August Butterflies

August is a great butterfly month, but listening to the rain splattering above my head as I write I should probably append the adverb ‘allegedly’. The weather forecast for the first week in August is for dull, wet and at times windy weather, not the right conditions for flying insects. Our butterflies are accustomed to our wet, erratic weather, and even brief sunshine is capitalized on, with often impressive abundance as butterflies have emerged from pupae and are waiting to fly.

The Peacock butterfly is especially numerous during August.

An impression of abundance is often noted on the first sunny day following days of rain, with exciting numbers evident. This population boom should not be confused with an increasing trend. Typically, butterfly pupae hatch over weeks, but if the weather is unfavourable, several days’ hatched butterflies will wait to fly together on the first good weather day, rather than fly in apparently reduced abundance over several days with good weather conditions. Similarly, pupae that have been ready to hatch for some days may delay hatching until a sunny day arrives.

The Small Copper has a second brood during August.

Assuming the sun shines in our wet and wild countryside, what can we expect to see?

This depends not just on the habitat but where in Ireland it is you are looking for butterflies. The Meadow Brown, on its ‘last legs’ in many areas in the east, is just beginning to emerge in abundance in many areas of the west and northwest during August.

Huge abundance can be encountered on coastal grasslands in Donegal and Clare during August, fresh and eager to mate, while their eastern cousins are faded or dead. By contrast, the Small Copper can be seen in suitable habitats throughout Ireland during August.

A wood White laying on a vetch in the Burren National Park.

A butterfly with a restricted Irish distribution that flew in spring should be around in August. The Wood White, a Burren and western limestone butterfly will be flying in its habitats. This second flight is a partial generation. The first flight, which occurs mainly during spring comprises all the over-wintered pupae.

Brown Hairstreak, male, on oak in Knockaunroe, Co. Clare.

The last Irish butterfly to emerge is the Brown Hairstreak. Our largest and rarest hairstreak exists in the Burren (Clare and Galway) and a small number of areas outside the Burren, such as west Tipperary, Ennis, near Lee’s Road, around Lough Corrib, Lough Mask (both Galway) and possibly Lough Carra (Mayo). This obscure but beautiful species is associated with wilder, bushy places typically (but not always) on exposed carboniferous limestone. It peaks in abundance during August and has been seen as late as October. Look out for it on bramble flowers, and on Creeping Thistle near scrub.

This exquisite female Brimstone has recently hatched from her pupa. After a few days of feeding, she will enter hibernation. Dense vegetation, such as ivy, low-growing bramble, holly and clump-forming sedges are used as overwintering retreats.

Another butterfly that is restricted in its Irish distribution but more widely distributed than the Wood White and Brown Hairstreak is the Brimstone. The new generation flies from July to September or even early October but its peak flight is in August. The Brimstone joins the Peacock and Small Tortoiseshell to gorge on nectar in flower-rich areas close to woodland before all three species go into hibernation. Bramble, Common Knapweed, Rough Hawkbit and Devil’s-bit Scabious are favoured flowers for all three.

Small Purple-barred, Timahoe Bog, Co. Kildare.

A lovely, small (19-20 mm wingspan) intensely coloured day-flying moth to look for on wild grassland is the Small Purple-barred. Ranked ‘Near Threatened’ on our Macro-moth Red List 2016. This moth breeds on Common Milkwort, which need semi-natural grassland untainted by agricultural chemicals. A common day-flying moth during August is the Shaded Broad-bar, a brown/buff grassland moth with a dark brown crosswise bar on its forewings.

Burren Green moth.

On limestone grassland, chiefly in the Burren, the Straw Belle may be encountered; it is a nocturnal species that is easily roused in daylight. Straw-coloured and brown-freckled with a brown diagonal bar on the forewing, this species is especially common on short Burren grassland. A moth confined to the Burren, the Burren Green, ranked ‘Near Threatened’ is abundant in the Burren during August. This moth is mainly nocturnal, but it will fly in daylight on muggy, over-cast days when it feeds on Common Knapweed. This moth is not found outside the Burren in Britain and Ireland. It breeds on Blue Moor-grass.

These are just a few of our August flyers. We will leave you to discover others, but remember to let us know what you see, according to the record submission procedures here:

All photographs are ©J.Harding.

