The Green Meshweaver gets to Hull

 

Another post on a rapidly expanding species, this time a small spider, the Green Meshweaver, Nigma walckenaeri. All started on Saturday, during the YNU AGM in the University of Hull, where spider expert  Geoff Oxford, showed us a specimen he had just collected from the university grounds, a tiny green spider that is currently expanding its range across the UK and now colonising Yorkshire. We all admired the individual, which had been collected with its web on a leaf. 

The first Nigma walckenaeri record from Hull.

After returning from the meeting, I searched the garden ivy, and since I've been searching ivy and holly, which are favoured leaves to weave it's mesh. Nigma walckenaeri  chooses curled leaves, so it can take advantage of this to hide under its little web. As other regular character in this blog, Amaurobius, Nigma is a cribellate silk weaver. 

Today, I took a local walk and came across a magnificent mature ivy, south east facing. I started searching its leaves, wondering if I had the wrong search image in my mind. It didn't take long to find a Nigma.

Nigma walckenaeri  habitat.

There it was! A light-touch web of blueish threads with the green spider sitting, well camouflaged, underneath.

I moved part of the web aside to have a closer look (also top shot). This is a female, with her whole body green. Once I had seen one, I found another, and another, almost every ivy leaf had it's little Nigma in it. 

A male, with its brown cephalothorax.

This individual was busy weaving its web.

I even spotted a male (with reddish-brown opistosoma) on the edge of a female's web.

Until 1993 it was only found in London and the home counties, and it is associated to parks and gardens. Since then it has been steadily spreading north. I'll keep a look for it in the garden!

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:

East Park Hull today: the best was single Willow Emerald in reeds (possibly 1st VC 61 record). Also Migrant Hawkers in cop high up in Willow & in reeds plus egg laying in reeds. Odd that no Darters seen. @aBugBlog @BDSYorkshire pic.twitter.com/7OULKL4diG

— Pond Watcher (@WatcherPond) September 8, 2019

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.

The Migrant Hawker unusual life history

Migrant Hawkers have been around since the beginning of August, and they should be around until October or early November. They are often seen flying away from water. They hawk at about 3 m high, patrolling an area repeatedly, occasionally rising to catch an insect, their abdomen held at an upward angle, with a slight droop at the end. 

Migrant Hawker in flight.

Their late summer appearance and social, non-aggressive behaviour are unlike other hawker dragonflies in the UK. When they hunt, it is not unusual to see several individuals together, either ignoring each other or not overtly aggressive. Occasionally they form large hunting aggregations in favoured habitats. They also often rest near each other in pairs or small groups in sheltered glades or hedgerows after hunting or during overcast conditions, hanging from branches orienting to the sun (top shot, a female in the foreground, with a male on the background). 

The reasons behind their distinctive life history lie in their ecology. Migrant Hawkers have a widespread distribution, from the Mediterranean to Asia to Japan. In the south of their range they often develop in temporary ponds, which recurrently dry in the summer. For this reason they have a very fast life cycle: after laying in the autumn, eggs undergo a period of winter rest (diapause). Larvae hatch in late winter and develop through spring, before metamorphosing and emerging as adults in late summer. The whole cycle lasts a single year, unlike other dragonflies that take at least two years to develop. When the adults emerge, the ponds where they developed quickly dry, so the adults do not mature immediately. They will hunt away from water for a while - up to 4 months in the south of their range in Algeria - and only return to ponds and mature when autumn rains may have fill the ponds, therefore they have one of the longest adult lifespans in dragonflies. During their immature stage they are great wanderers and migrate in both latitude and altitude to new areas where new ponds can be found or weather conditions more benign. In the summer, when they are away from water, immature migrant hawkers have no reason to be aggressive to others, there is no territorial behaviour or competition for receptive females, hence the congregations in feeding swarms on the plentiful insects. 

Male Migrant Hawker.

 Migrant Hawkers used to be rare in the UK before the 1940s, but they are now widespread in England and Wales, and also found in Scotland. In the early 2000 they colonised Ireland together with two other dragonfly species. It is likely that increasing temperatures are allowing this species to complete its cycle in the UK, where previously it was just a migrant species and their range has spread north as most other British dragonflies.

More information
Hickling, Rachael, et al. A northward shift of range margins in British Odonata. Global Change Biology 11.3 (2005): 502-506.

Samraoui, B., Bouzid, S., Boulahbal, R., & Corbet, P. S. (1998). Postponed reproductive maturation in upland refuges maintains life-cycle continuity during the hot, dry season in Algerian dragonflies (Anisoptera). International Journal of Odonatology, 1(2), 119-135.

