Tuesday, 10 September 2019

Willow Emerald twitch at East Park

When I posted the Hull Dragons August summary on the 6th of September, I noted that, regarding the Willow Emerald Damselfly "there is a possibility this rapidly expanding species may make it into Hull in the near future". But I was never expecting the near future to be as quick as 48 hours! @pondwatcher on Twitter:

How exciting was that!?
The Willow Emerald benefits from urbanisation, as it favours permanent garden and park ponds, surrounded by trees or bushes. Its is a late flying species, making the end of the dragonfly/damselfly season more exciting. It is the only Odonata species that oviposits into live wood, usually thin branches overhanging water, where eggs induce a diagnostic, gall-like reaction in the wood in a pattern of parallel lines.
 After a few sporadic records, the Willow Emerald became a regular breeding species in the UK in 2009, where many breeding colonies were discovered in Suffolk. Since then, it has steadily increased in range west and north, and this year it crossed the Humber for the first time.
 Today, there was a forecast of sunny spells and light WNW wind, and I decided to got on a damselfly twitch. I arrived at the park at 9:00 and walked to the eastern side of the lake, where the area around the boardwalk is favoured by dragonflies and damselflies. The first sunny spell took about an hour to arrive. When it did, Migrant Hawker males became active, with up to 5 males sharing the area, patrolling and resting over the large patch of marginal vegetation (above), a single female making a short appearance.
A female Common Darter (above) sat on the railings of the boardwalk, the first record of this species in the park this year. After walking up and down for a while searching for the Willow Emerald and with another large cloud looming, I moved onto the western side of the park to search for Small Red-eyed Damselflies. No luck, not a single damselfly on the west side of the main lake or boating lake.
 After a hot drink in the cafe I returned to the boardwalk. More searching of trees and marginal vegetation and walking up and down the boardwalk. The Migrant Hawkers were active so I watched them for a while. It was 12:20, the temperature quite pleasant in the sun, barely a breeze. Two male Common Darters were in attendance, chasing. After three hours in the park, I thought I had to content myself with a tandem pair of Common Darters, which were looking for an oviposition site. Maybe the Willow Emerald had succumbed to predation, of moved on. Another cloud was coming. I thought I'd stay for the next sunny spell. Then, a lovely, large sturdy green damselfly flitted about, checked the passing pair of hesitant darters in tandem, and sat on a leaf near the boardwalk: yes! the male Willow Emerald! It gave the impression of a sizeable insect, it is indeed as long or a bit longer than a common darter, and a stronger flyer than the common emerald. It sat on exposed leaves over the water, moving every now and then to another perch. It sat on alder leaves, on branched burr reed flower heads and leaves. I could take plenty of photos, as I watched it for about 20 min. A lovely damselfly tick!
This photo shows the 'spur' on the side of the thorax and the pale pterostigma with dark edges.
The pale appendages are also distinctive. No bluish pruinescence is apparent.

Willow Emerald males often sit on low branches of trees, overhanging water, which are the ovipositing sites chosen by females.

Friday, 6 September 2019

Hull Dragons: August summary

The dragonfly season has been in full swing, particularly during the very warm Bank Holiday weekend. Thirteen species and a total of 130 records from 23 km squares have been submitted so far to iRecord during August. Seven recorders have contributed to these records. During the last part of the month a number of species were still on the wing, although the records were dominated by Common Darters and Migrant Hawkers, two late, abundant and obvious species that also roam away from water. Both species add squares to the survey that don't necessarily hold breeding sites.
  Choosing highlights hasn't been easy, but these are some.
 Small Red-eyed Damselfly
To the thriving population at East Park we added Pickering park as a new site for the species, where many individuals and ovipositing was confirmed. A few other scattered records are evidence of the rapid range expansion of this species.
Migrant Hawker
A great year for Migrant Hawkers. They have been plentiful, with almost 50 records submitted to iRecord to 6th of September, compared to 18 last year. More excitingly, breeding evidence was obtained, with multiple patrolling males on several potential breeding sites with mating pairs at Pickering Park, Foredyke Stream, Beverley and Barmston Drain, and oviposition observed at Foredyke Stream.
Southern Hawker
A total of 22 records have been submitted for this species this year, compared to 4 last year. The Southern Hawker (top shot and above) is a recent colonist that has only been in the recording area since 2007, but is now well established, with records from 12 sites and evidence of breeding (oviposition and emergence) in several of them.
Male Black Darter near St Andrews Quay, 2012. Photo by Barry Warrington, used with permission.
Black Darter
A record of a male was submitted by Barry Warrington of this rare darter on the East side of the Yorkshire Wolds. This is a notable record as there are only a handful of records in the area, the first one from 2011 at Priory Fields, the second from the Beverley and Barmston Drain in 2013. Barry has found Black Darters in the same area, near St Andrews Quay, in 2012 (above) and 2016. Records of this species probably represent dispersing individuals, as the species breeds on boggy, moorland or heath ponds not present in our recording area.