Lullybeg Reserve Update

Butterfly Conservation Ireland’s reserve at Lullybeg is monitored using the transect system. This uses a fixed route to count butterflies on the reserve between April 1st and September 30th each year. The count includes all butterflies seen 2.5 metres to the left and right of the walker, 5 metres ahead of the walker and 5 metres above. The results of these counts are sent to the National Biodiversity Data Centre. In addition, counts are made on the reserve away from the transect.

Having systematic butterfly count data for the site since 2011 means trends in species and abundance can be tracked which means we can check the effects of changes in management, successional changes in the habitats, weather conditions and even climate on butterfly populations.

The Small Purple-barred moth flies in two generations at Lullybeg.

Here we give an indication of the reserve’s performance at the halfway stage in 2023.

The reserve has some species whose conservation is a concern nationally, and these are especially important monitoring subjects. The Small Heath, rated Near Threatened, has dramatically declined on the reserve. From a high of 231 individual Small Heaths in 2013, a slight drop to 229 in 2014, the trend has been of steady falls since. The years 2015, 2016, 2017, 2018, 2019, 2020, 2021 and 2022 saw 131, 62, 61, 63, 57, 44, 74 and 35 Small Heaths respectively.

This year the Small Heath count is 54, a healthy increase on 2022’s poor result but still much lower than its peak in 2013 and 2014. We are investigating the habitat conditions it likes on the reserve to inform decisions about how to manage these areas and how these conditions compatibly align with the ecological requirements of other important butterflies. This is quite a challenge in a relatively small reserve.

Dense grass tussocks are used by Small Heaths.

One conclusion is that the Small Heath dislikes heavy grazing. This is clear from the nearby reserve at Lullymore, where it has been eliminated by grazing designed to cater for the Marsh Fritillary, where donkeys were introduced to graze on Purple Moor-grass to prevent it from overwhelming the Marsh Fritillary foodplant. The Small Heath likes fescue grasses, and the growth form it especially likes is tall, shaggy, dense tussocks containing plenty of dead grass blades. Unfortunately, grazing livestock love this grass, and attack this before tackling the rougher species.

The Small Heath also likes smaller, sparser Red Fescue growing among sparsely vegetated ground, but semi-bare terrain is quite temporary on vegetated cutover peatlands. Thus, successional change is a threat, unless the fescues grow into dense tussocks needed, but this only happens if grazing does not occur or is very sporadic. The ground can be disturbed by cattle to create bare patches, but will Red Fescue colonise these quickly before more robust grasses assume control?

Small Heath, male, Lullybeg, County Kildare. J. Harding

The timing of grazing and topping of vegetation and the severity of topping are also important. Late autumn or early winter grazing and topping is probably best for the butterfly. These were used in 2022 and a population rebound was observed, but some of this recovery was in areas not grazed and not topped.

The Marsh Fritillary thrives at Lullybeg. It has shown a strong abundance over the last four years.

A priority butterfly that is doing well on the reserve is the Marsh Fritillary. A mere 15 were counted in 2011, followed by 19 in 2012, 41 in 2013, 92 in 2014, just 32 in 2015, 14 in 2016, 17 in 2017, 31 in 2018. After this numbers increased.

Totals of 287 and 207 in 2021 and 2022 respectively show this recovery. This spring and summer 246 Marsh Fritillaries were counted, great news that shows the management is working well for this threatened species. The control of scrub and light grazing is key to this success.

Another threatened butterfly, the Dark Green Fritillary is still on the wing, so the picture is unclear at this point. Only two males have been recorded so far. There are plenty of Common Dog-violet plants in parts of the reserve but the sward around the highest violet populations is probably too low for this species.

The Silver-washed Fritillary has shown lower numbers in recent years. In 2020, 2021 and 2022 the figures were 9, 6 and 8 respectively. Up to mid-July 2023, 16 Silver-washed Fritillaries have been seen and this number will increase, so that is very good news.

Ringlet numbers are showing huge increases on 2022’s figures.

So far in 2023, 1237 Ringlets have been counted (only 636 in 2022), and 168 Meadow Browns (204 in 2022). These two are still flying, so the totals will rise.

Twenty-two Dingy Skippers were found this year; its flight period is over for 2023. In the four years 2019-2022, the numbers seen were 10, 10, 47 and 31. The trend will be closely monitored next spring.

