Glowworms at Swinbrook

John C writes:

Who, late on a warm summer evening, doesn’t delight in finding a glowing female glowworm? The reaction of someone seeing one for the first time is always ‘Oh, WOW!’

Glowworm ‘hunts’ have been a fairly regular feature of WOFC’s summer programme for many years. ‘Hunt’ is a bit of a misnomer because we usually go to the same places where we’re likely to find them – Swinbrook and, more recently, Stonesfield. In the years when glowworms aren’t on the menu, a number of members will go out in June and July to look for them; there’e a certain competitiveness – who can find the most?

Last Friday – 29 July – Sue and I went to Swinbrook to look for them. We didn’t have high hopes because Maggie had been there a few days before and found none, and what had the extremely hot weather done to them? However, we were in luck and, after a fair amount of searching, found five in the churchyard although none in the adjacent lane.

As always, I reported our sightings to the UK glowworm survey ( which has day-by-day records going back to 2009. Most of the records there for Swinbrook (and Stonesfield) have been submitted by myself or other club members. Whilst browsing the website, I thought it would be interesting to see if there was any evidence of a collapse (or even increase!) of the population, or what was the best time to find them. I did a bit of Citizen Science and plotted some graphs – you have been warned!

Average number of GWs by year. (Zero means no record.)

The graph above shows the average number of glowworms recorded by year, from 2011 to 2022. There are so many variables that it’s pretty difficult to draw any firm conclusions except that – perhaps with the exception of this year – the population seems stable. A record from 1991 is for ten, which is the overall average number. With the exception of 2022, there is a very slight suggestion that numbers might be increasing – or simply that we are better at finding them.

The next graph shows the number of females by date, for all years in an attempt to discover if there is a best time for finding them.

Number of GWs by date, all years 2011-2022.

We usually visit – as the graph shows – in mid-July, but it also shows that glowworms can be found at least from any time between mid-June and early August. It would be worth going back there both earlier and later. However, there is the slightest suggestion that the best time to look for them is in mid-July. (The trouble with June is that it gets dark so late!)

That’s about all I could do with the limited number of records, but I would encourage people to go and look and, please, report numbers to the Glowworm Survey. Otherwise it will be difficult to monitor trends.

As far as I know, there is no evidence one way or the other that glowworm populations are declining, though are they probably declining like everything else. They were once fairly common, or at least commonly seen, but people don’t walk in the countryside late at night these days. Glowworms can turn up in unexpected places – I once found one in a field next to a pub car park. I have never seen any close to where I live but a 94 year old neighbour of mine told me that he used to see lots on the hill up from Newbridge on his way home from the pub (about three miles on foot). They may still be there for all I know, but I am not going to look for them alongside the now very busy A415!

We don’t know what the extremely dry and hot recent weather will have done to the population and it will take a year or two before we find out. It may not be good because the larvae live on (eat) snails and snails like it to be wet – time will tell.

One thing that has changed recently at Swinbrook is the amount of artificial light – every house now seems to have a security light – and very recent research has demonstrated that males are less attracted to females in the presence of ALAN (artificial lighting at night), which will undoubtedly reduce the breeding success.

I believe that there is no legal protection for glowworms as a species, which is a great shame for such a marvellous insect.

John Cobb, 30 July 2022