Big Butterfly Count: What do recent trends tell us about butterflies?

WARM, sunny weather is generally beneficial for butterflies, but can be a double-edged sword if persisting for prolonged periods with intense heat and long-lasting drought, and borne out in projections and predictions by experts at Butterfly Conservation (a British conservation organisation and charity). They say the heat and drought in 2022 hit many common butterflies and caused numbers to crash as the summer progressed.

Common butterflies including the likes of peacock (Aglais io) and small tortoiseshell (Aglais urticae), which overwinter in the adult (imago) stage, were seen in their usual good numbers in spring and early summer 2022, but subsequent 2022 generations were hit hard as heat and drought intensified throughout the summer months. This is because the food plants for their larvae (caterpillars) dried up, leaving fewer larvae to pupate and eventually emerge as adult butterflies in late summer and early autumn. 

Calculations and findings from the UK Butterfly Monitoring Scheme (started in 1976), led by Butterfly Conservation and the UK Centre for Ecology & Hydrology, strongly suggested that many generally common butterflies would be much rarer in 2023.

The question was – would the biggest impact be on species like the marbled white (Melanargia galathea), meadow brown (Maniola jurtina) and small skipper (Thymelicus sylvestris) which only have one generation per year? The situation became clearer as summer 2023 unfolded and the results from the Butterfly Conservation big annual survey for 2023 were released. 

What’s more, the heat and drought in 2022 may have exacerbated the situation because grasses which are the food plants for many butterfly larvae are typically the first plants to dry up during heat and drought. For instance, Yorkshire fog (Holcus lanatus) is the main food plant for the small skipper, while meadow brown butterflies feed on a range of grasses including cocksfoot (Dactylis glomerata), bents (Agrostis species) and meadow grasses (Poa species). Marbled white has a liking for cocksfoot, timothy (Phleum pratense) and red fescue (Festuca rubra) and sheep’s fescue (Festuca ovina).

Dr Richard Fox, head of science at Butterfly Conservation, said as a general rule warm and sunny weather is beneficial for butterflies because at such times they are active in locating food sources, mating and oviposition (egg lay). That said, drought is a major problem for butterflies because it may be difficult to find suitable locations to lay their eggs, or sufficient food plants for larvae, the drought having dried up and desiccated the plant species they rely on. The knock-on effect is that it leads to fewer individuals in the next generation.

Butterfly conservation says some species never recovered after the ‘infamous’ UK drought of 1976, although the situation could be far graver today. That’s because unlike 1976 the majority of British butterfly species are already declining. In fact, the 2022 ‘State of British Butterflies’ report produced by Butterfly Conservation showed how four fifths of species had declined in number or distribution or both since the 1970s. 

Butterflies in 2023

Holly blue butterfly is one of few native butterflies appearing to have had a good year in 2023, although the reason is not altogether clear. (Image: eA)

In March 2023, just as the first early flyers like small tortoiseshell, peacock and comma (Polygonia c-album) were about to emerge from winter hibernation, fears for some of Britain’s most colourful and common native butterflies were expressed.

Fast-forward four months to July 2023 and Butterfly Conservation’s worst fears were realised with spring populations of peacocks and small tortoiseshells significantly lower than normal. The caterpillars (larvae) of these species would have been feeding on nettles late last summer (2022). 

Dr Zoe Randle, a senior surveys officer at Butterfly Conservation, said: “What we’re really concerned about is survival rates of caterpillars from last summer (2022). Small tortoiseshells have had a dreadful year so far, as have peacock butterflies, and the whites are doing quite poorly as well.” 

The greatest fear was for grass-feeding species like ringlets (Aphantopus hyperantus) because grass species are the first plants to suffer from heat and drought, with meadows, fields, parkland and verges all drying up and browning during summer 2022. 

Butterfly Conservation’s ‘Big Butterfly Count’ got underway on July 14 2023 and was scheduled to last for several weeks, but even before the count closed Butterfly Conservation was able to comment on particular species. By the first week of August, there had been 170,000 sightings of the red admiral butterfly (Vanessa atalanta) representing a massive fourfold increase in numbers compared with the previous year. 

The red admiral butterfly traditionally migrated to the UK from the Mediterranean regions of southern Europe and North Africa. Red admiral butterflies arrived in spring to breed then flew back south to the Mediterranean area at the end of summer. In recent years the trend has been broken with increasing numbers of red admirals now overwintering in the UK because our winter weather is no longer too cold for their survival.

Dr Randle said: “We’ve been surprised to see the red admiral taking the lead.

"However, with the increased frequency of warm weather, the UK may well become a permanent home for this species.” 

Another butterfly which has apparently bucked the trend is the holly blue butterfly (Celastrina argiolus) but perhaps for a completely different reason. South Hertfordshire (my neck of the woods) was comparatively bereft of butterflies, with not a single early flying peacock or comma seen by me in March or April 2023. One exception was the holly blue, with hedgerows and gardens alive with these dainty-blue butterflies continually on the move. 

There could be a good reason why this tiny, azure-coloured, native British butterfly has done relatively well in spite of the heat and drought in 2022. Holly blue is almost unique in having two evergreen plants (English holly and common ivy) as food plants for its larvae. The first generation of adults lays its eggs on holly and the second generation on ivy, and, of course, neither of these plants would have suffered unduly during the extraordinarily hot and dry summer of 2022. However, my colleague Dr Roderick Robinson warns about jumping to conclusions about numbers of holly blue butterflies. He says numbers are predictably cyclical over a period of years, a phenomenon thought to be caused by parasitism by the wasp Listrodromus nycthemerus, sole host of which is the holly blue butterfly.  However, other species did not fare so well and particularly the small tortoiseshell.

Peacock butterfly was apparently hit hard by the heat and drought of summer 2022. There was a distinct dearth of early fliers in spring 2023 from the population that normally overwinters in the adult butterfly stage. (Image: eA)

Traditionally one of Britain’s most widespread and frequently occurring butterfly species, the small tortoiseshell started to crash some ten years ago and shows no signs of real recovery. An endearing memory of late summer in the late 1950s is a big purple Buddleia bush in my aunt’s garden covered with butterflies. I can still picture them today as masses of small tortoiseshell butterflies, but I have not seen any for quite a few years now, and definitely not in the summer of 2023.

Butterflies have been notably scarce in my garden, although Dr Robinson at Leavening, near Malton in North Yorkshire, reports how his garden was awash with butterflies, in stark contrast to mine in the south of England. But perhaps this is not unexpected because, as the UK climate warms up, native British butterfly species will logically move northwards, while the south of England may see ingress and establishment of butterfly species normally resident in continental Europe.