It is wonderful to see increases in the Small Heath, Marsh Fritillary, Silver-washed Fritillary and Ringlet, and an expected rise in Meadow Brown numbers. Without your help, none of this would happen.

These figures show what is possible elsewhere in our countryside if managed properly. Land managers and conservationists take note!

The Silver-washed Fritillary is showing a strong resurgence in 2023.

Photographs copyright J. Harding.



Where do we go from here?

Aesthetically pleasing, but this garden is little more than a plant collection, lacking the basic ingredient for animal life, that of native plants (except Foxglove).

At last month’s Bloom event in the Phoenix Park, Dublin, a question asked in the Irish Wildlife Trust (IWT) tent about the installation of industrial infrastructure on bogs was answered by the IWT’s Patrick Fogarty. He described the bleak condition of Ireland’s bogs and made it clear that plantation forestry, wind farms, solar farms, dumps, and roads should not be located on the nation’s bogs, which should be restored to allow them to recover from decades of mining and drainage.

Responding, a lady from the audience stated that as a Dublin resident who does her best to be green, she felt very depressed about the detailed description of the extent of the degradation of our bogs and other peatlands and habitats generally.

Ballivor Bog, Co. Meath. This drying, carbon-emitting dead zone is typical of the state of our bogs.

She does her best. She has a compost heap, plants flowers for pollinators, recycles her recyclable waste and probably uses public transport whenever possible and may use an electric vehicle for destinations that are not served by public transport. She does not burn fossil fuels; she has solar panels and has retrofitted her walls and roof space with insulation. She might also have had double-glazing units fitted and wears extra layers of clothing to minimize the amount of electricity used during the colder months.

She might be buying clothing made from organic cotton and eschewing ‘fast fashion.’ If she is a mother, she might use cloth or biodegradable nappies.

Surely, she pleaded, she was doing enough. What else is she to do?

As a society, we don’t know how diminished our environment is.

Let us probe a bit deeper.

She has a compost heap. Why does she need a compost heap? According to the Environmental Protection Agency  Irish households threw away an estimated 241,000 tonnes of food (31% of the total) in 2020. Excessive purchase creates an impetus to over-produce and encourages the practices of intensive farming that removes habitats and poisons our atmosphere, water, and soils. Shoppers need to think more about food quantity. Do we need to buy so much food? Consumer behaviour feeds back into food production practices, so choose less, and pay more for sustainably produced food.

If you don’t overbuy food, you will not generate the same amount of packaging waste either.

She plants flowers for pollinators. That sounds great, but it often makes matters worse.

Native flowers like Foxglove are needed for pollinators.

Where did the plant come from, what species is it, how much fertilizer was used to produce it and what is the growing medium?

The plant is most likely an import from Europe. It was not grown from native seed or native cuttings. It travelled here by boat or aircraft and by delivery vehicle, adding to transport emissions.

Regarding species, it might be a plant native to another part of the world. Plants sourced from other ecosystems often cause ecological damage when placed in our habitats and are often of very limited value to Ireland’s wildlife. A gardener’s favourite is Buddleia davidii and cultivated varieties of this plant. This plant is often called the Butterfly Bush. It attracts butterflies but it does not attract many species and its leaves feed very few moth larvae and no butterfly larvae.

Bramble over Buddleia. Bramble is native, flowers for far longer, produce edible fruit and is visited by many more pollinators. Its leaves and fruit feed a range of animals, and butterflies hibernate within its dense, semi-evergreen cover. The butterfly feeding here is a female Silver-washed Fritillary.

Despite its blooms being festooned with glorious Peacock, Red Admiral and Small Tortoiseshell butterflies especially, it is very limited in its overall value to pollinators. A recent study  Conserving Diversity in Irish plant-pollinator Networks (2022) concerning plant-pollinator relationships compiles a top 33 plant list. Buddleia is not on it.

Even worse is the choice of another flowering shrub, Fuschia.

This plant has escaped into the wild and is wreaking havoc on natural habitats, displacing native, insect-rich shrubs. Again, it attracts bees but its leaves feed very few native insects. Our native shrubs do have central roles in our ecosystems, sheltering and feeding the immature and adult stages of our insects. But these are ignored by many gardeners who think that buying visually appealing non-indigenous plants marketed ‘Bee Friendly’ or with a Peacock butterfly pictured on the label means their choice is nature friendly.

