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:

My final offering for #worldodonataweekend @Britnatureguide @BDSdragonflies: a unique image taken high over Gomatong Cave, Borneo showing a world first - 2 of about 10 migrating flocks of dragonflies! These can be identified as Rhyothemis obsolescens from their wing pattern. pic.twitter.com/zLlUez9jEV

— Dave Smallshire (@smallshire) May 10, 2020

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).

Emperor home ranges, habitat use and differences between sexes

Emperors (Anax imperator) have done very well this year in my local area, which has allowed me to become familiar with the different behaviours of males and females. Females are quite stealthy, entering ponds to oviposit on floating vegetation, often flying low over the water. In contrast, males display an obvious territorial behaviour, patrolling high over the water of the large pond, lake or slow running drains where they breed. Males move towards passing birds, and towards any intruders, with spectacular fights and clashes between the males, some times resulting on individuals falling on the water.

Ovipositing female emperor.

A male patrolling alongside the marginal vegetation of a large ditch.

All this means that we are biased in our understanding to these more obvious behaviours, ovipositing and territoriality. But many other questions remain about Emperor's behaviour and ecology, for example, where are females when they are not ovipositing? or males, when they are not patrolling a pond? how far do individuals roam or disperse? where do they roost? how is their behaviour affected by temperature? do they make use of the habitats around ponds? what about sex differences?

Capture-mark-resighting techniques have be used in dragonflies to study individual movements, behaviour or demographic parameters. Individuals of large dragonflies can be marked using alpha-numeric unique wing codes written with permanent markers, which could be read from a distance, with no need from physical recapture (for example using binoculars). This technique, however, requires an enormous field effort to relocate as many individuals as possible.

Another technique that has been used to study migration is to analyse location-specific isotopes in wing samples. These isotopes have signatures specific to geographic areas, which point at the area where the larval stage took place (as the wing tissue is formed during the larval stage). This method has been used to reveal the multi-generational migrations of the Green Darner (Anax junius) a North American relative of the Emperor that is a long-distance migrant. Hydrogen stable isotopes showed that the migration cycle comprises a north migrating generation, a south migrating generation and a resident generation that develops around the Gulf of Mexico. Another study using stable isotopes on the Global Skimmer, Pantala flavescens, revealed its multigenerational migration steps around the Indian Ocean. Although ranging from South Africa to Sweden, and still involved in natural colonisation towards the north, facilitated by global heating, the Emperor is a resident species, so this technique is not of much use.

Radio transmitters in dragonflies?

As technologies have resulted in the miniaturisation of radio-transmitters  in recent years, they have increasingly been used to study more local movement patterns in large dragonflies, like the North American Tiger Spiketail (Cordulegaster erronea) a relative of our Golden-ringed Dragonfly and Green Darners. The individuals need to be captured and fitted with tiny transmitter before release, and they have to be found in the landscape using scanning receivers fitted with an antenna. Size matters as the transmitter must be less than 30% of the weight of the dragonfly not to impede normal behaviour. The Emperor, one of our largest dragonflies, weighs about 1 g, and can be fitted with such small transmitters (check the photo here of an individual fitted with a transmitter). In an early small scale study (5 tagged individuals), researchers looked at home ranges and local movements between roosting sites and pond territories in male Emperors. The furthest moving male travelled 1.5 km from the tagging pond to another pond.

An article published recently sheds some light on home ranges and habitat use of Emperors using radio-transmitters. Marceau Minot and his collaborators chose five ponds in an urban/rural interface in northern France. Over the summers of 2017 and 2018 they marked 87 mature emperors with unique wing codes and visited the ponds at least once weekly to search for marked individuals. They also fitted 54 individuals with radio transmitters, and tracked them daily for up to 15 days.

Capturing individuals to fit radio transmitters or mark their wings has a cost. Both techniques increase mortality in the day after capture, probably due to the stress of the capture. 

Sex differences in behaviour

Females had larger home ranges than males. The furthest distance a female travelled was 1.9 km while males moved less, with the maximum male movement 0.5 km. This could be related to male's territorial behaviour. Presumably females oviposit in several ponds.

Reproductive behaviour of males, but not females, is positively related to temperature.

