Join us in tracking the remarkable spread of this enigmatic species...
Just a decade ago, the Willow Emerald Damselfly had only been reliably recorded in the UK on 2 occasions, in 1979 and 1992. A single individual was then recorded in southeast Suffolk during 2007, followed in 2009 by a sudden boom of 400 records of the species from this same general area (SE Suffolk/NE Essex). Since this time, the Willow Emerald has spread rapidly across the south-east of England, gaining footholds in new counties on a yearly basis.
The natural colonisation and spread of this damselfly in the UK is incredible. It is important we track the species in order to understand how it is spreading so rapidly and what might limit the species in the future. For this reason, the BDS have developed the ‘Willow Emerald Watch’ project.
The project so far
Thanks to the efforts of our volunteer dragonfly recorders, we have been able to closely follow the spread of the Willow Emerald Damselfly since those first mass sightings in 2009. The spread was steady at first but since 2014 the rate of expansion increased rapidly and in 2015 the species was found in 8 counties: Suffolk, Norfolk, Essex, Kent, Cambridgeshire, Hertfordshire, Surrey and west Sussex.
Map showing Willow Emerald Damselfly records up to 2015. (Adrian Parr)
In 2016, the species colonised yet further new counties, including Bedfordshire, Lincolnshire, Northamptonshire and Buckinghamshire with several sightings submitted:
Map comparing those Willow Emerald Damselfly records from 2016 only and those from before 2016. (Adrian Parr)
Update from 2017
Willow Emeralds were again reported in good numbers from southeast England during 2017, with records not just from their core areas, but also from most of the sites where they first appeared only the year before (e.g. Tattenhoe Park in Milton Keynes, Buckinghamshire, and Gibraltar Point in Lincolnshire). Unlike the situation over much of the previous decade, there was however little sign of significant further range expansion. Few new records were thus received that were more than 10 km from previous sightings. The only real exception to this was in Kent, where numerous records were made along the Royal Military Canal towards the far south of the county. Even here, the numbers of individuals seen and the length of canal involved do, however, rather imply that this area has been colonised for some while, but it is only now that the damselflies have been spotted! The lack of major range expansion did not mean that the species faired poorly though, with significant ‘infilling’ still being noted in Cambridgeshire. Several diligent observers also managed to fill in apparent gaps elsewhere in the species’ range. Quite why findings in 2017 should differ so noticeably from those of the preceding few years currently remains something of a mystery. Perhaps the Willow Emerald’s recent range expansion has been so rapid that the species has overstretched itself, and populations in outer-lying areas now need to build up before further expansion can take place. The next couple of years should tell!
All records currently to hand are mapped below:
Map comparing pre-2017 and 2017 Willow Emerald Damselfly records. (Adrian Parr)
We are keen to continue following the progress of this beautiful little damselfly and we need your help. We are recruiting as many volunteers as possible to get involved with recording the Willow Emerald Damselfly, in particular in those counties bordering the species’ range so far. You can give as much or as little time to the project as you like; all records of the Willow Emerald help to build on what we know of this fascinating new species in the UK.
What to look for
The Willow Emerald Damselfly is one of four emerald damselfly species which can be found in the UK. The species in the group are well known for sitting with their wings partly open when at rest, as compared to other damselflies which generally rest with closed wings. One of the emerald species is very common, the Emerald Damselfly, two are rare, the Southern Emerald Damselfly and the Scarce Emerald Damselfly, and the fourth is the Willow Emerald Damselfly. The species are fairly similar in appearance and it can be easiest to take photos of any emeralds you are not sure of and have a closer look at these at home.
The easily visible distinctive features of the Willow Emerald Damselfly (Chalcolestes viridis), below, are: the metallic green body without any sign of blue colouring on the male or the female, the brown eyes, the pale wing spots with a dark border and the large spur-shape dark green mark on the side of the thorax. The outer anal appendages of the male Willow Emerald Damselfly are pale with a dark tip while the inner are dark and very short. The females' ovipositor is dark with a pale patch on the underside and chunky. There are two triangular shaped dark marks on the top of the female's abdomen.
Willow Emerald Damselfly (Chalcolestes viridis) male (left) (Paul Ritchie), female (right) (gailhampshire). Click on the image to enlarge.
When mature, the widespread Emerald Damselfly (Lestes sponsa), below, in comparison, has dark wing spots, blue colouring and eyes on the male and a very small green spur on the side of the thorax of both sexes. The anal appendages of the male are all dark, and the inner appendages are long and finger-like. The female ovipositor is dark on the underside and less chunky than the Willow Emerald Damselfly's. There are two small rounded dark spots on the top of the females' abdomen. Immature individuals do, however, lack the blue colour and can have pale wing spots for the first few days, so some care is needed to correctly identify emerald damselfly species at the start of their flight period.
Emerald Damselfly (Lestes sponsa) male (left) (David Kitching), female (right) (David Kitching). Click on the image to enlarge.
The mature Scarce Emerald Damselfly (Lestes dryas), below, also has blue colouring and eyes on the male, a small spur on the side of the thorax of both sexes and dark wing spots on both the male and female. The anal appendages of the male are all dark, but the inner appendages are spoon-shaped and incurving at the tip. The female ovipositor is chunky, protruding beyond the end of the abdomen, and dark on the underside. The female also has two very small square, dark marks at the top of the abdomen. This species is rare in the UK.
Scarce Emerald Damselfly (Lestes dryas) male (left) (Alain Auffret), female (right) (António A Gonçalves). Click on the image to enlarge.
The Southern Emerald Damselfly (Lestes barbarus) has bi-coloured wing spots, little blue colouring and brown eyes on the male and no spur on the side of the thorax. The outer anal appendages of the male are also pale with a dark tip, but the inner appendages are paler than the Willow Emerald Damselfly's and curved outwards. The underside of the female ovipositor is pale all over. This species is very rare in the UK.
Southern Emerald Damselfly (Lestes barbarus) male (left) (Paul Ritchie), female (right) (Paul Ritchie). Click on the image to enlarge.
Willow Emerald Damselfly females lay eggs into the bark of trees; these eggs overwinter and then with the coming of spring they hatch and the larva falls into the water below. Therefore, also look out for the scars left behind by egg laying females on the branches of trees, particularly Willows and Alders, though a very wide range of species can be utilised. The presence of egg scars can be used to record the species even during the winter months, giving the most recent breeding season year as the date of the record.
The scars left behind on Goat Willow (left) and Crack Willow (right) by Willow Emerald Damselfly egg laying behaviour. The scars differ in appearance depending on the tree species used.
Where to look
The Willow Emerald Damselfly is unique in the UK due to its behaviour of spending the majority of its time basking on waterside trees, largely Willow and Alder. They are particularly fond of bare, leafless areas of trees which receive maximum sunlight. Willow Emerald Damselflies are usually found near ponds, canals and other still or slowly-flowing waters with overhanging trees.
Woods Mill, West Sussex: a Willow Emerald Damselfly site. (Helen Haden)
When to look
The Willow Emerald is a mid-summer to mid-autumn species, with most records arriving between July and October, and some individuals even being seen as late as early November. Warm, dry and windless days will offer the best views of the species.
What to record
Record what species you saw, where you saw it, when you saw it and who you are. It is also useful to include a photo with your record, making sure the distinctive characters of the species are visible. To see the female ovipositor, the photo should be taken side on. In addition to this, extra information is useful to record, such as the number of individuals seen. Breeding behaviour witnessed is also an important addition to your record, for example egg laying pairs of males and females.
Where to send your record
Thank you for taking part!
(photo top: Paul Ritchie)