This is slick marketing, not ecological truth.

Further strain on our world is evident in the plastic pot, the synthetic fertilizer and peat used in the growing medium the plant comes in. All these items require energy to produce, with oil also a component in the plastic pot and fertilizer. The production of these two materials adds to the atmospheric pollution driving climate change and habitat damage on a global scale. The peat was robbed from the environment. Robbed, not ‘harvested.’ This impoverished or completely removed the peatland habitat leading to the extinction of the Curlew from most of its Irish range with more extinctions to follow. Peat removal pollutes streams, rivers and lakes and atmosphere.

While I lack the data to prove this, there is a possibility that attracting butterflies by planting Buddleia damages the butterflies that benefit from the rich nectar provided.

Buddleias planted in sunny, sheltered situations will attract butterflies, sometimes in impressive abundance. It appears to be doing so much good. Day after day of Large Whites, Red Admirals, Peacocks, Small Tortoiseshell, Commas, and even Silver-washed Fritillaries. What’s not to like?

Native Guelder Rose in bloom. The berries are natural bird food, especially for Bullfinches. The flowers feed pollinators, the leaves support several insects.

Concentrating butterflies in a small area concentrates predators too. I frequently observe small birds such as Robins, Blue Tits and Wrens being drawn to Buddleias to attack butterflies. Both birds prefer to hunt in shrubs, so the presence of the butterflies on this bush makes them more vulnerable to avian attack. Furthermore, concentrating birds in a small area may make contagious diseases a greater threat. Buddleia is a bird table for butterflies, and insectivorous birds, but it may turn out that both groups end up sacrificed on the altar of our good intentions.

Common Hawthorn is a vital native plant, of great importance to birds and pollinators.

The answer is to grow our native herbs and native trees widely. Garden centres grow according to demand. Much better, harvest native seeds from wild places nearest to your home and create compatible habitats in gardens and green spaces. By ‘compatible’ I mean the natural habitats that are represented locally and that belong together. If you farm, farm with nature. Get advice from your farm advisor about the best ways to do this.

This unfertilized field in County Meath supports hundreds of Meadow Brown butterflies, along with Small Coppers, Common Blues, Green-veined Whites, and Orange-tips.
This field, located near the field above, was re-seeded with one grass species, is sprayed with slurry, and probably fertilized. It supports no grassland butterflies.

Electric vehicles are not a panacea for Greenhouse gas (GHG) emissions from the transport sector. We still generate power by burning fossil fuels so powering up electric vehicles contributes to emissions. Unless we get our power from clean energy sources that do not involve habitat destruction we are doing no real good. Even when clean, renewable energy is produced sustainably, the construction process for EVs is very severe on the environment with some dreadful consequences in many areas mined for the materials used for the EV batteries.

Insulating one’s home, and installing double-glazed windows and solar panels will help to reduce energy consumption and lower GHG emissions. Where you can, get rid of hard surfaces such as tarmac, brick drives and concrete. New builds should use as much natural material as possible. Natural slate (not always expensive, depending on the source), clay roof tiles, natural stone and clay brick should replace concrete, the production of which requires enormous energy and generates massive CO2 emissions.

More native woodland, like this one in Tullamore, County Offaly, is needed to support a stable climate and ecosystem.

But to take this article back to where it began, we must re-wet our bogs and other wetlands, re-plant native woods and allow existing native woods to expand, reduce the pressure on land by producing and consuming less, grow food with reduced or no chemical inputs and generally allow nature to return. The crisis must be tackled at every level, in one’s home, garden, neighbourhood, nationally, and globally.

The soaring temperatures across southern Europe and the United States and flooding and drought elsewhere are alarms we must not ignore. Those who rejected the EU Nature Restoration Law should consult their conscience and intelligence.

Removing nature removes us. Action now.

Photos copyright J. Harding







Climate Change and Butterflies

The signs of the warming climate are evident in a range of climate statistics. In this article, we look at the most recent data and explore the effects the changes are having on our butterflies, moths, and the places they need for survival.

A sunny day in early June 2023. There was no rainfall recorded in northwest Kildare between May 8th and June 10th, 2023. This was replicated across much of Ireland.

The world’s average temperature reached a new high on Monday 3 July, topping 17 degrees Celsius for the first time.