Flying behaviour in females is positively related to temperature, while resting high in trees is negatively related to temperature.

Both sexes were mostly found on ponds or pond marginal areas, although males spend more time near water.

Resting happened in ponds and trees. Females tend to roost high on trees, more than males. While males tend to rest low in vegetation.

A male emperor resting on marginal vegetation.

The researchers estimated the effect of marking protocol on survival of the dragonflies. The manipulation of the individuals affected their survival, possibly due to the stress during manipulation: 76% of individuals survived 24 after capture and wing marking, while just 56% survived 24h after being fitted with a radio-transmitter. Individuals with proportionally larger wings (not larger body mass) and younger in age survived better throughout the experiment.

Habitat management

The study also highlighted that both rural and urban ponds will benefit from the presence of neighbouring trees as suitable roosting sites, and emperors will benefit from the presence of a network of ponds, rather than isolated ponds.

More information

Hallworth, M. T., Marra, P. P., McFarland, K. P., Zahendra, S. & Studds, C. E. Tracking dragons: stable isotopes reveal the annual cycle of a long-distance migratory insect. Biol. Lett. (2018).

Hobson, K. A., Anderson, R. C., Soto, D. X. & Wassenaar, L. I. Isotopic evidence that dragonflies (Pantala flavescens) migrating through the Maldives come from the northern Indian subcontinent. PLoS One 7, e52594 (2012).

Knight, S. M., Pitman, G. M., Flockhart, D. T. T. & Norris, D. R. Radio-tracking reveals how wind and temperature influence the pace of daytime insect migration. Biol. Lett. 15, 20190327 (2019).

Levett, S. & Walls, S. Tracking the elusive life of the Emperor Dragonfly Anax imperator Leach. 27, 59–68 (2011).

Minot, M., Besnard, A. & Husté, A. Habitat use and movements of a large dragonfly (Odonata: Anax imperator) in a pond network. Freshw. Biol. 46, 207 (2020)

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).

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).

Marmalade flies everywhere!

Marmalade flies, Episyrphus balteatus, are the most common British hoverfly, and can be seen year round as they are able to overwinter as adults. However, at this time of the year, they are at their most abundant, as I walk around the garden, every flowers seems to have a few males hovering nearby or females feeding on the flowers. These slim, small hoverfly is able to migrate, and this occasionally bumps the numbers of British individuals. They are very distinctive, as the pattern of double black lines in the abdomen is unique to this species. The male above (eyes meet at the top of the head in many male hoverflies), settled briefly on an Agapanthus flower bud, and rubbed its feet together, keeping its wing open for a few seconds.

A surprise migrant moth

The afternoon was sunny and pleasant as me and my daughter were walking home from school. We had stopped to watch a tight cluster of garden spiderlings on a hedge, when, from the other side of the street I spotted what I thought was a butterfly moving to feed on a large clump of Red Valerian in the full sun. I kept watching and then I realised it was a Hummingbird Hawkmoth! We crossed the street, let some people walk pass and watched the beautiful moth as it hovered from tiny flower to tiny flower, its long tongue outstretched, sucking the nectar. The light was lovely and the background hard to beat.
 The Hummingbird Hawkmoth is a migrant from southern regions, it overwinters as an adult and it wouldn't withstand British winters. It tends to appear in times of warm weather. I found it a bit surprising, given how cool this spring is being, but maybe migration may happen in response to warm weather in the areas where they come from, and it's been very warm around the Mediterranean. If you see one you can report it to Butterfly Conservation Migrant watch, where you can also see an interactive map with all sightings submitted.
What a treat this was for day 11 of #30dayswild.

Angle Shades moth

This Angle Shades moth, Phlogophora meticulosa, caught the attention of my 8 yr old while it rested on dark moss. Angle Shades are very distinctive moths, as they rest with their wings folded lengthways, but the disruptive wing colour pattern and their crumpled appearance makes them difficult to spot when they rest on dry leaves. I moved it to the fallen leaves of Virginia Creeper, which match the moth's subtle colours, to take its portrait. The species belongs to a group of large noctuid moths - which include the Lesser Yellow Underwing and the silver Y - that are regular migrants to the UK from continental Europe, although the Angle Shades is able to overwinter here as well.

