Tuesday, 23 October 2018

Painted Ladies, the world travellers

I kneeled down and crept closer. The butterfly stood tiptoeing exposing its wings fully to the low afternoon sun, warming up. I was on the coastal path by the low cliff top at Spurn Head, a well known bird migration hotspot. This was my last Painted Lady of that year, 5th October 2014. The butterfly's faded colour and worn wings suggested that it was old and had been on the wing for a while, migrating. It still had a very long journey ahead.

Cosmopolitan and warmth loving
The Painted Lady is one of the most cosmopolitan butterflies, only absent from South America, Australia and the polar regions. It is cosmopolitan in more than one sense, as it is a constant traveller, moving in search of good caterpillar food sources and away of the cold. Unlike other butterflies, which have a cold-resistant stage - which can be the egg, the caterpillar or the adult - no life cycle stage of the Painted Lady can survive cold temperatures. In the UK, most sightings occur from May to September, with a strong peak in August.
This fresh individual from 25th July (Hornsea, East Yorkshire), feeding on buddleia, is likely to have been born locally.
A male Painted Lady hilltopping in the Swiss Alps (12th August 2015).

Through the desert and back
Painted Ladies migrate north in the spring to arrive as the summer starts in Europe, and they do the reverse flight in the autumn - just like many migratory birds - although the facts and routes of their migration have been only recently been pieced together. Unlike birds, migration is achieved in several generations - individual adults live for about four weeks - so it will be different individuals who carry out each leg of a migratory flight. Scientists used to think that the Northern European individuals overwintered around the Mediterranean and in North Africa, and that if any made it across the Sahara they might reach a dead end. Even a return journey from northern Europe was doubted, as migration is less obvious in the autumn, when the butterflies fly at high altitude to take advantage of tail winds, where they are out of sight. Research in the last few years has revealed that their migratory journey encompasses sub tropical Africa in a circuit of about 12,000 km. The migratory journey of the Painted Lady is actually the longest of any butterfly, with individuals being able to cover over 4000 km journeys, easily surpassing the Monarch! Gerard Talavera and Roger Vila, from the Institut de Biologia Evolutiva in Barcelona, travelled to four countries in the African Sahel (Senegal, Benin, Chad and Ethiopia), a band of savannah environments just south of the Sahara desert in October-November, just after the rainy season, with the vegetation is at its lushest. They visited tens of sites in each country in search of Painted Ladies. They found evidence of southbound migration, with worn individuals and directional movements, and local breeding. This is evidence that the European populations must actually cross the Sahara en masse during their migrations, to spend the winter months in tropical areas after the rainy season. Overall, they estimated that between six to ten generations are involved in the yearly cycle. 

Stable isotopes and the autumn migration
Is there an efficient return autumn migration in the Painted Lady? How can we find out? Scientists can fit migratory birds with miniature geolocators before migration, and these can be retrieved when the birds return to their breeding grounds, and the collected data allows to reconstruct their migratory routes and strategies. Unfortunately this is not feasible with small insects. There is, however, an alternative technique is at hand: to use stable isotopes. Stable isotopes, which have a lot of regional variation, leave permanent signatures of the natal origin of an insect in their bodies. The tissue development of a butterfly takes place in the caterpillar and pupal stage - often occurring in a single plant, and therefore reflects precisely the isotope composition of the particular place were the butterfly developed. If the adult emerging then migrates and is captured at its destination, or en route, the stable isotope composition of its wings provides a signature of where the butterfly developed. 
In a different study, Talavera and his multidisciplinary team collected specimens from around the Mediterranean from Morocco and Spain to Israel and Egypt just as they reappeared in early spring (February to April). They then analysed the stable hydrogen isotopes on samples of the butterfly wings. These were consistent with the presence of some locally born individuals but many others had a sub-Saharan origin, providing evidence for a return migration from the Sahel and subtropical Africa in spring.
Another likely migrant, this one a spring one, found in Flamborough head, 29th June 2015.

Next time you see one of these tattered Painted Ladies, spare a thought about the incredible distance that this small insect might have covered, and that it might have spend the last week or two flying across the Sahara, crossing the Mediterranean and moving through Europe in search of food for its next brood.

More information
The Vanessa cardui project. Here.

Talavera, G. & Vila, R. Discovery of mass migration and breeding of the painted lady butterfly Vanessa cardui in the Sub-Sahara: the Europe–Africa migration revisited. Biol. J. Linn. Soc. Lond. 120, 274–285 (2017).

Talavera, G., Bataille, C., Benyamini, D., Gascoigne-Pees, M. & Vila, R. Round-trip across the Sahara: Afrotropical Painted Lady butterflies recolonize the Mediterranean in early spring. Biol. Lett. 14, (2018).