And according to Met Eireann, June 2023 was the hottest June on record in Ireland, breaking an 83-year-old temperature record. The increasing temperature is not a one-off, but a trend towards a warmer climate. Met Éireann climatologist Paul Moore states:

An average monthly temperature of greater than 16C has been seen in July and August but never before in June. June 2023 was well above normal due to persistent warm days and nights.

Further afield, Spain experienced extreme heat as early as April when temperatures exceeded 38 Celsius. According to the World Meteorological Organization, Europe is warming twice as fast as other continents and Ireland is feeling the effect.

According to Met Éireann’s TRANSLATE report, Ireland’s climate has shown changes in temperature and rainfall. Daily annual average precipitation has risen between the two baseline periods, 1961-2010 and 1991-2020. Ireland is wetter, with the western half of the country showing the biggest increases. Temperatures have risen throughout the country with the highest temperatures recorded in the centre of Ireland.

The report also deals with climate projections:

As the world warms it is clear that Ireland’s temperature and rainfall undergo more and more significant changes, for example on average Summer temperature could increase by more than 2°C, Summer rainfall could decrease by 9% while Winter rainfall could increase by 24%.

Butterflies can tell us a great deal about how climate is affecting our environment. Butterflies and moths respond quickly to changes in the environment because they are highly sensitive to temperature, precipitation and habitat change. Their rapid reproduction rate means that populations can rise very quickly when favourable conditions prevail but abundance crashes when conditions are less suitable.

Abundance is not the only response butterflies show to a warming climate. Distribution changes, earlier emergence, extended and shorter flight periods, changes to brood structure, and even appearance can occur in butterflies.

An extreme weather (EWE) is an event that is rare at a particular place and time of year.   Extreme weather events, such as extreme heat, extreme drought and extreme rainfall are expected to be more common, and these will have an impact. I will look at extreme weather events first.

Extreme Weather Events

Met Éireann’s TRANSLATE report states that as the climate warms, extremes become more widespread and pronounced. A study by McDermott Long et al. (2017) looked at the impact of extreme climate effects on all life stages and the over-wintering period of UK butterflies. This study showed that the pupae of several single-brooded species (univoltine) are vulnerable to prolonged precipitation. This study also found that extreme heat during the period starting on November 1st and ending on February 28th has a negative impact on the population of the over-wintering stage of single-brooded species (whether this is the egg, larva, pupa or adult).

However, extreme heat during the adult life stage outside winter causes positive population change for 21% of species and extreme heat during the pupal stage outside winter is also positive, while during over-wintering it is associated with negative population change in 45% of species. The study discovered that extreme cold during the adult life stage of single-brooded butterflies has a negative effect on 35% of species.

The study looked at the impact of extreme climate effects on butterflies that produce two or more broods (multivoltine) annually. It found that extreme heat during over-wintering and extreme precipitation during the first and second generation adult life stages are the biggest cause of population declines in multivoltine species (67%, 58% and 50% of all multivoltine species affected, respectively). Extreme heat during the adult life stage is associated with positive population change in 42% of species. Drought plays a much more important role in multivoltine species than in univoltine species. Drought negatively affects 50% of species during their second larval life stage but has a positive impact on 25% of the species during their first ovum life stage.

Drought particularly affects vegetation on thin, free-drained soils, like those in the Burren uplands.

In summary, butterfly population changes are primarily driven by temperature extremes. Extreme heat is harmful during over-wintering periods and beneficial during adult periods (outside winter) and extreme cold had opposite impacts on both life stages. Harmful effects were identified for extreme precipitation during the pupal life stage for univoltine species.

Distribution Changes

One species that has undergone a range change is the Comma butterfly. In fact, it had not been recorded in Ireland before 1998.

In The Irish Butterfly Book (2021), I explored the role of climate change in its colonisation of Ireland since 2000, a process that has accelerated since 2014:

Asher et al. (2001) and Thomas and Lewington (2014) state that climatic factors appear to be relevant, particularly climate warming. The factors driving the recent expansion of range into Ireland are likely to be a warming climate and closeness to an expanding British population. Unlike the gregarious larvae of the Small Tortoiseshell and the Peacock, which bask communally to heat up, solitary Comma larvae are not able to raise their own body temperature above that of their surroundings. This dependence upon external temperature might explain the absence of the Comma from Ireland in the past when the climate was cooler.

