Identification

The Downy Emerald, Cordulia aenea, is one of three Emerald dragonflies found in Britain. It is moderately sized¹ with a downy thorax² and, as the name suggests, it has a metallic green body with a copper sheen. Adults have noticeable bright apple green eyes³, which develop as the insect matures; by comparison immature individuals have brown eyes¹. The male has a slightly club-tailed abdomen⁴ and can be seen patrolling the edge of water bodies with a characteristic rapid flight, interspersed with fairly prolonged periods of hovering³. In flight, the tip of the abdomen is held slightly higher than the thorax, which gives this species a unique head down appearance. Despite this, the Downy Emerald is very similar to the other British Emerald dragonflies and unless very good views are obtained, specimens should be netted and the facial markings and anal appendages examined to confirm identity⁴ where it occurs with other Emeralds. Separation from Brilliant Emerald (S. metallica), should present little difficulty.

The larvae and exuviae are spider-like and can be distinguished by their rounded abdomens, long, striped, cream and dark brown legs³ and a distinctive black stripe on the side of the thorax¹. Separation from the other Emeralds can be made with the shape and number of mid-dorsal spines which are diagnostic.

Key Identification Features⁵

• Dark greenish-bronze abdomen and downy green thorax
• Mature specimens have bright green eyes
• Males have a constricted waist and a club-shaped abdomen

Status

The Downy Emerald is nationally scarce according to the British Red Data Book on Insects, although where suitable habitats exist it is widespread⁴.

Distribution

On the continent the Downy Emerald occurs from France throughout central and northern Europe to Siberia⁴. In Japan it is replaced by the sub-species C. a. amurensis Selys 1887. In southern Europe it is restricted to mountainous areas^6,7. It has a southerly limit in the mountains of Algeria and the Caucasus⁸. It is absent from the Iberian Peninsula, most of the Mediterranean and much of north Scandinavia^3,4

In Britain the Downy Emerald is one of three surviving corduliids⁴ with a scattered distribution from Devon to the Scottish Highlands^9,10. It appears to have a stronghold in the south east of England, but evidence suggests that the populations in East Anglia, the Shropshire and Cheshire meres and on the Hertford Essex border are declining³. It is locally common on the wooded heaths of Surrey and Hampshire^4,7, Berkshire, Essex, Sussex, and the New Forest³. Small populations also occur in a few localities in Dorset and south Devon, the Bristol Channel, Cheshire, Norfolk, Cumbria and north west Scotland^3,4.

Distribution map from the National Biodiversity Network Gateway website.

Ecology and Habitat Requirements

Observations of the Downy Emerald have shown that is it associated with woodland across its range in Britain¹⁰ and it appears that ancient deciduous woodland is particularly important³. The species seems to show a preference for still water habitats¹⁰, predominantly ponds and sometimes canals. Breeding colonies are invariably found in or near woodlands^3,10, and large well established ponds appear to be preferred^7,11. In areas with a good population, new ponds can be colonised fairly rapidly.

Despite this, it seems that only small areas of a pond are used¹². Males are characteristically seen patrolling the shoreline of tree lined ponds rarely crossing open water¹³. They favour sunny bays at the edge of the pond and avoid heavy tree shade, dense emergent vegetation and unvegetated areas^1,3,7. Females are commonly found along woodland rides², although both sexes can be found away from ponds feeding, roosting and mating in the tree canopy as well as in woodland clearings and rides^1,3.

Tree density around the water can be quite low, but deciduous woodland is often close by. Dappled shade is provided by bankside trees or large stands of reedmace or yellow flag around the margins. The broken nature of the shoreline due to the stands of emergent vegetation, bushes or fishing points can lead to quite large numbers of males holding territory¹⁸.

Mating appears to be a timely affair lasting up to an hour and a half. Females are seized and then taken directly to the treetops were copulation takes place. Females then oviposit alone, generally at a time of day when few or no males are present⁸ often, but not exclusively, near dawn and dusk¹⁴. Brooks et al⁷ noted that sparse stands of emergent vegetation in sun or dappled shade near the pond edge were the favoured oviposition sites. The female repeatedly dips her abdomen into the water whilst in flight³ releasing gelatinous clusters of eggs² into unshaded shallow water amongst relatively sparse emergent and submerged plants¹. In Scotland, Yellow Water-lily (Nuphar lutea), Common Reed (Phragmites australis),Water Horsetail (Equisetum fluviatile) and the mosses Fontinalis antipyrecta and Sphagnum subsecundum were noted as plant species common at oviposition sites¹⁵. In the Sussex Wealds, studies at 30 breeding sites have indicated that oviposition takes place in dappled shade over thick leaf litter at "traditional" woodland sites, or in front of or amongst stands of reedmace at more "parkland" sites¹⁸.

