The Common Club-tail (Gomphus vulgatissimus) is a medium sized species with a distinctively club-shaped abdomen as suggested by its common name. Females and teneral males are mainly black¹ with extensive yellow markings on the thorax and abdomen. The shape of the hind wing differs between the male and female and can be used to help distinguish the sex of insects even while they are immature. As the male matures most of the yellow markings turn green although the spots on the sides of abdominal segments 7-9 remain bright yellow². Unlike most dragonflies the eyes are separate and do not meet at the top of the head, which aids identification, and in mature adults the eyes are dull green². This dragonfly is elusive as an adult, but can be seen most frequently during emergence or as a teneral insect. Once it has settled in the vegetation or upon stones, its cryptic colouration makes is difficult to observe³. It can be mistaken for the Golden-ringed Dragonfly Cordulegaster boltonii, but it is much smaller and the pattern of pale markings is very different⁴.

Larvae of the Common Club-tail are unique, and unlike other British species have short bulbous antennae. The shape of the exuvial case is diagnostic² and easy to identify with the head appearing triangular due to the way the antennae always form an apex at the front of the head.

Status

The Common Club-tail is a nationally notable species (Nb), being found in less than 100 10km-squares in Britain. It is an extremely local insect, but where suitable habitats occur it can be found in very large numbers². It is the subject of three local Biodiversity Action Plans, in Worcestershire (this county having more 1km-square records than any other nationwide), Shropshire and Cheshire.

Distribution

The Common Club-tail is the most widespread Gomphid species in Europe¹, inhabiting many central and northern countries. It occurs from Southern Scandinavia, eastward to Russia and South to France and Northern Italy. However, it is scarce in the Mediterranean⁴.

In Britain it is confined to Midland and Southern counties⁵ with the River Dee supporting the most northerly population⁶. It is found on the mature stages of seven river systems and their tributaries; five of these rise in the Welsh Uplands (the Dee, Severn, Wye, Tywi and Teifi) and two in Southern England (Thames and Arun)⁴.

Distribution map from the National Biodiversity Network Gateway website.

Ecology and Habitat Requirements

In Britain the Common Club-tail is a riverine species typically associated with moderate to slow flowing water⁵. It breeds in unpolluted, meandering rivers, which have a depositional nature⁴. Silty substrates are favoured over stretches with rock beds⁵ as the larvae are burrowers, living in the mud and silt. Inhabited reaches are typically adjacent to woodland, as it provides cover for the adults², rather than more open habitats⁵. Common Club-tails spend most of their adult life away from water⁷ perching on the ground, on bushes¹ and in treetops². They are often found in woodland and use the tree canopy more than most species^{1, 2}. They are elusive insects and consequently knowledge about their terrestrial requirements is limited.

Mature males are territorial, preferring to fly low over open water rather than along the shoreline, to defend territories or to hunt^{8, 9}. They appear to show a preference for stretches where the banks are steep and where there is some tree cover².

Copulation is rarely seen, but has been witnessed some distance from water^{2, 10}. It can be seen in woodland glades, where males will display over bracken to encourage females to fly down to mate.

Females oviposit alone, favouring quieter stretches in an attempt to avoid male attention^{4, 5}. They fly low over the water, dipping their abdomen below the surface several times before disappearing².

The larvae of the Common Club-tail live within the silt and mud on the riverbed for 3 to 5 years², foraging actively in the sediment at night1¹. Research suggests that they prefer clumps of emergent vegetation⁹, although it is not a necessity. This explains why the species continues to thrive along the Thames regardless of the tremendous build up of pleasure boat traffic¹². The larvae are usually found on meanders where silt deposition is greatest² and research indicates that they favour the inside of these⁹. Research also suggests that the larvae prefer gentle banksides and require water at least 20cm deep which is slow flowing⁹. It has been suggested that both the eggs² and larvae may be carried a short way downstream during their development5. It is likely that the preference for slow flowing water prevents them from being washed too far downstream² and they appear reluctant to wander, except at low current speeds¹³. Ole Müller¹⁴ found that the angle of the front of the head adopted by Gomphids helps to prevent the larvae from being dislodged.

The Common Club-tail is a spring species with a tightly synchronised emergence⁴ triggered by increasing day length and rising water temperature in May and June¹⁵. The larvae wait for the sun to come up before leaving the water and may travel some distance to find a suitable emergence site¹⁶. Distances of 20m or more have been recorded^{16, 17} and observations on the continent suggest that in some situations stones are chosen in preference to plants on lake margins^{18, 19}. This probably reflects the anatomy of the larvae as the legs, which are widely spread, are not adapted for climbing reeds^{12, 20}. Unlike most British Dragonflies, Common Club-tails can emerge in a horizontal position, although observations suggest that the numbers of larvae actually doing this is very low¹⁵. Collision with overhanging vegetation at the beginning of the maiden flight can knock teneral adults into the water¹³. Common Club-tails are famous along reaches of the Thames, Severn, Wye and Arun, where mass emergence occurs. There is a tendency for dense concentrations of emerging individuals to occur at well-spaced sites, with small number of exuviae found in between⁵. One survey carried out in Worcestershire over the last 18 years has shown that emergence rates at the same location can vary from 1.5 to 6.0 per metre per year (M. Averill, pers. comm.).