The Comma usually breeds on Stinging Nettles, on mainly unshaded plants, especially using plants growing in damp soils near trees and shrubs, where the Comma benefits from warmth, shelter and succulent foodplants. In May 2023 I confirmed its use of elm in Ireland, probably Wych Elm Ulmus glabra, as a foodplant for the caterpillar. The Wych Elm and Stinging Nettle are the optimal foodplant choices for successful larval development (Braschler and Hill 2007).

Comma caterpillars on Elm, Prosperous, County Kildare. The larva on the right is in the fourth instar (stadium) and the larva on the left is in the fifth (and final) instar.

We have gained other species, and species already here that had a limited range here are extending their Irish range. Moths such as Blair’s Shoulder-knot (first recorded in 2002) and the White Satin moth are colonists extending their range, while moths like the Yellow-tail and Marbled White-spot, formerly quite restricted in Ireland are in expansion mode. The Bedstraw Hawkmoth might be colonising Ireland, with adults and larvae being recorded in various places across the country.  Lime Hawkmoth is another recent arrival, appearing now to be well-established in parts of Dublin, breeding on lime trees on tree-lined roads.

Earlier Emergence

One way of tracking the response of living things to climate events is the study of phenology. Phenology is the study of periodic events in biological life cycles and how these are influenced by seasonal and interannual variations in climate, as well as habitat factors.

In Ireland, the Orange-tip, largely a spring butterfly, usually starts to fly shortly after the end of the first week in April until the end of June. Sometimes an early July sighting is made. The peak flight time is in the first half of May. It has one brood in Ireland and throughout its vast European range. It is likely that some of the pupae over-winter twice, a strategy to cope with prolonged inclement conditions during their flight period.

The trend in Britain, where records of butterfly phenology are more extensive than in Ireland, is toward earlier emergence. Here I look at just one example of earlier emergence trends. Throughout most of the twentieth century, the first British Orange-tip of the season was recorded, with very few exceptions, in April.

The Orange-tip is now emerging earlier in response to warmer springs. Before 1989 it was rarely seen in March in Britain. Now the first sightings in Britain are usually in March, with only one first sighting in April during the period 2011-2021 (05/04/2018). The earliest record in Britain is the 27th of February, 2013.

But since 1989, most of the earliest emergences have been in March (Oates 2015). The average first sighting date 2011-2021 for the Orange-tip in Britain, compiled by Butterfly Conservation UK, is March 17th. This indicates the impact of warmer springs. The average first sighting date 2013-2017 for the Orange-tip in Ireland, compiled by Butterfly Conservation Ireland, is later, on April 10th, reflecting the cooler temperatures here. However, the mild spring of 2019 saw an Orange-tip recorded on March 29th, in County Donegal, while in 2021 the first Orange-tip was recorded, again in Donegal, on April 3rd.

The Orange-tip, like several other butterfly species, passes the winter in the pupa, so it is well placed to emerge earlier when spring has sustained warmth.

Earlier adult emergence dates are being recorded for several species such as the Marsh Fritillary, Large Heath, Narrow-bordered Bee Hawkmoth and others.

These Marsh Fritillary caterpillars in Lullybeg, County Kildare have hatched a month earlier than expected.

But earlier life cycle events are not limited to adult emergence dates. The hatching dates of larvae might also be taking place earlier than was recorded in the past. On June 29th 2023, I found two Marsh Fritillary nests containing newly hatched caterpillars in Lullybeg, in County Kildare. Judging from the leaves they consumed, hatching occurred a few days before June 29th. This hatching date is up to one month earlier than recorded in this area in the past.

Extended and shorter flight periods and abundance

The effect of increased warmth is already being expressed in the lives of our moths and butterflies.

The Red Admiral, a migrant, typically arrives in Ireland in March and departs during autumn. In some places, like Howth, County Dublin, it is remaining during winter, taking advantage of the warmer winters to breed on nettles that now survive because winter frosts rarely occur there now.

Sustained summer warmth and dryness do not suit all species. The Small Blue breeds on just one foodplant that grows on thin soils, such as sandy soils. The plant, Kidney Vetch, can shrivel during drought, which means the larvae will starve or be under-sized. Droughts have already stricken the species, such as when the prolonged drought in 2018 was followed by a population crash on the sand dunes at Portrane, County Dublin, in 2019, and presumably elsewhere.