Wednesday, 29 August 2018

Late August shieldbug gallery

While spending some time in the garden pruning trees, deadheading and generally pottering around I have come across several species of shieldbug. The top shot shows a Corizus hyoscyami, a species which is expanding north in range in the UK. I found it on the seed heads of a purple toadflax and I briefly moved it indoors for a white background photographic session.

This large and colourful bug is a Hawthorn shieldbug, Acanthosoma haemorrhoidale, which was feeding on a meadow crane's bill.
I'm not sure how I spotted this cryptic Sloe or Hairy Shieldbug, Dolycoris baccarum on a buddleia seedhead.
This forest shieldbug, Pentatoma rufipes, was crossing the path. 
Finally, a late instar nymph of a Green shieldbug, Palomena prasina, under a budleia leaf.

Click here for a great resource for the identification of British bugs (Hemiptera).

Tuesday, 31 July 2018

Egg laying Southern Hawker

On the way back from work I popped in my local wildlife garden. I was looking for dragonfly exuviae in the small pond when I gasped: A female Southern Hawker had materialised like out of nowhere and was hovering inches from my face. This is a large and inquisitive dragonfly, with a striking apple green, yellow and black pattern, which often approaches humans when they walk on their flight path. She took no notice of me and landed just by my feet, on the side of the pond-dipping platform. She was egg laying! I watched, mesmerised, how the tip of her abdomen appeared to direct proceedings, making searching movements on the pond lining, in a caterpillar-like fashion. After a few moments she flew low across the rushes growing in the pond, her wings clashing with the leaves. She landed nearby on the shore and started egg laying again. Taking her time, her abdomen appeared to 'look' for crevices amongst the mosses and litter, just a few centimetres over the pond surface.
 There was no male about. Hawkers oviposit on their own. 

Oviposition sites
Hawkers often lay their eggs endophytically, which means inserted into plants, mud or debris or wood on the pond margin, as opposed to exophytically, meaning dispersed on water. However this species appears to often chose odd places as substrate, and indeed, in the space of 10 minutes the female today had probed or laid in several places in the pond:
Probing on pond liner...
on mosses at the pond margin...
...on the decaying leaves of aquatic plants.
on the wooden panels of the pond-dipping platform.
 Having recently read the New Naturalist volume on Dragonflies, I wasn't too surprised. Philip Corbet and Stephen Brooks compiled a list of totally bizarre substrates chosen by Southern Hawker females to lay eggs. These included:
-A brown woolly jumper
-A wellington boot
-Someones ankle
-A dog's rump
-The skin of a Yellow-bellied toad

Corbet and Brooks remarked that in some of these cases the female might be just investigating the suitability of the particular substrate chosen, as opposed to ovipositing.

The Southern Hawker female hovered a few times by the pond and then moved on, hawking low along the garden path by the hedge a few times and exploring the bushes in the typical fashion of the Southern Hawker. This low flight is a behaviour that unfortunately, makes this species a frequent prey of cats.

More information
Klaas-Douwe B Dijkstra. 2006. Field Guide to the Dragonflies of Britain and Europe. Illustrated by Richard Lewington. British Wildlife Publishing. 320 pp.

Philip Corbet and Stephen Brooks. 2008. Dragonflies.  The New Naturalist Series. harper Collins Publishers, London. 454 pp.

Monday, 30 July 2018

Small Red-eyed Damselfly range expansion

This morning I watched several males of the Small Red-eyed damselfly at North Cave Wetlands, a YWT nature reserve with a diversity of large lakes, shallow reed bed lakes and dragonfly ponds (top shot). It was the first time I see this species in this site, where is now established. The Small Red-eyed is quite a distinctive damselfly, but it can be confused with its relative the Red-eyed Damselfly. The Small Red-eyed has an later flight season (end of July-August, although they do overlap), is smaller, has more blue in the tip of the abdomen and tends to sit with the abdomen curved upwards. Both species like to sit away from the shore, on floating aquatic plants, and individuals come back to the same spot after hunting. Given their habits and small size binoculars are a must to identify it!
Female Small Red-eyed Damselfly at Clubley's Scrapes (Spurn NNR, 21/07/18)
 The Small Red-eyed has been a British species for less than two decades. After a range expansion within Europe culminating with the colonisation of the Netherlands and Belgium, the species was first recorded in coastal sites in the south of the UK in 1999 and rapidly expanded north and west at a pace of 28 km per year. Today it is present up to North Yorkshire, but the rate of expansion is reducing. In 2006 it was first found in East Yorkshire, in a fishing lake in Hull, Oak Road Lake, and now it is present in several East Yorkshire sites.
 Colonisation can be associated with loss of genetic diversity, especially if the species in question is a poor disperser or population growth is slow after establishment. Given the speed and recency of colonisation of the Small Red-eyed Phillips Watts and colleagues investigated the genetic relationship and genetic diversity of nearby European populations and UK ones. It was presumed that the waves of migration came from NW France and Belgium, but there was another colonisation centre in the UK around the Isle of Wight, which hadn't expanded as much. Watts screened Small Red-eyed populations with 10 very variable molecular markers, similar to the markers used in forensics. The Isle of Wight population had less diversity than the remaining British populations, but the populations involved in the main expansion had a similar diversity to European populations investigated, with no evidence of population bottlenecks. This indicated that the waves of colonisation from the continent likely involved large numbers of individuals, and or colonising populations grew rapidly after establishment precluding losses of genetic diversity. Although it might appears surprising that this tiny insect can be capable of long distance dispersal and rapid colonisation, but migration is a increasingly acknowledged feature of many insects.
Small Red-eyed in flight.
Spot the tiny Small Red-eyed underneath an ovipositing pair of Common Darters and a Common Blue Damselfly.