Following emergence, the maiden flight of teneral insects aims away from water, usually into vegetation at the top of the riverbank. Thus, woodland or scrub within close proximity of inhabited rivers provides a local source of food and shelter for adults, particularly during their maturation period⁵. Dispersal is invariably away from the river and adults can be seen flying in woodland rides or along hedgerows often several kilometres away from the breeding site. Maturing insects can be found feeding in woodland clearings up to 10km from the river².

Summary of the Main Habitat Requirements

The Common Club-tail inhabits and breeds in moderate to slow-flowing river systems in which there is sufficient silt deposited for the larvae to burrow. Bankside tree cover and adjacent woodland are important requirements for adults².

Flight period: early May to end of June/early July

Current Threats

• Water pollution: - Poor water quality is generally considered to be a factor affecting this species^{2, 3}. Gomphidae are one of the pollution-sensitive taxa assigned a value in the biomonitoring indices used to assess water quality. Under the Biological Monitoring Working Party (BMWP) system Gomphus is assigned a value of 8 (10 being the most sensitive) illustrating its vulnerability to pollution²¹. Toxic inputs from domestic sources and industry in the form of organic pollution and toxic chemicals are indeed likely to affect Club-tails. The Common Club-tail has been recorded in some polluted lakes outside of the UK, but was able to survive where turbulence caused by boat traffic increased the amount of dissolved oxygen²².
  
  
• River Maintenance (Excessive dredging and vegetation removal): - The impact of dredging is not completely understood, but it is considered to have a significant effect on the larval stages as the insect spends 2-3 years as a larva in the river bed silt, particularly as damage to riparian vegetation makes the emergence difficult⁷. Limited research and survey work indicates that although adults do not avoid the bare ground which dredging causes, larvae avoid dredged sites⁹.
  
  
• Sediment Deposition: - If a thin layer of fine sediment is deposited by the current over the substrate in which a larva of G. vulgatissimus is buried, the larva can rapidly re-establish respiratory contact by pushing the anal pyramid through the layer, but a larva cannot penetrate a layer several centimetres thick and is killed when sand particles enter the rectum^{13, 23}.
  
  
• River Improvement Schemes: - Major river works that fundamentally alter the hydrological nature of the river have a detrimental impact on riverine Dragonflies. These alterations may lead to excessive scouring of the bed, loss of bankside vegetation and subsequently to a single habitat type bereft of additional ecological niches. The suitability of rivers for species has also been impaired by 'improvement' schemes that create steep banks and deep straight watercourses⁹.
  
  
• Recreational Use of Rivers: - The passage of fast moving boats can be detrimental to riverine species, particularly as it exposes emergence sites to wave action. Although the Common Club-tail thrives on the River Thames, despite boat traffic, the waves generated by boats sometimes dislodge emerging adults^{13, 24} and can drown large numbers³. There is also the risk of a local pollution event where boats occur.
  
  
• Weather: - The synchronised emergence of the Common Club-tail over a short period in the spring means that it is vulnerable to changing weather conditions. Bad weather causes reduction in the numbers emerging² and in recent years wet cool weather during the emergence period may have affected the success of the species slightly. The literature also suggests that hot weather can be detrimental to burrowing species, such as the Common Club-tail. Insects can fail to undergo or complete ecdysis due to the drying of the mud, which encrusts the larval skin^{20, 25}.
  
  
• Habitat Loss: - The loss of woodland within easy reach of the river is considered to be a threat to the Common Club-tail and limits the suitability of riverine environments particularly as this species spends most of its adult life away from water and scattered bankside trees are usually⁷ present in the best sites. Therefore, the loss of woodlands not only reduces valuable shelter², but also has serious impacts because the secretive pairing of the adults actually takes place in the woodland glades.
  
  
• Habitat disturbance: - The trampling of large sections of river banks by sheep and cattle that sometimes occur in riverside meadows can adversely affect emergent vegetation and consequently have an impact on emergence sites for the Common Club-tail.
  
  
• Changing Hydrology: - Changes in hydrology which lead to lowering of the water table and drying of the habitat have serious implications for most species of dragonfly, particularly as it usually destroys suitable habitats and causes fragmentation. Over- abstraction of water may pose a serious threat, particularly as the Common Club-tail has a three-five year development period. In addition, during survey work Hinterman⁹, found no larvae in water less than 20cm deep, which suggests that excessive surface water abstraction from lowland rivers might be a concern for this species.
  
  
• Predation birds, ants:- Wagtails are birds that pick up opportunistically during peak emergence times. Bank voles may also take some emerging larvae.

Management

General Guidelines

• Survey Work: - Before any management is undertaken, surveys should be conducted to assess the species using the site and the impact that any management might have. This is particularly important where dredging operations are required. The Environment Agency has detailed records of the location of the Common Club-tail and considers these when river work is planned. The species is listed in the Local Biodiversity Action Plan in each area where it occurs⁷.
  
  
• Maintaining Inhabited Sites: - In localities where the species occurs and thrives, it is vitally important that the habitat does not deteriorate from its current level in relation to the outlined threats.
  