The larvae are sedentary⁸, living and sprawling among the leaf litter and debris at the bottom of the pond, close to the water margin^1,7,16,17. They are found among submerged aquatic vegetation, resting inverted on the underside of leaves in the leaf litter¹⁰ and they avoid exposed silt or gravel. Surveys indicate that most congregate in areas of shallow water near the pond edge⁷. They are most active at night¹ probably feeding on a range of macroinvertebrates including water hog-lice (Asellus), alderfly larvae (Sialis) and oligochete and chironomid larvae³. Development takes two or three years before the larvae emerge at the water's margin. Consequently the pond must hold permanent water⁷. Bankside plants and trees are necessary as emergence supports and to provide the important carpet of leaf litter^3,7. Mapping emergence sites will often show that small areas of a water body can be very important and need to be carefully managed¹⁸. On their maiden flight individuals disperse into the tree canopy where they mature away from the water, feeding and resting in woodland rides and glades¹⁰.

Emergence usually begins in early May and adults are on the wing until early August, and exceptionally late August. Research suggests that males do not move between ponds frequently and adults appear to be reluctant to fly out of the woodland canopy⁷, although in some cases males can prospect ponds up to a mile from the nearest colony¹⁸. The species may not easily re-colonise former sites or areas that have become suitable for the species. This poor dispersal means that conservation of extant sites is very important³.

The Larvae are known to occur in water ranging from pH 4.5 to 7.5. The species appears to be restricted to fairly oligotrophic waters but this requirement had not been extensively studied throughout its British range. It is thought that eutrophication of sites in the Norfolk Broads could be the reason for its decline in that region³.

Summary of habitat requirements

A pond within or close to deciduous woodland with scattered bankside trees, sparse stands of emergent vegetation and a carpet of leaf litter on the pond floor will offer the best conditions for breeding.

"Ideally a breeding pond will be situated in or on the edge of extensive woodland, hold permanent water have a largish surface area and a long undulating shoreline, have scattered bankside trees and emergent vegetation, sections with open sunny margins, a moderate accumulation of leaf-litter and areas of open water with floating-leaved aquatic plants"⁷.

Current Threats

• Habitat destruction and Fragmentation - The loss of woodland ponds in addition to the destruction and fragmentation of deciduous woodlands are thought to be major threats to this species. "Loss of woodland habitat from the British countryside may explain the disjunct distribution"¹⁰.
  
  
• Inappropriate Habitat Management - The dredging of ponds has a detrimental impact on the larvae of this species, eliminating the larval habitat and the nymphs^1,3. This practice is known to have exterminated the species from a number of sites.
  
  
• Inappropriate Vegetation Management - The excessive clearance of bankside trees has a negative effect by making sites less attractive to adults¹. Equally, dense tree shade will deter the species. Inappropriate management of the emergent vegetation can also have a detrimental impact on the species. Sparse stands are necessary but dense beds of emergent plants will deter the species.
  
  
• Predation - Sticklebacks are predatory and feed by pecking amongst the substrate, and consequently early instar larvae are at risk in ponds with high densities of sticklebacks.
  
  
• Pollution - Nutrient enrichment is thought to be one of the factors that influenced range contraction, particularly in East Anglia¹. Ponds with large populations of wildfowl are likely to be unsuitable as a result of eutrophication of the water.
  
  
• Altered Hydrology - Over abstraction of water may pose a serious threat, particularly as the species has a two to three year development period. Consequently, seasonal drying of the water body will exterminate the population.

Adapted from EA Species and Habitats Handbook³

Management

General Guidelines

• Survey Work should establish what other species are present and ensure that management is undertaken sensitively. By establishing the distribution of the Downy Emerald and the areas being utilised management can be targeted to appropriate areas¹² that will not disturb breeding populations. This can often be achieved by mapping the emergence site of exuviae.
  
  
• Metapopulations dynamics - The Downy Emerald appears to have metapopulation dynamics and consequently any management should be undertaken with this in mind. Where populations exist it seems that activity is centred at one important locality with 'satellite' populations at neighbouring ponds. Therefore it is important to identify the main population centre and focus conservation activities on this site. This is important as once the main site is lost it is likely that the other peripheral sites will also go and therefore efforts to improve the 'satellite' sites may be wasted.
  