More information
Watts, P. C., Keat, S. & Thompson, D. J. Patterns of spatial genetic structure and diversity at the onset of a rapid range expansion: colonisation of the UK by the small red-eyed damselfly Erythromma viridulum. Biol. Invasions 12, 3887–3903 (2010).

Tuesday, 17 July 2018

How do female dragonflies avoid male harassment?

After egg laying, female damselflies and dragonflies can be exposed to males trying to mate. Females will then be unreceptive and in order to avoid a lengthly and potentially costly copulation, they have evolved different strategies to avoid male harassment. These are only some of these strategies:

1) Avoiding water. This strategy appears to be quite general in the group. Males will return to the breeding ponds first after they mature and will be very obvious as they spend a lot of time patrolling the pond or sat on prominent perches, lake or river or hanging around near the water. In contrast, females often stay well away from the water, only returning to the breeding site to mate and oviposit. Staying away from the water allows females some control about when to mate.

2) Sneaking in. Females can be quite secretive when approaching an oviposition site, or they may lay during cloudy or cold weather - especially the larger species which can generate heat by whirring their flight muscles. Males are more likely to be roosting during dull weather or early in the morning, so females might be able to oviposit uninterrupted if they time their visit to the pond well.

3) Adopting an oviposition posture. If a male flies overhead, a female may try and repel him curving her abdomen down, like she was ovipositing. A great photo showing this behaviour in an Emperor female is here.

4) Looping the loop. Female dragonflies can fly faster than a chasing male, or do a loop the loop or even dive under water to avoid harassing males!

5) Androchromes. In many damselflies, such as the Blue-tailed and the Common Blue damselflies there is genetic variation in female colour with some of the colour forms strongly resembling males. I have covered this topic recently in Blue-tailed Damselflies.
A female Emperor with a very blue abdomen (16/07/2018). 

6) Age-related male mimicry? Females have an ability to store sperm and the sperm from a single mating should be enough to fertilise all her eggs for two weeks. In some species of dragonflies, females change as they age to resemble males (e.g. Emperor, Common Darter). The abdomen of mature female emperors is green, but it may turn blue - like a male's- when they are about 2 weeks old (it is unclear, however, it this is a purely age effect or a temperature response to warmer weather as the season progresses). If this was age-related, then male mimicry might reflect a different evolutionary response to the same selective factor than the genetic androchromes. When the female is young it is in her interest to attract males and mate, but as she ages she is likely to have already mated and the colour change might make it easier to avoid male attention. More research is definitely needed!

7) Playing dead. One of the most striking strategy of male avoidance is that of the Common (or Moorland) Hawker Aeshna cyanea (top shot). Ovipositing females often chose sheltered spots with denser vegetation in ponds to avoid male detection. As females leave ponds after ovipositing, males chase them. Rassim Kheliffa carefully documented that in such occasions, females dived into the tall grass surrounding the pond, staying motionless, often upside down, and so avoiding being grabbed by the males. The females were alert and responsive, and 87% (out of 31 attempts) where able to successfully avoid being picked by Rassim. He hypothesised that death feigning has evolved by females co-opting a pre-existing antipredator behaviour into a male avoidance strategy to avoid undesired mating attempts

More information
Corbet, P. S. The Life-History of the Emperor Dragonfly Anax imperator Leach (Odonata: Aeshnidae). J. Anim. Ecol. 26, 1–69 (1957).
Khelifa, R. Faking death to avoid male coercion: extreme sexual conflict resolution in a dragonfly. Ecology 98, 1724–1726 (2017).

Credit: Top photo of Common Hawker by Robert, used with permission.