  
• Water quality: - This should not be allowed to deteriorate in habitats supporting the species, particularly in terms of nutrient enrichment, organic pollution and toxicity. Where water quality is considered to be a factor limiting the species, attempts should be made to improve the situation.
  
  
• Boating Activity: - Wherever possible, the intensity and speed of boating activity should be controlled to avoid extensive disruption to larval habitats. Zoning the activity and imposing speed limits are likely to be beneficial.
  
  
• Management of the Surrounding Landscape: - Although further research is required to ascertain the habitat requirements of the adult Common Club-tail, the fact that they spend most of their adult life away from water⁷ means that management of the surrounding landscape is important. A report on the Wise Use of Floodplains²⁶ suggests that only a complete river restoration of the entire lowland will see the return of those typical lowland river dragonfly species that would normally be expected, illustrating the need to consider the surrounding landscape.
  
  
• Monitoring: - It is important that once management has been carried out, whether specifically for the Common Club-tail or for general maintenance, monitoring should be undertaken to ascertain the degree of success of these measures and detect any changes in the populations, positive or negative, to provide feedback for future management efforts.
  
  
• Recording: - On the continent, the Common Club-tail is not confined to rivers, but also breeds in the static water of ponds and lakes^{4, 27}. In fact, in Switzerland it is more common on large lakes than rivers²⁸ and in France it survives in pools stocked with fish²⁷. Kemp²⁷ recorded a very teneral male, which had only recently emerged at Berrington Pool in Shropshire. Although there was no absolute proof of successful breeding at this site it raises the question of whether the species uses still water habitats in Britain. Further recording efforts are required to assess whether the species does breed in a wider variety of habitats than previously thought and therefore if habitat management should reflect this.
  
  NB: There is one record of a Gomphus having successfully emerged in Worcestershire on a non flood pool - and where the entrapped silt was quite red compared to the usual grey colour (M. Averill pers. comm.)
  
  
• Research: - Work is required to investigate the habitat requirements of the Common Club-tail in both the larval and adult stages. The species appears to have a complex ecology and research is needed to ascertain more exactly its habitat requirements to enable habitats to be managed appropriately²⁷. There is still much to learn about the activity of adults away from the water. We know very little about the elusive insects at this stage in their life cycle. There are also questions regarding the larval requirements and the impact that river management may have.

Best Management Practice

• River Engineering: - Any operations should provide areas of low current velocity in the channel. Within these, emergent and bankside vegetation should be encouraged and managed to ensure that a mosaic of habitats is retained. In rivers which support the Common Club-tail, engineering work should attempt to retain meanders, as the larvae appear to favour these areas. Furthermore, continuity of the preferred habitat is paramount and should be maintained. Any in river work should attempt to minimise silt movement and the use of silt mats can help in this respect.
  
  
• Management of Channel and Aquatic Vegetation: - Where dredging or weed cutting is required along rivers, this should be undertaken selectively to avoid extensive disruption to long stretches of the watercourse. Dredging must be undertaken with great care, as the larvae prefer muddy substrates, which increases the impact that dredging can have. Where possible dredging of meanders should be avoided, particularly as larvae appear to favour these areas where deposition of silty sediment is high. It may also be necessary to identify and consider areas of gravel substrate and any associated channels. These sites are important breeding areas for some species, specifically the Common Club-tail, which has suffered following dredging operations on the Thames, in the past (Moore, pers. com). Perhaps fortunately, very little dredging is carried out nowadays on the River Severn due to the lack of commercial craft (Averill, pers.com).
  
  Ideally, no more than one-third of the area of a given habitat should be cleared in any one year ensuring that there is a refuge for the larvae and emergent vegetation for the adults. The spoil from dredging and weed cutting should be left on the bank for several hours in order to allow evicted larvae to find their way back to the water. It should then be removed to ensure that runoff from plant decomposition does not feed directly into the watercourse.
  
  
• Scrub and Tree Management: - This should be done with care and consideration as Common Club-tails require woodland in riverside meadows or areas adjacent to inhabited stretches of the river. Excessive tree removal should be avoided around known emergence sites. A careful balance must be struck between adequate shrub/tree cover and its shading effect upon the water. Scrub and trees should be managed as discreet clumps and at no time should shading exceed 50 per cent of water between 11:00 and 16:00 hours during the period from May to July. Furthermore, it should be recognised that although the Common Club-tail requires woodland at a reasonable distance away from the river, where they will disperse immediately after emergence, this is not a requirement of all dragonfly species. A mosaic of habitats is the key to a diverse dragonfly fauna as it provides suitable areas for adults and a variety of niches for larvae.

Habitat Restoration and Recreation

• Habitat Restoration - In areas where suitable habitats exist and where conditions can be improved with respect to potential threats, the species should have the opportunity to extend its range. It seems likely that populations are already expanding in suitable areas. Therefore, habitat restoration may be a useful strategy in combination with better water management.
  
  
• Buffer strips are popular for general riverside habitat protection and this will benefit the Common Club-tail as well.

Case Study

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References

Management file updated on 17 April 2007