On insect migration and fairy tales

When it comes to invertebrates, very odd ideas can circulate even in scientific circles. One of such is called 'The Pied Piper' effect, and states that many insects migrating to northern latitudes in the spring do it hopelessly, as their offspring will be incapable of making the return trip to the region of origin, where they would be able to successfully overwinter: a one way migration trip. Of course this makes no evolutionary sense. If in a population some individuals were genetically predisposed to migrate north each spring - while the rest stayed put - and the offspring of such migrants had no chance to make the return trip successfully, natural selection would select against migratory behaviour: migrants will quickly be purged out of the population, while the offspring of sedentary individuals would be more successful and propagate their sedentary tendencies resulting in a non-migratory species.
  No matter how ridiculous this idea might seem to us, it has persisted for decades, in a similar way that Aristotle believed that swallows and other birds hibernated in the mud at the bottom of ponds or that Redstarts turned into Robins in the winter. An enormous body of research has confirmed that birds are capable of migrating back and forth to distant regions and have a magnetic compass that allows them to navigate even in adverse meteorological conditions. Bird migration is now widely accepted. Could it be that our prejudices with invertebrates made us find impossible to believe that some of them also could be able of regular back-and-forth seasonal migration? It appears so. But an online early paper by Jason Chapman and co-workers, from Rothamsted Research Station demonstrates that migration in an insect is an adaptive behaviour. Migrating insects use resources that are geographically and seasonally partitioned: in the summer they are available in more northern latitudes than in the winter and a two way migration is the most profitable way to use these resources. They collected data on the Silver Y, a common noctuid moth which is known to migrate from the Mediterranean to temperate latitudes in northern Europe. This species breeds continually in up to five generations a year and cannot survive the winters of northern Europe. The very active adults (above, adults often vibrate their wings when resting and when feeding) feed on nectar-rich plants and their caterpillars grow on a range of plants, including crops, where they can become a pest.
Building on a large body of previous work of the group on insect migration, some of it I already covered in BugBlog, these researchers analysed long term moth trap monitoring data at a national scale and entomological radar data. Both sets of data agree beautifully, complementing each other. The authors estimate that between 10 and 240 million Silver Y moths migrate annually to the UK, There strong inter annual variation in numbers and three years between 2000 and 2009 with exceptionally large immigration events in 2000, 2003 and 2006, likely due to benign ecological conditions in their wintering grounds. Their results provide solid evidence of the astounding population increases - and therefore potential reproductive benefits - experienced by migrating Silver Y moths in their northern breeding grounds. As the summer progresses, adult populations increase on average four-fold and for each moth that arrives in the spring, three will start the autumn return flight. There is a clear two-way movement: northern in the spring and southern in the autumn, with the autumn migration being of larger magnitude. But are they able to migrate all the way back? Simulations of expected moth return flights using flight and wind speeds and orientation of flying moths detected using the radar support the hypothesis that most of these moths could reach to their wintering grounds in the Mediterranean successfully. The Silver Y now joins the Painted Lady and the Monarch butterfly as models of insect migration, banning the 'Pied Piper effect' to the realm of fairy tales.

More information 
Chapman JW, Bell JR, Burgin LE, Reynolds DR, Pettersson LB, Hill JK, Bonsall MB, & Thomas JA (2012). Seasonal migration to high latitudes results in major reproductive benefits in an insect. Proceedings of the National Academy of Sciences of the United States of America PMID: 22927392

Migrant Common Darters

At this time of the year, my garden - which hasn't got a pond - hosts occasional dragonflies as they migrate to warmer climates. The most regular one is the Migrant Hawker, which often hunts in small parties 2-3 m high, occasionally stopping to rest hanging from branches. Rarer species are the Darters. In the last few days, I have watched female Common Darters, Sympetrum striolatum, resting and hunting. The first day it banked atop my parasol and I couldn't get good shots. Today I was luckier and the darter sat on the fence. With the help of my new Canon Powershot G12 flip out LCD display, and perched on a chair, I could hold the camera high enough to get its portrait.

This species, as other darters, is known to migrate regularly. David and Elizabeth Lack reported their pass south in the Pyrenees together with migratory birds, butterflies and hoverflies in October 1950 and reviewed the information available on dragonfly migration until then. Today, although the morning was warm with sunny spells, a large front with thunderstorms passed through all the afternoon, so it could be that the dragonflies were moving away from the coming rain. 

