Tuesday 10 October 2023

Ivy bees in Hull

 I have spent some time watching mature Ivies lately. It is Ivy peak flowering season, and the couple last days have been warm and sunny, bringing the insects out, the ivies humming with insect activity. Droneflies, Red Admirals, Comma, Wasps, honeybees are attracted to flowering bees. I was actually looking for Green Mesh Weavers, on an Ivy in an untarmaced tenfoot (alleys between houses) when I noticed two stripy bees rummaging around the leaf litter at the bottom of the ivy. I couldn't believe they were Ivy Bees, Colletes hederae! I have previously seen Ivy Bees in east Yorkshire, at North Ferriby and at Flamborough, but I thought we wouldn't get them in Hull due to our clay soils, as this bee needs loose soils for nesting. 

Two bees exploring the soil under the ivy, this is probably loose enough and sunny enough for a nest site?

A winner from climate change

Ivy Bees started colonising the UK from 2001, after expanding in northern Europe, and in the last couple of decades its distribution range has rapidly expanded northwards, with the first Scottish records coming in 2021. The mail pollen source for its larvae is Ivy, and the bee flight period coincides with the flowering season of ivy, from September to early November, with a single brood. They are solitary bees but they tend to nest together, sometimes forming large nesting aggregations in suitable habitat. Females will excavate a nest and line the walls with a cellophane-like substance, which explains another name of the bee, Cellophane Ivy Bee. Ivy pollen is brought to the nest and an egg is laid atop a mound of pollen, before the cell is sealed and another load of pollen is collected for the next egg.

One of the Ivy Bees near the ground after some exploring.
A female laden with pollen having a rest to clean its tongue.
A foraging female showing the banded abdomen. This bee is relatively large, the size of a honey bee, with orange hairs on the thorax, and contrasting broad buff bands in the abdomen, unlike its close relative the Sea Aster bee, which is smaller and has a white banded abdomen. Habitat, timing and foraging flowers can also help distinguish these bees.

Tuesday 18 October 2022

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, AmaurobiusNigma 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!

Thursday 4 August 2022

Urban Purple Hairstreaks

I had a most unexpected and delightful walk to work this morning. As I was about to cross Westbourne Avenue, where a water leak has been repair, a passer-by flushed a small butterfly from the barriers. I thought it was probably a Holly Blue, but when it settled with wings closed on a privet hedge nearby, I realised it was a hairstreak! I fumbled for my camera as it fluttered along the hedge into a front garden, where it sat with wings open. With shaking hands, I was able to take a photo (below), confirming the id as Purple Hairstreak! 

The first Purple Hairstreak

The Avenues is a very leafy part of Hull, with large gardens often holding large trees. In fact, there is a mature, spectacular oak at the back of the Adult Education Centre on Park Avenue not far away, and, as I walked past, I wondered if the wind might have blown the small butterfly away. I carried onto Jack Kaye fields. A Speckled Wood by the entrance to the fields, by an oak, reminded me to do a butterfly count, and I set an alarm on my phone to 15 min. I counted six Speckled Woods, as they squabbled around the trees. Then I moved onto the first glade. A small butterfly settled on a Whitebeam to bask in the sunshine. As I focused my camera, I got the feeling of being in a dream: it was another Purple Hairstreak! It has its wings closed and I took plenty of photos. 

Tree canopy butterflies

Purple Hairstreaks are canopy specialists, they spend much of their time on tree tops, around oaks (the larval food plant) and nearby trees, where they feed on honeydew. Although other species that obtain most of their food from honeydew, like Speckled Wood and Holly Blue, can occasionally be found feeding on flowers, or resting or fluttering low down, Purple Hairstreaks rarely come down to the lower level of trees, but they have been seen feeding on bramble or hemp-agrimony. They live up in the tree tops, and it is for this reason that they are easily missed. There are only 18 records of this species in East Yorkshire, mostly in nature reserves, but also at Snuff Mill Lane, just outside Hull, where I saw them in July and in Beverley Westwood, which has a sizeable population. They are not only found in woodland, but also along lanes and in parks with oaks. But, as these lovely, tiny butterflies, spend their adult lives up in tree canopies gorging on honeydew, they are likely to be overlooked and under-recorded. In the last few weeks I've been looking for them at Jack Kaye, staring into that small oak by the entrance, so although I was beyond pleased to find it, I was a bit astounded that it had actually happened. 

I will keep an eye on this area to check if these are isolated individuals or if there is a colony, and I plan to keep looking at oaks, wherever they are!

Thursday 12 August 2021

Do Migrant Hawkers migrate in groups?

This summer it appears to be a bumper year for Migrant Hawkers. Their sudden appearance in several places with several individuals present, and the huge numbers seen around makes me think that these are migrants, rather than locally bred individuals. Last week I took a clip of a large group feeding by a nearby wood, a photo would not have conveyed the feeling of watching these dragonflies hunting in groups, paying little notice to one another, in contrast to the two feisty Brown Hawkers in attendance, who squabbled every time they met.