List of species recorded in August
  1. Migrant Hawker.
  2. Southern Hawker.
  3. Brown Hawker.
  4. Common Darter
  5. Emperor Dragonfly.
  6. Common Blue
  7. Blue-tailed Damselfly.
  8. Small Red-eyed.
  9. Red-Eyed Damselfly.
  10. Azure Damselfly
  11. Ruddy Darter.
  12. Emerald. Foredyke Green.
  13. Black Darter.
In the wider area: Willow Emerald expansion
Emerald Damselfly. Mature male.
Willow Emerald male. Note lack of blue 'pruinescence', pale pterostigma with black margins and pale abdominal appendages. On side view a spur on the thorax is distinctive. Note eyes are not blue. 

Records of Willow Emerald Damselfly from Lincolnshire and a record from North Yorkshire means that there is a possibility this rapidly expanding species may make it into Hull in the near future. Fortunately, it is a late flying species, active until October, so it is worth while keeping an eye for it. The photo above was taken in a London park during the bank holiday weekend.

Tuesday, 3 September 2019

Migrant hawkers scramble competition

I'm used to watching migrant hawkers foraging over gardens, leafy streets and sheltered woodland rides, some times in groups. They are immature individuals, gathering energy away from water. Migrant Hawkers, unlike other hawkers, mature slowly, and will move to suitable breeding sites after their long immature period. During this past week I've watched them in their breeding sites in lakes and drains, where mating and egg-laying takes place.
A mature male rests briefly between bouts of searching (Pickering Park, 27th August) 

Males at the breeding site 
At one of my local parks (Pickering Park) last week, dozens of Migrant Hawkers sat or patrolled alongside marginal vegetation around the lake. The males, now fully mature and showing their bright blue spots and eyes and side yellow-green stripes often hovered in a spot, or explored the vegetation, flying well into it, searching for females.
A typical hovering male in a clearing at the marginal vegetation (Pickering Park, 27th August), offering them good views.
I saw no females, until one was captured by a male: no preliminary or courtship, the male just tackled her and positioned himself to grab her by the head. The female is then able to curve her abdomen and mate, retrieving sperm from the male's secondary genitalia at the base of the abdomen, forming the 'wheel position'. They may fly in wheel position very fast, zigzagging alongside the marginal vegetation edge or briefly rising into the air, before settling on vegetation (above).
Ovipositing female (Foredyke Stream, 1st September)
The moment when the male passes by, and sees the female.
The pair, mating.
A couple of days ago I watched a patrolling male on a ditch doing its usual patrolling routine, rising to inspect any passing individual, even paying attention - briefly rising - to birds flying over. A mated female arrived, unnoticed, and started laying eggs on live leaves well above the water line, I'd say over one metre over the water. She checked leaves and unsheathing her ovipositor, started laying into them (the eggs will overwinter inside the plant leaves, where they are protected from predation). After a couple of minutes, the male noticed her, tackled her and mating ensued. This time the pair settled briefly on plants, which allowed me to take a shot (above). Mating in Migrant Hawkers is longer than in other territorial relatives.
Two males resting near each other (Pickering Park, 27th August)
Nonterritorial males
This species is notoriously non-aggressive, even at the breeding sites. Males will even rest within view from each other (above). A patrolling male will swiftly rise to check a passing one, but the interaction is suggestive of them 'checking' that they are not a female, and letting the other individual go their way if it's a male. There is no defended territory, just males congregating on suitable ovipositing sites and searching for females, a type of mating tactic called 'scramble competition'. This appears to be the reason behind the long copulation. A territorial male mating for a long time may lose the territory to an invader, or miss extra mating opportunities. A nonterritorial male has less to lose, and therefore makes sure that he fertilises as many as possible of the females' eggs, possibly by taking time to remove any previous sperm before transferring his transfer. These patterns have been shown to stand when the copulation duration of territorial and nonterritorial drogonfly species were compared, but I haven't found specific data on the Migrant Hawker. To illustrate the pattern, the Emperor, a territorial species, copulates for an average of 10 min, while the Common Hawker, a non-territorial one, copulates for an average of 67 min. Of course, this lengthy copulation is not necessarily to the benefit of the female, who may be already mated, as the female already laying eggs, and therefore this sets the stage for the evolution of female counter-tactics, such as visiting the water as little as possible and avoiding males if they can, something I have covered before at Bugblog.

More information
C√≥rdoba-Aguilar, A., Serrano-Meneses, M. A. and Cordero-Rivera, A. Copulation Duration in Nonterritorial Odonate Species Lasts Longer than in Territorial Species. Ann. Entomol. Soc. Am. 102, 694–701 (2009).