Changes to Brood Structure

Brood structure refers to how many generations an animal can produce per year. In Ireland, most butterfly species produce one generation a year.  Some can produce two or even three generations, while some species, such as the Red Admiral, are continuously brooded.

The Small Copper, typically a bivoltine butterfly in Ireland, appears to benefit from sustained warm weather. This butterfly can fly in three generations in Ireland in years with warm summers, and in two broods when the summer is cooler. We have seen third broods in recent years, such as in 2021 when the recorded flight period was May 7th to October 9th. It appears that only two generations flew in 2020 when summer was cooler and the recorded flight period was May 8th to September 6th.

A species that might be vulnerable to increasing warmth and decreasing summer rainfall is the Small Tortoiseshell.

The Small Tortoiseshell is not benefitting from climate change despite being a nettle feeder, like the Comma, which is expanding its range thanks to the warming climate. The Small Tortoiseshell’s larvae are gregarious, unlike the Comma; this might make Small Tortoiseshells easier targets for parasites. The warming climate is creating opportunities for new parasites to colonise Britain and Ireland. One new arrival in Britain, a fly named Sturmia bella, is already taking a large toll on British tortoiseshells, and it may colonise Ireland. The social habit of the larvae, which require greater food might also make it more vulnerable to drought.

The Small Tortoiseshell can fly in two or even three generations, but the second might be a partial generation while any third brood will be very small. Adult Small Tortoiseshells can usually be found flying from March-May (over-wintered adults), June-July (first brood) and late August-early October (second brood, small third brood).  All final brood Small Tortoiseshell butterflies enter hibernation, often in houses.

An interesting phenomenon has been recorded in a Small Tortoiseshell hibernaculum in St Albans, Hertfordshire, where the butterfly went into early hibernation in some years (Hull, 2019). In 2017 and 2018, all the Small Tortoiseshells that hibernated in Malcolm Hull’s unheated shed were hibernating by July 7th and August 5th, respectively. In 2016, the last five did not enter hibernation until mid-October. In 2017 and 2018, the butterfly had effectively abandoned the second generation and entered reproductive diapause. The weather during June and July 2018 was hot and dry and the Stinging Nettles needed for breeding may have become unsuitable.

Frank Smyth, a very experienced observer has noted the absence of Small Tortoiseshells at Howth, County Dublin during August and September in some years but their reappearance the following spring. The butterfly may be skipping its second generation at Howth too, and this may occur in other dry east coast locations during dry summers. A skipped second generation might also occur during cold summers. This ability to modify its brood strategy suggests a level of adaptability, much needed by a widely distributed species in an era of climate change.

Changes in appearance

Butterflies, moths and other insects, such as dragonflies, show morphological responses to climate and weather conditions. Thus, some butterflies produce smaller offspring during or following drought. Common Blue butterflies that emerge in summer following drought are often greatly reduced in size. This is the result of a reduction in the quality and or the amount of edible foodplant. In years with good rainfall and warmth, adults are larger.

The Dark Green Fritillary (to take one example) present in Ireland, Scotland and other cool climates are darker than their counterparts in sunnier, warmer climates, an adaptation to overcast, cool conditions. As our climate becomes sunnier and warmer, it is likely that our darker Dark Green Fritillaries will become paler.

A Climate Warming Success

One butterfly that has successfully tracked climate change is the Comma. This butterfly is a fascinating example of a butterfly whose development and brood structure are adapted to daylight length. Increasing hours of daylight see eggs laid in late April and early May producing direct breeding adults that breed soon after they emerge from their pupae in June and July. These are paler, well designed for bright sunshine. Eggs laid later in spring produce mainly dark adults which postpone reproduction until the following spring. Even more intriguing is the ability of the caterpillar to switch to producing the adult form best adapted to the conditions. Swedish research found this is influenced by two main factors: the length of daylight experienced by developing caterpillars and whether days become shorter or longer as they grow (Nylin, 1989).

The light form of the Comma butterfly. This form breeds directly after hatching in summer. This photograph was taken on July 03 2023 and shows a female basking on bramble. She will soon lay eggs and will die long before winter.
This is the dark (normal) form of the Comma butterfly. She will not breed until spring. She passes the winter by taking shelter in wooded areas. She is darker overall on her uppersides, but her undersides are very much darker than those of the light form. The dark undersides are appropriate for an over-wintering butterfly. A photo of the underside of the dark form and light form is shown below.
The underside of a dark form Comma. The underside of the light form can be seen below.
The underside of a light form Comma.