A sunny spell earlier today brought out at least 10 individuals over the garden, the largest number I've had. They are not aggressive or territorial, but, is there more to it? When roosting, they actually appear to choose to rest near other individuals, despite an abundance of sites nearby with a similar aspect, even when breeding (top shot, two mature males basking close to one another at Hornsea Mere, 24 September 2018. 

The following, much more spectacular image was shared on twitter by @jwood_t on the 2nd of August shows 12 Migrant hawkers roosting side by side:

Given that they appear to be a sociaI species, feeding in groups and roosting close to each other, I wonder if they are attracted to one another during migration. Do they actively seek each other, migrating in a compact flock? And why are they social, is there an antipredator advantage, or a better timing and orientation of migration, a quorum decision on directions, or even an energetic advantage, when migrating in groups?

Migration in groups would not be a new phenomenon in dragonflies. A striking example is this amazing photo of migrating dragonflies shared by Dave Smallshire:

The Migrant Hawker is indeed known for migrating in large swarms - hence its name - but little experimental research has been carried out on this species. Recent research conducted at a bird observatory on the Baltic coast in Latvia starts to address this gap. Aline Knoblauch and collaborators took advantage of dragonfly 'by-catch' in Heligoland traps during a few weeks in August and September. These large funnel-like traps covered on wire-netting, widely used to investigate bird migration, also capture dragonflies, unharmed. The researchers also used field 'orientation experiments, releasing freshly caught dragonflies into a closed round arena made of mesh and analysing recorded videos to determined if they had a preferred flight direction. All dragonflies were released after the short trials. Their results showed that individuals captured during autumn migration orient themselves to a southerly flight direction irrespective of the prevailing winds, even though there were more captures in the Heligoland trap when the prevailing wind were northerly wind, indicating that the dragonflies were choosing to fly when wind direction was favourable. These results indicate true migration, rather than random accumulations of feeding individuals in suitable areas. This experiment, however, doesn't answer the question of orientation to other individuals.
An observational study on the related species, Southern Migrant Hawker, Aeshna affinis, provides some intriguing clues about group migration, likely to apply to Migrant Hawkers as well: 
"In the late afternoon, thousands of immature individuals were flying above the swamp, mainly at a height of 5-10 m. They formed a huge compact bubble, which was spatially clearly defined on the outside, but with unorganised bee-like swarming inside. Individuals showed a distinctly jerking and dancing flying style, with a minimum space kept between individuals."
This indicates not just agglomeration due to being in the same area with the same intent of migrating, but active flock formation behaviour. The insects also appeared to follow geographical features, a river, to migrate. 
Another interesting feature of Migrant Hawker migration has been noted in a study recording the numbers of Migrant Hawkers migrating on the Danube delta. A strong male-bias of 2 males to 1 female was found, which was also noted in the first study. Although sex-biased dispersal and migration is well known in many animals, it is unclear if this is due to different timing of migration of males and males, or to different geographical or altitudinal pattern of migration of sexes. Note that observational studies are limited to dragonflies flying low, and Heligoland traps only capture dragonflies flying just above ground (within about 3 m).
We are starting to understand Migrant Hawker migration, but there is still a lot of questions to answer.

More information

Knoblauch, A., Thoma, M. & Menz, M. H. M. Autumn southward migration of dragonflies along the Baltic coast and the influence of weather on flight behaviour. Anim. Behav. 176, 99–109 (2021).

Schröter, A. A mass migration of Aeshna affinis in southern Kyrgyzstan: attempt to provide a spatial and temporal reconstruction (Odonata: Aeshnidae). Libellula 30, 203–232 (2011)

Dyatlova, E. S. & Kalkman, V. J. Massive migration of Aeshna mixta and Sympetrum meridionale in the Ukrainian Danube delta (Odonata-Anisoptera: Aeschnidae, libellulidae). Entomol Bericht 68, 188–190 (2008).

Saturday 31 October 2020

Budapest slug mating

It's the last day of October: damp, windy and mild. As I go out into the garden, pondering upon the impending winter lockdown, I find two Budapest slugs circling on a cherry leaf on the ground. Budapest slugs remind me of hedgehog poo, dark and shiny and all the right shape. If they had been on the path pebbles I would have easily missed them. I watch them for a few minutes. They keep slowly circling, head-to-tail for a while as they follow each other's mucus trails. This is slug courtship and it happens at slug pace. It's 10:40 and it doesn't look like much is happening any time soon. I regularly go out to check on them for the rest of morning and afternoon. As I'm typing this, I grab a torch and decide to check on them again. They are still there, exactly in the same position as 3 hours ago. 

11:06. The pair have moved onto the path pebbles now, the circling has stopped. They are now finding each others genital openings at the right side of their heads.
11:20. Copulation proper appears to start.
17:20. Very little change in the last few hours. The slugs are now entwined and practically immobile.
I pick the Slugs of Britain and Ireland and check the species account for info on their mating behaviour. Of note is that they are mainly subterranean and active year round.
This quote is fitting: 
"in winter adults are often found almost motionless, mating. Mating lasts many hours and involves the production of elaborate spermatophores"
I wonder if they'll be there tomorrow.

More information
Rowson, Ben, James Turner, Roy Andreson and Bill Symondson Slugs of Britain and Ireland. 2014. FSC Publications. AIDGAP.