The Willow Emerald

 

The Willow Emerald Damselfly Chalcolestes viridis

Due to previous commitments I was relatively late to the Willow Emerald party, which is no real excuses as this species has occurred within the local area since at Harwood Dale since 2019.  Having redefined my ecological priorities I quickly began to search suitable sites whilst there was still the possibility of some on the wing. This species favours ponds with overhanging Salix, of which I am not sure there is any preference between Salix species, but I intend to find out. This is a key requirement for successful breeding sites, as this is the only UK Odonata species which oviposits into live wood, producing distinctive galls in the lower overhanging branches of Salix. It is however also worth noting this species will also use other woody species for egg laying, particularly Alder Alnus and Birch Betula, and has even been recorded egg laying in Nettle Urtica stems. The former match lake at Wykeham Lakes has been very productive this year for the species, although I found none (16th October) with only Migrant Hawker Aeshna mixta and Common Darter Sympetrum striolatum recorded, albeit in good numbers, I was starting fear I may be a bit late for this species. 

A break in the weather closer to home, tempted me to go and look at neighbouring Seamer Tip Pools, which was quickly rewarded after about 10 minutes of checking Salix, I am not sure whether the species has been recorded here before (I would assume it has given how regularly it is watched) but it certainly provides the right habitat structure. The species has a tendency to roost around head height within Salix trees, making it perfect to photograph, and no crawling on the floor like with other damselfly species. 

The story of the Willow Emerald in the UK, is a fascinating one any yet another of a recent colonist to the UK. Prior to 2009 there were very few records of this species, which was considered a real rarity. In 2009 it began to colonise en masse in the south east with large numbers recorded in south-east Suffolk, south east Norfolk and north Essex. Large numbers were found the following year in the same areas as well as south Essex and north Kent, with strong evidence of successful overwintering and breeding. From this initial colonisation even in the following 10 years or so it had spread all the way up to North Yorkshire by 2019, which is still around its most northerly point in the UK. 

Interestingly its spread nationally hasn't been a uniform south to north spread as one might perhaps expect. The spread has been far quicker north up the east coast than it has to the west. It has still not colonised the south west or central sections of the south coast, being scarce or absent west of the Isle of Wight. Presumably the spread north up the east coast matches climatic and micro-habitat preferences. There is certainly no consistency with temperature as warmer areas occur in the south-west, although interestingly the current distribution reflects areas with the lowest mean annual rainfall in the country, does this show a preference for ephemeral/shallow ponds which are more abundant in these areas? Or has less cloud cover allowed more rapid colonisation in these areas? The local pools in which it is recorded are not ephemeral and range from acidic peaty moorland edge ponds to shallow open lowland pools with a range of pH levels. As a generalist in Europe (perhaps the most generalist of the Lestes/Chalcolestes species) it is likely to eventually spread west and north-west one would assume.