A number of recent studies have offered an interesting glimpse into how warming temperatures are affecting the UK’s butterfly population – and how the actions of arborists and gardeners can make a difference.
CLIMATE change and the impacts of a progressively warming climate over the British Isles can push in both directions, with native butterflies as a prime example.
Ask anyone with an interest in butterflies about which species evoke the most vivid back garden memories from childhood, and the answer will almost certainly be small tortoiseshell, red admiral and peacock.
The annual UK Butterfly Monitoring Scheme (UKBMS) recently released the results of its annual survey for 2023, which showed worrying declines for some and heartening increases for others. Small tortoiseshell, once Britain’s most frequently seen butterfly, falls into the former category with the lowest numbers recorded in 48 years and a decline of 82 per cent since the UKBMS began in 1976.
On the other hand, the red admiral attained its highest-ever recorded count and is now common in all habitats, having increased by 318 per cent at monitored sites since 1976.
If climate warming is the bedrock underpinning these changes then how exactly is it operating and in diametrically opposite ways on these two species? The first thought coming to mind is the possible effect on larval food plants and flowers on which the adult butterflies nectar. That does not look at all promising, because the larvae (caterpillars) of both species feed on members of the Urticae (nettle family – e.g. stinging nettle), while flowers borne by the Buddleia bush are the favourite sources of nectar for the adult butterflies of both species. Stinging nettles are no less frequent than they were 50 years ago, while Buddleia, the invasive weed is, if anything, more frequent now than half a century ago.
Butterfly Conservation has looked in detail at the predicament of the small tortoiseshell and tried to come up with some answers. Its best guess appears to be the effect of climate warming on the relationship between small tortoiseshell and its natural enemies, in particular an exotic parasitoid (Sturmia bella), a tachinid fly, which arrived in Europe in the late 1990s. The fly lays its eggs on nettle leaves, which are subsequently ingested by small tortoiseshell larvae. Fly grubs hatch from the eggs inside the butterfly larvae which is killed after the larva pupates.
However, it is clearly not as simple as that, even though small tortoiseshell is doing much better in Ireland than Britain which could indicate a differential climatic effect. That’s because Sturmia bella also parasitizes other native British butterflies, including the peacock, which have not suffered the same catastrophic decline.
A contributing factor to the fall in numbers of small tortoiseshell could be periods of dry weather occurring in mid-summer, reducing the quality of the butterfly larvae’s food plants. This in turn could result in extended development times for the larvae or cause the adult butterfly to ‘cancel’ reproduction in the year of its birth and subsequently go into a reproductive diapause until the following spring.
This would require an extended hibernation with adult butterflies going into their dormant state during July instead of September/October. This in turn helps the small tortoiseshell butterfly to avoid having a second brood. Being later in the season, this would leave it much more at risk from the effects of malnutrition and the Sturmia bella parasitoid than its first brood, which was produced under much more favourable conditions during spring.
The situation for the small tortoiseshell is clearly complicated with no immediate, ready and clear-cut remedy, but in direct contrast to that for the red admiral butterfly. Migrating to the British Isles from North Africa and continental Europe each spring, the immigrant females lay eggs which hatch to give a generation of fresh butterflies from July onwards. Although most red admirals traditionally fly back to North Africa and Europe in the autumn, an opportunistic few always pushed their luck and hibernated in Britain. They survived in mild winters but were wiped out in harsh ones. Climate warming in recent years means many more are staying on, hibernating over winter and surviving to emerge in spring. If only everything to do with climate change was so clear-cut.
COUNTING BUTTERFLIES IS GOOD FOR THE SOUL
Counting sheep gets you off to sleep, but counting butterflies has more wide-ranging and deeper benefits by reducing anxiety and boosting your sense of connection with nature. In other words, counting butterflies is good for your soul.
Surveys carried out on a sample (382) of the participants in the Big Butterfly Count 2022 before and after a 15-minute period counting butterflies revealed a nine-per-cent reduction in anxiety and a greater sense of connection with nature.
Follow-up surveys conducted on 345 of the original 382 participants after five weeks showed the increased awareness of butterflies had been maintained.
The Big Butterfly Count is carried out annually by scientists from Butterfly Conservation and University of Derby to track and monitor how these insects are faring.
This is exactly the type of story which is picked up by the national press. The spread I saw illustrated the story with a picture of a small tortoiseshell butterfly (Aglais urticae). Most of the participants would have been waiting around a long time to even see one of these butterflies today, let alone count them, although just a few years ago things would have been different. Once one of Britain’s most frequently seen garden butterflies, especially on Buddleia blossom, the small tortoiseshell has become increasingly uncommon with numbers falling by almost 75 per cent over the last half century.
KICK MOWING INTO THE LONG GRASS
Gardeners are bombarded with pleas from biodiversity campaigners – especially lepidopterists – to adopt more relaxed lawn maintenance, specifically through the NoMowMay campaign, assured they will be rewarded with much more butterflies.
Now the analysis of six years of butterfly surveying across 600 British gardens has furnished the first scientific evidence that wilder lawns with grass allowed to grow long does indeed boost numbers. The benefits were most pronounced in gardens within intensively farmed landscapes, with up to 93 per cent more butterflies found and a greater range of species. Gardens with long grass in urban areas showed an 18 per cent boost to butterfly abundance.
Another bonus revealed by the study was flowering ivy growing up trees, walls and in hedges. The presence of flowering ivy increased the numbers of the holly blue butterflies for which ivy (and holly) are food plants for the larvae, while red admiral and comma butterflies nectar on the flowers when they appear at their perennially peculiar time of September and October.
Butterfly Conservation says the benefits of long grass to butterflies and other invertebrates is likely to be found in other grassy public spaces too, such as parks, school grounds, allotments and road verges. Through its Wild Spaces programme, the charity aims to transform 100,000 such areas across the UK to enhance and sustain populations of butterflies.
However, to furnish and sustain an amenable habitat for butterflies and their breeding activities, the grass must be left in a long condition through spring and summer and until late September or October, and then cut lightly, so as not to cut short butterfly breeding which differs markedly between species. Taking part in NoMowMay and then starting up the mower in June will clearly defeat the whole object of the campaign.
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