“This is part of a project run by Purbeck Natural History Forum on National Trust Land between Durlston and Winspit, and as volunteers we go out once in July and once in August on dry, still nights to walk the transect and count as many glow worms as we see.
“They start to come out in May and June, but July should be peak season for them, though they will continue through August and into the early part of September as adults. During that time, they will mate, lay eggs and die – the adults are alive for about two to three weeks.
“They are beetles, not worms, but they don’t have mouth parts as adults, so they can’t feed and basically mate and starve to death.
“Their eggs will hatch after about three weeks and as a larva, they will live for two years before turning into a chrysalis and transforming into the final beetle.”
The larvae of glow worms feast on snails in a quite disgusting way
“It is a bit gory”
Mike added:
“In the larval stage they will live on the surface of the ground, or just under the ground and feed on snails – they bite the snail, inject some digestive enzymes into them, and wait around until the snail turns to mush, which they will then eat.
“It is a bit gory – we do take children out on glow worm walks occasionally, and they love the grisly detail – but when you see one you will realise why we love them so much.
“It is only the females which glow, and they do it to attract a mate. They don’t have wings, and although they are insects and have six legs, they are not very mobile.
“During the day, they will be at the bottom of the vegetation canopy hiding from predators, but when it is dark they will elevate themselves and glow for a couple of hours until the chemical reaction inside their bodies runs out.”
The light emitted by a glow worm is quite powerful
Unmistakeable and magical
The light from glow worms is a strong, green form of bioluminescence, caused when an enzyme in their body – luciferin – reacts with oxygen to emit the glow.
It is far more efficient than most light sources, although the size of the glow, on only the final two segments of the female’s tail, is not large as the insect itself is no more than 3 centimetres in length.
But once the green glows are spotted in the hedgerows or fields, they are unmistakeable and magical, and can be seen from quite a distance away.
The nocturnal insects live on grassland, woodland edges, heaths, verges and hedgerows, and need a mix of short and long grass and scrub to complete all their lifecycle – conditions the National Trust is returning to its lands from Durlston Castle to Winspit Quarry.
Barn owls, glow worms and grasshoppers under the light of the stars during one evening in Worth
“It feels very exciting”
Kathy Hollidge, of Sustainable Worth, said:
“The numbers of glow worms are incredibly down on what they used to be, there are stories about people being able to read from the light of glow worms in a jar. Some of our members have been walking these lanes for years and there are definitely fewer than there used to be.
“But there are reasons to be optimistic in Purbeck, they seem to like living by the coast and on limestone soils which we have down here – it’s because the larvae like eating snails and the snails like limestone, because then they have the calcium for their shells.
“A lot of people will see a white flower in fading light and think it’s a glow worm, or maybe drops of rain on a leaf catching a bit of light, but when you see the bright green glow, you can’t mistake it for anything else and think that perhaps you might be able to read a letter by it, as Hardy wrote.
“It feels very exciting – I wish we had fireflies as well in this country, but we don’t. But on these walks we will be able to hear grasshoppers and crickets, a barn owl or two if we are lucky, and seeing glow worms is amazing, there’s nothing else like it in this country.”
A red light underneath a plastic bowl attracts the flying male glow worms
Humane ‘traps’ set up to attract males
On the journey down the Winspit transect, more than half a dozen glowing females were spotted, and when humane ‘traps’ were set up – a red light under a plastic bowl – they attracted almost double that number of winged males, drawn down by the light.
When the results of the second year survey were finalised in autumn 2024, volunteers counted a total of 186 glow worms, as opposed to just 53 in 2023.
The highest counts in one evening were recorded around Weston Farm, Worth Matravers, an area managed for nature recovery by the National Trust.
And the latest ever recorded adult female glow worm recorded glowing anywhere in Britain was over several days until Friday 8th November 2024 at Durlston Country Park.
After the insects mate, the female will lay her eggs and die
Indicator of a healthy ecosystem
Michelle Brown, ecologist for the National Trust, said:
“We started glow worm monitoring in 2023, as it’s crucial to get the data to show how glow worms are faring, year on year.
“This will help guide our management on the sites we care for, and also help us establish whether and where it might be feasible to reintroduce glow worms in other locations.
“The use of pesticides has a detrimental effect on glow worms and their prey, and they are also affected by light pollution and changes in land use. So, if we have glow worms in the landscape, it can be used as an indicator of a healthy ecosystem.
“Few things are more magical than the sight of glow worms lighting up our hedgerows on summer evenings, and thanks to the help of our volunteers, we can see that numbers of these enigmatic creatures are growing in some areas of Purbeck.”
In the light of a torch, the size and the power of the glow worm can be clearly seen
Further information
- Find out more about these fascinating insects at the Glow Worm Survey website
- Join local teams surveying species from glow worms to puffins at the Purbeck Natural History Forum
- Use the What’s That Bug? to identify insects