A new hornet-mimic hoverfly

 I found this large, colourful hoverfly feeding on Yarrow (Achilea millefolium) in the garden this evening. It is a male Volucella inanis, a hornet mimic which is also found in nests of wasps and hornets, where its larvae develop. Spot on, its numbers peak at the beginning of August. The entry in Stubbs and Falk's British Hoverflies notes that its distribution range is expanding north, and being the first time I come across this species, I checked the NBN Gateway: no records for East Yorkshire or anywhere north of the Humber. Another species expanding its range due to climate change? I should start keeping a list.

UPDATE 4/08/2013. Barry Warrington e-mailed me with some more info about the species in East Yorskshire: "V. inanis is scarce in our County. I recently found one a couple of weeks back and having liaised with the YNU Diptera recorder and checked the Watsonian Checklist, my find was the first for East Yorkshire. It is clearly spreading well and quickly, with it being most common in VC63."

Bug heads

Dead heading is one of the rules of gardening, isn't it? You are told to remove the faded flowers so that seeds don't form, as they extract lots of energy from the plant. But what will you miss? you want to look at seedheads. In a walk around my local wildlife garden I found that meadow cranesbill's seedheads were busy with shieldbugs of several species, feeding on them. Among the bugs there were some striking black and red ones (above), which I had never seen before, they were very flighty and the light conditions were not right, but I managed some shots. Upon looking into the British Bugs site I identified them as Corizus hyoscyami a species that during the last decade has expanded throughout England from a few coastal locations in Wales, another species that, according to the NBN gateway, wasn't known to have crossed the Humber before.

Corizus hyoscyami feeding on developing cranesbill seeds

Nymph Green Shieldbug, Palomena prasina

Rhopalus subrufus, one of the most abundant today

The first Green Shieldbug Palomena prasina adult of the year

Likely Corizus hyoscyami nymph. Thank you to Dave who identified it in a comment.

Dolycoris baccarum Hairy Shieldbug

This dark Forest Bug, Pentatoma rufipes, had a ride home on my trousers 

I must remember to leave my geranium seedheads for next year, as true bugs (hemiptera) are one of my favourite insects. One job less to do, and plenty of opportunities to enjoy bugs.

Winners of climate change?

The words 'climate change' tend to have negative, almost apocalyptic, connotations, we struggle to think on its positive effects (unless you live in the UK and you dream on Mediterranean weather to come, of course). Some species, however, thrive on warmer termeratures, so we could expect their distributions to be climate-limited and to increase their range north with increasing temperatures. This is true for many butterflies in the UK. In the first half of the twentieth century, many butterflies were restricted to the southern corner of the UK, whereas nowadays some are steadidly expanding into the north. Expanding butterflies include charismatic garden species such as the Comma, Polygonia c-album, the Speckled Wood, Pararge aegeria and the Holly Blue, Celastrina argiolus. Before the eighties they were absent from East Yorkshire and in the nineties these species become common north of the Humber. All good then? No, there is a problem: for a species to expand its range, suitable habitat needs to be present nearby - within the usual flying range of the species. Throughout the last century, with agriculture intensification, there was widespread loss of wild habitats in the UK. The combined effect of climate change and habitat loss was investigated in depth in a paper by Warren and co-workers. They used a dataset of 46 non-migrating butterfly species with northermost European ranges within the UK and detailed distribution records compiled in the last 40 years. Although all these species seem to develop faster and have denser populations in warmer temperatures (within UK limits) only a quarter of the species actually increased their range as predicted. Species increasing their range tended to be mobile habitat generalists: the species that are more likely to use gardens and other human-related habitats. Most habitat specialists, which tend to be more sedentary declined. This study illustrates the interplay of climate change and habitat availability: in our highly fragmented landscape, generalist, highly mobile species will be the winners of climate change, and we should be able to enjoy them more widely in our northern gardens, but biodiversity as a whole will, sadly, decline.

Comma

Speckled Wood

Holly Blue

Ringlet

Reference

Warren MS, Hill JK, Thomas JA, Asher J, Fox R, Huntley B, Roy DB, Telfer MG, Jeffcoate S, Harding P, Jeffcoate G, Willis SG, Greatorex-Davies JN, Moss D, & Thomas CD (2001). Rapid responses of British butterflies to opposing forces of climate and habitat change. Nature, 414 (6859), 65-9 PMID: 11689943