UK Glow Worm Survey 


Some of your glow worm questions answered

I think there were attempts by the Henry Doubleday Research Association, who champion organic gardening, to rear glow worms for snail control, but though I don't have any details I think they failed. I don’t believe that glow worms are ever prolific enough to control snails -- the reproduction rate of snails is probably much greater than that of glow worms. So though they would have some effect, it wouldn't be enough for gardeners. It tends to work the other way round – we tell people that they have to have a lot of snails if they want glow worms.

I don’t know of any successful attempts to start a colony of glow worms where they have not previously existed. Many people ask if there is a source of glow worms, or whether they can import them to their garden, but in general glow worms have fairly sparse populations and numbers vary tremendously even in a good site in the wild. Some years there are plenty in one little area, other years none at all there. It is a bit like sowing wild flower seeds – often nothing comes up because conditions are not quite right.

If you are in a suburb, forget it. If you have a garden bordering an area where glow worms might be found, it’s a possibility.

There is currently no commercial breeder of glow-worm larvae.

There is no perfect way to find larvae. Turning over stones is one way, looking for their brief and faint glows on very dark nights is another. Some people set up pitfall traps (eg plastic beakers sunk into the ground). But no method seems to find any but a small proportion of the larvae which must be there. One tip from site wardens is to put out flat pieces of wood bark, just heavy enough that they won’t be easily dislodged, and to turn them over occasionally to see what has made its home underneath.

During the glowing season you may also find male glow worms and maybe females by looking under stones, bits of wood and so on. But bear in mind that the larvae may well not be in the same area as the females. They are more likely to be around the undergrowth, whereas the adult females make their way into an open area where they can be seen by the males. (Human behaviour is much the same!)

The (glowing) adult females live for only a couple of weeks, as they cannot feed and they just live to mate, then die. Each year you are seeing the ones which were hatched out from eggs two years previously. Yes. Because they don't feed, they conserve energy as much as possible and their activity is restricted to crawling up a grass stem each evening, then descending after a couple of hours if they haven't mated. They spend the day in the understorey then go up again, either the same stem or a nearby one, in the evening. Click here for details of how to use convenient websites to find Ordnance Survey map references (grid references) within the UK.

Not really. I do get occasional reports of fireflies from the UK, but the sad fact is that we don't get fireflies in this country, though I wish we did. Fireflies are a different species from glow worms, though they are both members of the same overall family. The closest to Britain that fireflies are seen is Belgium.

Fireflies glow while they are flying, but glow worms just sit where they are and glow, though female fireflies may not actually fly but just sit there and wait for the males to find them, just as the glow worms do. Sometimes people talk of fireflies and actually mean glow worms, which remain static.

If your colleague saw apparently glowing insects flying around, it's most likely that they were actually ordinary night-flying insects of some sort illuminated in a beam of light from something – maybe a security light. You see much the same sort of thing when driving at night and moths suddenly seem to light up in the headlamp beam.

Solar lights are probably not a good idea where there are glow worms, because male glow worms are notoriously attracted to artificial lights and the solar lights are right down at ground level where they are looking. They could spend all their time crawling over the solar lights instead of finding females. Experiments suggest that even though the colour and appearance of the light is different from that of female glow worms, males are still attracted to artificial lights. Having said that, I do know of sites with quite bright overhead lighting where glow worms are seen from year to year, and the jury is still out over the extent to which artificial light actually harms glow-worm colonies.

If you have glow worms in your immediate area I would strongly advise you to turn the solar lights – and any other garden lights – off during the glowing period, usually June to August but varying from place to place.
There is very little info that I know of – maybe it would be a good topic for an academic paper if you fancied doing the research!

There is a Thomas Hardy book – I think it may have been Return of the Native – in which a group of men are playing with dice on the heath as it gets dark. Finding that they can’t finish their game, they hit on the idea of collecting glow worms. After a few minutes they have found a dozen or so, and can continue their game.

Two things emerge. One, they seem to think it is a novel idea to do this; and two, it takes them several minutes even in Dorset in the 19th century to find even a dozen. This suggests that glow worms, and the idea of using them for a light, were not very common.

Occasionally elderly people say that when they were young they used glow worms in their cycle lights, and I was told of one chap who got hauled up before the judge for doing this. However, he did get booked, possibly because the law requires a certain wattage of power in a cycle light!

Gilbert White, writing in the 18th century, only briefly mentions glow worms, and not in the context of giving light, so one assumes that they didn’t have this purpose.

I also feel that glow worms give off so little light that you would need an awful lot of them to get any useful illumination. If they were common, you could probably get enough to fill a whole jar with them and they might give you enough light to read a letter by, but I don't believe they were ever that common. Probably if someone did collect enough for that purpose, there would not be enough of them in the same place in subsequent years to be able to repeat the experiment!

So, from the above, I believe that in general they were not widely used for illumination. But it is a very interesting question, and maybe you can find out more.

Pam of Flintshire asks this, as there are no other sites (that I know of) from miles around. As only male glow worms can fly, and then only rather reluctantly, the species can only really spread on foot unless humans have put them there. There are many isolated colonies, not just in the popular glow worm areas such as chalk downland, but in remote Welsh and Scottish hillsides. The fact is that most glow worm colonies have probably been close to where they are for a very long time indeed – maybe many hundreds of years, if not thousands. Keith Alexander comments:

Following the last glaciation, species gradually recolonised Britain as
conditions improved for them. The subfossil evidence suggests that the
ranges of individual species ebb and flow in response to climatic change, so
glow worms would have spread north and uphill as conditions made this
feasible. But once people started to fragment the natural landscape, and
create vast sweeps of inhospitable land, populations have become
increasingly fragmented and isolated, and prone to local extinction with
little hope of recolonisation.

Of course, accidental introductions are possible, albeit unlikely – a viable
population needs to be established, which is more difficult for a beetle
like the glow worm than it is for many other species, eg New Zealand
flatworm or some of the exotic slugs and snails. Attempts to introduce glow
worms to Ireland have failed, which must indicate something!

I strongly suspect that records for the "lesser glow worm" Phosphaenus
hemipterus have been the result of casual introductions which have failed
after a few years. The most recent record was associated with a dump of
building rubble to help drainage along  a track - the glow worms could have
been present in cavities within the bricks etc and been moved with the
material. Certainly all records for this species have been one-offs, with no
subsequent sightings.