Common Darters are known to form occasional irruptive, massive migratory swarms, although the reason for these are not well known. A large irruptive swarm of this species was reported in Ireland in August and September 1947 by Cynthia Longfield from the British Museum. She compiled the letters of many observers amazed by the sudden, unprecedented invasion, including interviews with schoolchildren, which helped document the spatial extent of the invasion. As, she was visiting Ireland at the time, she identified the dragonflies and further documented the invasion with her observations. She concluded that this mass invasion event probably came from Spain and Portugal, as no invasion was reported in France or England at the time. She calculated that the dragonflies must have flown over the sea overnight. It is hard to imagine what being there at such massive influx event would have been:

'They were flying low in compact formation. . . . The flight was orderly, in a northern direction, as if they were intent on reaching some known destination.'
'They seemed more to be drifting with the breeze than flying, but were keeping to a straight course, without zigzagging or undulating, coming in from-the sea to the south and going north. What was so remarkable was the witnessing of the actual arrival and passing of the swarm.'

'They seemed to be coming in from the sea and flying about two feet above the ground, though some were higher, to about eight feet up. All were flying in the same direction, northwards. It seemed a never-ending stream, but we had to move on, and passed out of their track, so I could not tell the duration of the flight. They flew considerably faster than we were walking. The density was about one foot, or rather less, apart.'

For the next day, 3rd September, Mr. Allen writes: 'Tuesday's invasion was nothing to Wednesday. Wind south-west. Before noon they commenced and continued until evening. They not only came singly, drifting as on Tuesday, but in great, thick black masses, like dark clouds. They really frightened the people.'

Here he found 'the people looking out to sea, and I saw a sight which gave me a fright at first. I heard a hum like bees and yet not quite like bees. The next thing the whole sky filled with large flies that resembled aeroplanes, millions of them'

 Cynthia hypothesizes that the reason these dragonflies were emigrating was the drying up of the marshes where they regularly bred. That these dragonflies somehow were prevented from completing their reproductive cycle in the region of origin is illustrated by the fact that many of them paired on arrival, and some were even observed laying eggs on the sea. Although these spectacular irruptions appear to be extremely rare occurrences, they illustrate that the more regular, seasonal migratory behaviour of some dragonflies, can possibly be exacerbated by unusual weather conditions in their region of origin - in a similar way that occurs with Painted Lady irruption years.

More information

Cynthia Longfield (1948). A vast immigration of dragonflies into the South coast of Co. Cork The Irish Naturalists' Journal, 9 (6).
David and Elizabeth Lack (1951). Migration of Insects and Birds Through a Pyrenean Pass. Journal of Animal Ecology, 20, 63-67 DOI: 10.2307/1644

How do bugs cope with winter?

We've had a couple of frosts in the last few days. Despite the die-hards which allow me to post occasionally these days, most insect species are nowhere to be seen throughout winter. How and where are bugs surviving the cold weather? There are various ways in which invertebrates go through the winter.

Far, far Away
Some - I'd say the lucky ones, avoid winter altogether - they are far, far away during winter, having migrated to warmer areas around the Mediterranean. These include several butterflies, moths, hoverflies and dragonflies (Red Admiral, Painted Lady -above - Silver Y and the Migrant Hawker are examples) which often lack a frost-resistant stage in their life cycle.

Slowing Down
Others stay to brave the elements, a few of them even carrying on more or less as normal, but at a much more sluggish pace. These are the invertebrates that enjoying the occasional sunbathing during rare winter sunny spells: Bluebottles (above), Winter Gnats, the window spider (Zygiella x-notata). Some moth caterpillars, like the Ruby Moth caterpillar, are found active in the middle of winter. A few bumblebee populations are starting to behave this way as the climate warms and they can make use of winter flowering resources.

Dormancy
Many insects enter a dormant state or diapause: this is the scientific term to describe what we often call 'hibernation': organisms enter a physiological state, dormancy or diapause, in which growth and feeding pretty much stop, and metabolism is very reduced. To protect their bodies against the damaging effects of freezing, some chemicals (glycerol and anti-freeze protein are two of them) are produced in their bodies to maintain cell stability and aid survival under low, often freezing, temperatures. Overwintering can occur in any stage of the life cycle depending on the species or taxonomic group  - egg, larva or nymph, pupae or adult (imago).