  
• General Habitat Management - Each stage in the life cycle utilises the available habitat in different ways. Therefore, to conserve the species effectively this must be taken into consideration⁷.
  
  
• Over Management - care should be taken to ensure that sites are not over managed as the excessive removal of aquatic and bankside plants or dredging an entire pond is likely to be detrimental. Care must be taken during management particularly as the Downy Emerald is fairly poor at dispersing. Sites are unlikely to be recolonised, in the short term, if poor management practices eliminate a population. If dredging is essential, no more than a third of the water body should be cleared in any one year (i.e. management on a three-year rotation at least).
  
  
• Maintenance of the terrestrial Habitat - As adults appear to be reluctant to fly out of the woodland canopy, they tend to be contained within areas of continuous woodland. Thus colonisation of ponds remote from strong breeding populations can often be unlikely in the short term⁷. Consequently, woodlands supporting these species should be maintained and managed appropriately.
  
  
• Maintaining Water Levels - Any activity that results in reduced water levels in the pond, such as abstraction, diversion or drainage, should be avoided.
  
  
• Wildfowl - Feeding of wildfowl by the general public should be discouraged to avoid nutrient enrichment.
  
  
• Carp - The introduction of carp into ponds can have a detrimental impact on the Downy Emerald and should be avoided. Carp are bottom feeders that up root submerged aquatic plants. This causes turbidity and can encourage algal blooms. In addition carp fishermen often use bait, known as 'boilies', that are high protein baits that can lead to eutrophication of the water if not eaten.
  
  
• Research - Systematic research is needed to determine more exactly the habitat requirements of this species, particularly as nearby pools that seem to provide apparently suitable habitat remain uncolonised.

Best Practice Guidelines

• Provision of leaf litter - Bankside trees and emergent plants should be ensured at all times and although submerged aquatics seem to play no part in any stage of the life history it is likely that they provide shelter from predators. Some of the Epping Forest ponds are apparently unsuitable because they have too many submerged plants, but managers should not remove submerged aquatic in an attempt to try and encourage the colonisation of this species. The removal of aquatics should only be undertaken if the pond supporting the main population is in danger of becoming overgrown.
  
  
• Physical Habitat Management - If dredging of a pond is essential, no more than one-third of the pond bed containing leaf litter should be dredged at one time. Areas of bare substrate or areas under heavy shade are less likely to contain larvae and so could be cleared without a negative impact.
  
  
• Shading - Where shading by bankside trees is becoming too intense, pruning of overhanging boughs should be considered so that patches of sun can reach the water's surface. Dense tree shade will deter the species as it restricts the amount of open water. It is considered that this makes sites less attractive to adults by reducing the amount of suitable patrolling and ovipositing sites⁷. However, scattered, large bankside trees are required to provide plenty of leaf litter throughout the pond and therefore management must be carefully balanced.
  
  
• Managing Emergent Vegetation - A careful balance must be struck to ensure that the emergent vegetation does not become too dense or that over zealous management does not remove too much. However, thinning of dense beds of emergent vegetation may be necessary to improve oviposition sites. Observation by Brooks et al⁷ indicate that dense stands of emergent vegetation and heavily shaded pond margins are avoided by sexually active males and ovipositing females. Bank vegetation can however be very important as emergence support and is best cut back, at the earliest, after the end of the adult emergence period.

Habitat Restoration and Recreation

• Habitat restoration and creation may be worth trying, although Downy Emeralds are generally poor dispersers. They do occasionally colonise sites naturally. The creation of woodland corridors between ponds might be beneficial¹⁰.
  
  
• Research suggests that "larger ponds appear to be more attractive to adults". Brooks et al⁷ suggest that "ponds with a long shoreline incorporating deep narrow bays would maximise the number of males present at one time" and could be an important part of habitat management and creation.
  
  
• Translocation could be a sensible option in some instances, considering the usually poor dispersive abilities of adults. It is probably unnecessary in most areas but could be considered for East Anglia where the species was once more widespread, provided suitable oligotrophic sites were still available.

Adapted from EA Species and Habitats Handbook³

Management Case Study

Management has been undertaken at Burnham Beeches 'Top Pond' with the aim of controlling the growth of Bogbean and expanding suitable habitat for the Downy Emerald¹².

References

Management file updated on 18 October 2004