Willow Emerald (Chalcolestes viridis)

John Solomon, October 2022.

The Emerald Damselflies are often called the ‘Spreadwings’, as they habitually perch with their wings open in a delta position. There are five European species, including the Common Emerald (Lestes spoons), Willow Emerald (Chalcolestes viridis), Scarce Emerald (Lestes dryas), Southern Emerald (Lestes barbarus) and Winter Damselfly (Sympecma fascia). The Common Emerald is our only native species, but in recent years, thanks to global warming, the Willow, Scarce and Southern species have been gradually colonising our shores.

The very first record of the Willow Emerald Damselfly was way back in 1979, when a dead one was found in East Sussex, near Pevensey. In 1992 an exuvia was discovered at Cliffe Marshes in Kent, and a female was found on the Suffolk coast in 2007. From this it is reasonable to assume that at least some small, if isolated, colonies were slowly being established in the South-East of the country.

The numbers reached a tipping point in 2009 when around 400 were reported from 35 sites in east Suffolk, north Essex and south Norfolk. Proof that they were breeding here was that among these were a number of teneral, or immature, individuals which must have hatched out in this country. The damselfly rapidly began to increase its territory. By 2012 it was well established in North Kent, and by 2015 had colonised Cambridgeshire,
Herefordshire and West Sussex. More recently it was discovered in Yateley in the North-East corner of Hampshire. In Andover, as 2020 came and went, our eyes were out and searching, but none were found and there were no sightings any closer.

This was about to change thanks to a local birder, by the name of Brian Cartwright, whose habitual haunt is the local Anton Lake. He frequently sends photos he has taken through to me, particularly of Odonata. On the 14th September, he sent through a dozen or so photos and the set included this:

Needless to say this invoked much excitement, but despite regularly searching of the area of the Lake where he had found it, no further sightings were made. However, as this year’s season approached the hunt was well and truly on.

Willow Emeralds are late emergers. They are a species of standing water, of ponds and lakes, ideally with healthy reed beds. In late summer and early autumn the females inject their eggs into the bark of the slender, young twigs of trees that reach out to overhang the water. The insect is not as fussy as its name implies and will use a variety trees and bushes, including Alder, Ash, Elder, Birch and even Hawthorn, but its preference for Willow very obviously gives the common name of the insect. When laid the eggs do not immediately hatch, instead entering a period of arrested development known as diapause. It is important that they are not laid too early otherwise the warm weather of late summer will stop this happening and they will hatch. If they do the nymphs are vulnerable to the coldness of the water of the lake or pond over winter. There is also the very real possibility that the protolarva would fall onto bare ground revealed by the water of lake or pond drying up during the summer. Come spring the eggs hatch and the protolarvae drop into the water. Here they will undertake their first moult and start eating, hunting voraciously, and growing rapidly to emerge as adults around 3 months later. They often fly away from their birthing pond or lake, to become sexually mature, before returning to ensure the next generation. Their normal flight period is from early August through to late September, or even early October.

And so it was that on August 5th I was wandering along the bank of a small private lake, set just away from the River Anton, when this landed beside me:

This is another male Willow Emerald, and the pose I’ve caught makes the delta position of the wings very clear. Our rather more native Common Emerald rests with the wings in the same delta position, as mentioned it is also a ‘Spreadwing’ species, but the two are easily told apart. Here is a male Common Emerald for comparison:

The first difference, obviously not evident in the photographs but very much in the flesh, is that the Willow Emerald is a larger insect. However, the first characteristic that will catch the eye is the colour of the pterostigma. These are the spots towards the end of the wings. Notice that on the Common Emerald they are very dark, almost black, while on the Willow Emerald they are a pale buff, almost a cream. You will also notice there is a lot of blue pruinescence on the Common while there is none on the Willow. Another identifier, although rather less obvious and requiring a decent photo to spot, is that, viewed from the side, on the Willow Emerald the pale underside colour extends a ‘spur’ into the darker colour above:

Willow Emeralds exhibit rather different behaviour to other Damselflies. Most Damselflies are fairly sociable, often being present in large numbers together. Even when preferring to keep their distance from others there is rarely any confrontation. Willows, on the other hand, act much more like the Skimmer Dragonflies, with males selecting a territory. Here they will perch, often on the end of a prominent twig or branch, and drive away other males even as they search for a female.

Over the next few days I searched the other lakes around Andover for the species. I was lucky enough to see a mating pair at Charlton Lake, although not so lucky as to be able to get a photograph of them. On the 2nd of September, I saw my first one at Rooksbury and, on the same day, Brian sent through a photograph of a mating pair he had managed to capture at Anton:

Notice how much stubbier and blockier the female is in comparison to the rather slim male. This meant that I had now established that the species was present on all four of Andover’s local lakes. Unfortunately, I still hadn’t managed to get a shot of a female, the only ones I’d seen being involved in mating, either in tandem or in copula. This species usually oviposits when the male and female are linked up in tandem. At this point, it seemed that numbers were still good and I held out hope for finding and photographing a female myself, but as the end of September drew ever closer I realised that wish might not be granted.

Today, 30th September, I have finally admitted defeat. It’s possible that I might still come across the elusive female I seek, the species can be found into October, but there is a chill in the air and I don’t think I’m going to be that lucky … this year. But …. next year … when numbers should be higher …

HOMEPAGE is http://www.nwhwildlife.org. Go there for 150+ ad-free wildlife articles.