The beautiful blue silk cocoon of a Enoplognatha spider, ready for winter.

For example grasshoppers, aphids and many spiders overwinter as diapausing eggs. Many dragonflies and damselflies overwinter underwater as nymphs. Many solitary bees overwinter as pre-pupae in their nest cells.

A fully developed Noctua moth caterpillar in early spring

Some butterflies and flies overwinter as pupae, whereas some butterflies (Comma, Small Tortoiseshell, Brimstone), Ladybirds, queen wasps and queen bumblebees hibernate as adults. Many solitary bees are already imagos in their cells, waiting for the right time to emerge.
 I will keep my eyes open for bug life throughout the winter and be on the lookout for any stirrings indicating that springs is coming.

The Migrant Hawker

The Migrant Hawker, Aeshna mixta, is one of the commonest dragonflies found away from water. It regularly visits my pond-less garden, with a clearly marked peak in the second half of August (the male on the top shot from 15th August this year). This species is a fast developer, their larvae take just a year to become adults, but it is sensitive to the cold. Adults are not territorial and are often found hunting together, at high, patrolling over the garden hawking for insects. Early in the morning they rest hanging from vegetation, sunbathing, at about 1-2 m high, often a pair not far from one another, and they then allow close approach.

A female Migrant Hawker basking, with another individual on the background (16/08/06) - which I only noticed when checking the photos.

Although dragonfly migration has been known for over a century, it is still a little known phenomenon. Several British dragonfly species are regular long-distance migrants, like the Four Spotted Chaser and the Common and Ruddy Darters, and others occasionally appear as vagrants from the Mediterranean or even from North America. Where do they go and why do they do it? Do they migrate back and forth or do they just wander? Is this behaviour to avoid overcrowding or drought, in search for good sites for reproduction? David and Elizabeth Lack (1951) favoured the existence of large scale two way migration in dragonflies, like that of many birds and butterflies, after observing large numbers of Common Darters passing over a high mountain pass in the Pyrenees. American migrants tend to be species breeding in ephemeral or semipermanent ponds, which dry frequently, so migration might also be a wandering in search of suitable ponds. Recently, a combination of miniaturised radio transmitters, isotope analysis and genetic markers have shed light into the migratory behaviour of the American Common Green Darner, Anax junius. Although the Migrant Hawker has been seen forming large swarms, unfortunately, we know very little about the migration in this species. In any case, its migratory behaviour makes it a very good coloniser. In the early 20th century it was regarded as a rare immigrant - in fact it was called Scarce Hawker -, but since an invasion in 1935 it has been steadily and rapidly expanding its range northwards and westwards in the UK, reaching Scotland in 2004. The species also colonised Ireland in 2000 and is now well established. I can't wait to find out more about these fascinating garden visitors.

References
Wikelski, M., Moskowitz, D., Adelman, J., Cochran, J., Wilcove, D., and May, M. (2006). Simple rules guide dragonfly migration Biology Letters, 2 (3), 325-329 DOI: 10.1098/rsbl.2006.0487

Lack, D. and Lack, E. (1951). Migration of Insects and Birds Through a Pyrenean Pass The Journal of Animal Ecology, 20 (1) DOI: 10.2307/1644

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

Brian Nelson, Colm Ronayne and Robert Thompson (2003). Colonization and Changing Status of Four Odonata Species, Anax imperator, Anax parthenope, Aeshna mixta and Sympetrum fonscolombii, in Ireland 2000-2002 The Irish Naturalists' Journal, 27 (7), 266-272