I suspect the Comma’s development might also be accelerated by increasing night warmth. It is known that caterpillars can develop into the direct breeding form even when day-length is declining if the temperature is warm, and they are feeding on very nutritious food (Thomas and Lewington, 2014). In captive breeding indoors during spring, I observed Comma larvae feeding at night, in darkness, likely benefiting from indoor warmth. Despite darker conditions indoors, all the larvae I reared in spring produced pale, direct-breeding adults. The adults hatched shortly before and around 18 June 2014. Warming night temperatures may increase the percentage of direct-breeding Commas hatching in June and July, even when day length is declining after the summer solstice (June 21st).

Furthermore, warmer daytime spring weather arising from climate change will likely result in more direct-breeding adults for two reasons: earlier adult emergence and faster development of eggs and larvae. Over-wintering adults emerge earlier if spring weather is warm and sunny, and breeding may occur earlier, followed by earlier egg-laying on earlier developing nettles, earlier hatching and faster development of larvae under warmer conditions, meaning that more larvae will be developing when daylight is still increasing.

The direct-breeding adult Commas (light form) that hatch from pupae in June and July reproduce and die weeks before August ends. Their offspring develop mainly during July, August and September, when day length is declining, and nettles are less lush than they were in spring. The resulting adults are dark and these delay breeding until spring after hibernation. However, Nylin (1989) states that some exceptions occur with some dark male adults breeding in summer (under laboratory conditions) and some pale adults over-wintering (Eeles, 2019). (Nylin also found that only dark (normal form) adults are produced in the wild in Sweden.)

The Future

Braschler and Hill (2007) investigated the Comma’s expansion and point out that polyphagy (the ability to use several foodplant species) may enhance the ability of species to track climate change although the females sometimes chose suboptimal foodplants (Gamberale-Stille et al. 2014). There are species (such as the Small Blue, Small Tortoiseshell and Marsh Fritillary) that use just one foodplant, leaving them vulnerable to changes that negatively affect their food. Prolonged summer drought will affect many moisture-dependent species, such as the Ringlet and Green-veined White. No longer will these be the abundant, widespread species we know today.

Nobody told me there’d be days like these. Extreme warmth and dryness can cause grassland butterflies to emerge crippled from their pupae, like this unfortunate Ringlet butterfly.

There is evidence that climate change is challenging efforts to protect rare and endangered butterflies. Climate change and nitrogen deposition, or a combination of the two, are considered likely candidates for driving habitat change leading to longer growing seasons and increased grassiness on the Morecambe Bay limestones in northwest England, leading to 50% less food (violets) and a cooler micro-climate for the warmth-loving larva of the High Brown Fritillary (Ellis et al. 2019). This effect may be occurring in the Burren, impacting our very rare Pearl-bordered Fritillary butterfly.

Lying Bracken trash among violets creates a warm, dry micro-climate for Pearl-bordered Fritillary caterpillars. Climate warming and atmospheric nitrogen deposition leads to increased grassiness, lowering temperatures just above the soil, making the larval micro-site less suitable or unsuitable for the caterpillars.

The future picture of our environment under the range of potential climate change scenarios is not clear. Predictions at the level needed to anticipate the ecological impacts on butterflies and moths may not be possible. The most immediate answer is to protect the habitats we have and restore what can be restored. Large-scale habitats with the best conservation management and the fullest range of conditions that apply to habitats are the best safeguards against our changing climate.

Landscapes must be protected for nature, not small nature reserves which by virtue of size and the range of species they can protect are of little value against biodiversity loss and climate change.

Much of the information in this article is from The Irish Butterfly Book by Jesmond Harding. For more information about the way climate and related factors are hitting our butterflies, read The Irish Butterfly Book. Contact the author by email:

All photos © J. Harding


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Nylin, S. (1989). “Effects of changing photoperiods in the life cycle regulation of the comma butterfly, Polygonia c-album (Nymphalidae)”. Ecological Entomology, 14(2), 209–218. doi:10.1111/j.1365-2311.1989.tb00771.x

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