These curious insects are not worms, but beetles and only the females glow. The sexes look very different - the males look like a regular black beetle, and the females look more similar to a grub. She has no hard wing cases and cannot fly. Glow-worms are not fussy and are found in a variety of habitats. They eat slugs and snails, so anywhere with a ready population of prey could be suitable. Glow-worm larvae have a gruesome way of eating their prey, biting into their soft flesh, and injecting a toxic chemical into them, paralysing the slug or snail and dissolving their insides. Glow-worms have even been seen sitting on the shells of snails waiting out of the way of their sticky slime. Adult glow-worms are only around for a short time and do not eat.
For a few weeks in June and July, the adults emerge, and this is the only time of year you will see the glowing females. The female will climb up into the grass or gorse to attract a flying male. Adult glow-worms only live a few weeks, so once the female has mated, she will lay hundreds of eggs on the ground. Once hatched, the larvae take around two years to mature and develop into their adult stage.
Silchester Common, in north Hampshire, has historically been home to a population of glow-worms. Stephen began monitoring their numbers in 2019 as numbers have been declining. Carrying out annual walks is a great way to record the number of breeding females, which can help inform management of the area and raise awareness locally.
The walk began just before 10pm, with a small group of interested locals along for the hunt. As it got darker, we were treated to the churring of nightjars on the common. This ground-nesting heathland bird calls at night and is far more often heard than seen, as they are well-camouflaged to their surroundings.
It wasn’t long before someone spotted the first faint glow of one of the females, shining from within a clump of heather. After that, the findings came thick and fast, with one of the more popular females spotted with three males!
In total, we saw nine glowing females and four males. This year's number is down on a total of 18 seen in 2019. However, given that this year has been very late for a lot of species, is it possible there were more around. It has also been a very wet summer and lots of wildlife had a late start which could be another factor in the lower numbers this year. It was encouraging to see such a good number in a small area, including mating pairs.
Sadly, glow-worms are declining in many of the areas they were once found, and have disappeared from some locations completely. The reasons behind this are not fully understood, but given that larvae and females cannot travel far, habitat fragmentation is likely a big factor. Light pollution and pesticides are thought to be another cause of their decline, particularly the use of slug pellets in gardens.
Given that these little beetles should be a gardener’s best friend - and are an amazing sight to behold - it is important to raise awareness about them to help prevent local extinctions.
Sustainable Silchester is a group dedicated to positive action for our planet. They encourage and support residents to take practical action to live sustainably and to help wildlife. I would like to thank them and Stephen for inviting the Trust to join their walk.