Little riders on the wind

The mass influx events of Painted Ladies - such as during the summer of 2009 - and wildlife documentaries on the Monarch Butterfly have made butterfly migration familiar to everybody. Many butterfly species are now known to carry out bidirectional migrations, for those of the northern hemisphere, towards the north in the spring and towards the south in the autumn. Surprisingly, the extent of fascinating behaviour was only recognised relatively recently. Up to the 1930s, the only butterfly that was acknowledged to migrate was the American Monarch, Danaus plexippus, with evidence for migration in other species dismissed even from experienced entomologists and lepideropterists as "overflow": one way dispersal movements from successful, very dense populations. One early researcher was set to change matters: Carrington Bonsor Williams (widely known as C.B.), from Rothamsted Experimental Station in the U.K. He was passionate about insect migration and amassed large amounts of data on the subject, in Britain and abroad. In his 1930 book Migration of Butterflies and later work, he presented evidence for migration for over 200 species of butterflies worldwide, including support for return migration. He also helped organise the collation of data collected by naturalists across the U.K. on insect migration. His analyses of these data gave strong evidence for migration for six species of British butterflies: the Painted Lady, Small Tortoiseshell- which also hibernates in the U.K. - , Red Admiral, Large White, Clouded Yellow and Pale Clouded Yellow. For the Red Admiral, the records  showed clearly a dominant proportion of individuals flying in a northerly direction up to the end of July, and a southerly direction during September and October:

(Figure from Williams 1951)

In the case of the Painted Lady and the Large White, the lack of evidence for a return migration puzzled Williams:

I have already (Williams et al. I942, p. 250) pointed out the difficulty of accounting for the persistence of a habit of movement in one direction only on any theory of evolution. If we accept that directional movement in butterflies is simply an overflow from an over-populated area, and that none of the emigrants ever return; then the species must be perpetuated by the offspring of those individuals which do not emigrate. Thus a habit must persist for countless generations, in spite of the fact that all  individuals which develop it die without contributing to the continuity of the species.

He was optimistic though, and correctly predicted that return flights would be found in all species once enough research effort was put into it, making butterfly migration as important a phenomenon as that in birds. Indeed, recent research - some of it carried out in Rothamsted - including meteorological analysis of prevailing winds when influxes of Painted Ladies occur, and entomological radar research has shown that the Painted Lady taked advantage of tailwinds as a migratory aid and that the reason that these flights were not detected is that they fly at high altitude.
 Migrating butterflies are tracking suitable breeding grounds, areas where larval food plants are abundant. Red Admiral caterpillars feed on fresh, rapidly growing nettles. At the end of spring, nettles start to wither in the Mediterranean winter grounds, so the spring Red Admiral adult generation flies to Northern Europe - and also up mountains - where nettles are luscious. They mate and lay eggs in the north and the resulting autumn generation of adults migrates due south in the autumn.

(Figure from Mikkola 2003)
Unlike birds, butterflies breed both in the winter and summer grounds, and also unlike birds, each generation makes a one way trip, leaving the return journey to their offspring.
(Photo above, a fresh, autumn generation Red Admiral feeding on Cherry Laurel, 19 Aug 2011)

References
Williams, C. (1951). Seasonal Changes in Flight Direction of Migrant Butterflies in the British Isles The Journal of Animal Ecology, 20 (2) DOI: 10.2307/1537

Kauri Mikkola (2003). The red admiral butterfly (Vanessa atalanta) is a true seasonal migrant: an evolutionary puzzle resolved? European Journal of Entomology, 100, 625-626


Stefanescu, C., Alarcon, M., & Àvila, A. (2007). Migration of the painted lady butterfly, Vanessa cardui, to north-eastern Spain is aided by African wind currents Journal of Animal Ecology, 76 (5), 888-898 DOI: 10.1111/j.1365-2656.2007.01262.x

Chapman, J., Nesbit, R., Burgin, L., Reynolds, D., Smith, A., Middleton, D., & Hill, J. (2010). Flight Orientation Behaviors Promote Optimal Migration Trajectories in High-Flying Insects Science, 327 (5966), 682-685 DOI: 10.1126/science.1182990

