Blog: Glowing reports wanted!

Glow worms in the UK need our help - if you would like to support this lesser-known native species, start with this blog from Robin Scagell, who runs the UK Glow Worm Survey. 

If you look at the map of where glow worms are found around Britain, you might be surprised. First, perhaps you didn’t even know that we had glow worms, which seem to be quite exotic. Second, they seem to be so widely distributed around Britain (though unfortunately not Ireland) that you might wonder why you’ve never seen one. And you could ask why we need to survey for them, since they seem to be all over the place.

But the fact is that glow worms are regarded as being Near Threatened in Britain by the International Union for Conservation of Nature, and there are many things that we don’t know about their requirements that would help us to conserve their habitat. Some apparently ideal sites where there were large numbers as recently as the early 2000s now seem to have very few, if any. Yet at other sites that look quite unsuitable they seem to be thriving. So the more we can learn about the habitats where we find them, the better.

Glow worms are beetles in which only the wingless adult females glow brightly, and over a fairly short season mostly in June and July, to attract the flying adult males. Once mated, they retreat below ground to lay eggs. The larvae take two or maybe three years to reach maturity, feeding mostly at night on snails, slugs and possibly carrion. 

The adult females are up to 25 mm long and their glow can be seen for up to about 50 metres. They are a magical sight when seen along roadside verges, unimproved grassland such as downs and coastal areas, woodland clearings and even railway lines. At most UK sites you may see only two or three scattered around on summer nights. Some hillside sites could have dozens or even hundreds, but this is rare. They seem to be quite particular about where they want to be: you can walk for miles at night over suitable territory and not see one, then come across a group of them. There is no one type of vegetation that they need. So what makes them stick to one small area? That’s one of the things we have little idea about. 

I first got interested in glow worms in the late 1980s when I saw fireflies in Germany, in a park near Frankfurt. I was captivated and wondered whether we can see them in the UK. It turned out that we don’t have any fireflies (which glow as they fly) but we do have glow worms, which are part of the same family. There was no up-to-date information available about where to find them, so in 1990 I started the UK Glow Worm Survey, which you can now find at glowworms.org.uk 

I suppose glow worms are no more special than any other insect, other than their delightful appearance, but they have one major advantage when you want to monitor them – the adult females are in effect calling ‘Yoo-hoo, here I am!’. Although we are seeing only the unmated females, there are usually some that don’t find a mate each night so simply counting them is a reasonable guide to the size of the population.

There are, in my view, misguided attempts to ‘rewild’ glow worms where they are declining. I think it’s vital to know the reasons for the decline before we subject more individuals to the same fate. There are the usual suspects – agrochemicals, climate change, changes in land use, light pollution – but we don’t really know much about glow-worm predators or decline in prey, or perhaps soil structure or mineral content. These are all subjects that are waiting to be studied, so there’s an opportunity to carry out research on a local scale. 

Finding and monitoring your local glow worms is the start of the process. So I hope BIAZA members will spend a few late nights this summer searching likely spots and maybe finding them closer to home than they realise!

- Robin Scagell, UK Glow Worm Survey. 

Robin is kindly hosting a webinar with further glow worm surveying expertise. Taking place 12 May 2026, BIAZA members can sign up via this link.

All blogs reflect the views of their author and are not necessarily a reflection of BIAZA's position