On the Western Conifer Seed Bug and Christmas trees

This large, striking bug entered the house at night through an opened window. It is the alien species Leptoglossus occidentalis, the Western Conifer Seed Bug, a native from western North America, where is a minor pest of conifers. This species has long antennae and large rear legs, a pale zig-zag mark on the wings, and distinctly flattened and enlarged tibia, which gives the name to its group, the leaf-footed bugs. During last century, this species spread across North America and during the last decade has also been introduced in Europe, where it has quickly spread from the initial introduction point in Italy. It was accidentally transported in timber, and its spread could also possibly been aided by Christmas tree shipments. Nymphs and adults feed on the sap of cone seeds and buds, and therefore the damage is restricted to seed production. In continental Europe, where there are established populations, it does not appear to cause much damage. Every now and then, there are migratory influxes to the U.K. from the continent, as occurred in 2008. The bug often falls in moth traps, and is a good flyer. It is most obvious in the autumn, when it moves about looking for suitable hibernation sites, often entering buildings and sometimes forming large aggregations, attracted by pheromones produced by males. Nymphs have been found in a few sites in the U.K., so it appears to be establishing.
 If you have seen this bug, you can report it here.

References
Lis, Jerzy A., Barbara Lis & Jerzy Gubernator (2008). Will the invasive western conifer seed bug Leptoglossus occidentalis Heidemann (Hemiptera: Heteroptera: Coreidae) seize all of Europe? Zootaxa, 1740, 68-68
Taylor, S.J., G. Tescari & M. Villa (2001). A nearctic pest of Pinaceae accidentally introduced into Europe: Leptoglossus occidentalis (Heteroptera: Coreidae) in Northern Italy. Entomological News, 112, 101-103.
Blatt, S. & Borden, J. (1996). Evidence for a male-produced aggregation pheromone in the Western Conifer Seed Bug, Leptoglossus occidentalis Heidemann (Hemiptera: Coreidae) The Canadian Entomologist, 128 (4), 777-778 DOI: 10.4039/Ent128777-4

Swift hoverfly settled down for the night

This hoverfly species, Scaeva pyrastri, is a challenging photographic subject. I find it particularly flighty, and many a time I have chased it across the garden as it fed on Verbena bonairensis, Buddleia, Hebe or Red Valerian. It is very aware of shapes approaching, and it cunningly and swiftly flies out of range. You really have to stalk it for close ups. S. pyrastri is a migrant hoverfly which flies from the continent to rear a generation in northern Europe and all my records are of July and August, of single individuals. Their numbers vary greatly from year to year. It is a large, handsome hoverfly, males giving the impression of having a disproportionately larze head (as above) due to their enormous eyes, which are distinctively hairy, and like in other hoverflies meet on top of their heads. The species has very characteristic white markings in the black abdomen. Adults are found in a range of flowers, while the larvae feeds on ground aphids. This male had chosen to settle in the startlingly red rowan berries and it allowed me to approach to the minimum focusing distance of my camera.

A business of hoverflies

A business and a swarm, are the collective terms for flies, in this case, hoverflies. In the last few days, hoverflies, particularly the Marmalade fly, Episyrphus balteatus (above) have descended covering every flower, bush and branch with a stripy fuzziness. The group of Marmalade hoverflies on the flowering teasel above greeted me first thing in the morning. As I have said before, many hoverflies are migrants: numbers are swelled in the summer by migrants from southern Europe. I have taken shots of several species today, including Scaeva pyrastri, which I had only seen once in 2006 before, and now seems to be everywhere.

Syritta pipiens on fennel

Eristalis sp.

Syrphus sp.

Scaeva pyrastri on Knapweed

Myathropa florea on fennel

More information
Stubbs A.E. & Falk, S.J. (2002) British Hoverflies. BENHS.
Hoverfly Recording Scheme.

The Hawkers

For a week I have been watching hawker dragonflies hunting over the garden well into the evening. They fly high (3-5 m) and never seem to stop, so I could not identify them. My luck changed today, when early in the morning I came across a male Migrant Hawker (Aeshna mixta) sunbathing on a fence outside. This is the only species of hawker I have identified in my garden and I usually see it in August and September. It is quite approachable and getting close-ups is relatively easy.

The male was a little later joined by another individual (a duller male I think) and both sunbathed quite close to each other high up in a buddleia for a while.

My usual basic photographic equipment was joined by a chair, so that I could take the pictures. At 11:30 both dragonflies started hunting and they didn't stop again.

This species of dragonfly is very sociable and regularly hunts in groups. Is also, as suggested by its name, a regular migrant species. Before the 1950' it was quite a rare species in the south of the UK but steadily increased its range and is now found also in S. Scotland and Ireland.