Analysis: Improved weather conditions this summer have led to butterfly populations recovering after the crash in numbers in 2024
By Jesmond Harding, Butterfly Conservation Ireland
After a wet summer in 2023 and a cold summer in 2024, when butterfly numbers crashed, a rebound in butterfly numbers in 2025 was certain to be noticed and appreciated by nature lovers. The decline in 2024 was strongly evident in butterflies that visit our gardens and parks so the extent of the 2025 increase must be considered against our experience during the summer of 2023 and 2024.
However, the increase in populations in 2025 is real. There are several reasons for this; some are obvious, some are more speculative and hard to verify. The improved weather is a key influence. Research carried out in Britain shows that extreme heat during winter and extreme rainfall in spring and summer damage butterflies.
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From RTÉ Radio 1's Mooney Goes Wild in 2024, Jesmond Harding discusses why 2024 saw such a collapse in butterfly numbers in Ireland
Mild winters can damage populations; cold winters and hot summers are much better for butterflies. While summer 2025 is yet to be assessed, June was warmer than usual, with above-average rainfall, while July was warmer than usual with near-average rainfall, so the weather this summer has been highly favourable for butterflies.
Caterpillar foodplants received the moisture, warmth and direct sunlight to promote healthy growth. The warmth has accelerated the development of eggs, caterpillars and pupae making them available to predators for a shorter period. Species that are able to produce more than one generation of adult butterflies in one year are likelier to do so in these conditions.
The warmth has enabled butterflies to fly, feed, mate and lay eggs on healthy foodplants. In 2025, we avoided extreme winter heat, and prolonged extreme rainfall, and drought, which also damage butterflies. Warmth with normal rainfall during spring and summer is generally a positive for Irish butterflies. Overall , the weather conditions during 2025 led to population recovery after the crash in 2024.
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From RTÉ Radio 1's Countrywide, the importance of protecting butterflies
Why is the rise so dramatic?
Some butterflies can produce more than one generation. The Small Tortoiseshell, a garden favourite was at a shocking low in 2024 (between 2008 and 2024, it declined by 63%), but survivors laid their eggs on healthy nettles in good spring weather, developed rapidly producing a new generation as early as late May and in June. These in turn gave rise to a second generation that flew during July/August, some early in July, and some of these are likely to have produced a third generation, which is abundant now (mid-August 2025).
A butterfly that has never been known to produce a second generation in Ireland has done that this summer. The Dingy Skipper, a spring flyer, produced a small second generation in August 2025, to the amazement of seasoned observers. It is likely that this historic development is the result of the high levels of warmth, sunshine and moisture during March to August.
The Cryptic Wood White is another butterfly that has proved single-brooded in Ireland. It flies mainly from May to mid-July. The appearance of Cryptic Wood White butterflies in a few locations in Northern Ireland and Inishowen Head, Donegal in late July and early August is hard to explain. The tendency to produce a second generation in Ireland is higher further south, but only one Cryptic Wood White was seen in the south of Ireland (Kildare) in late July.
The weather in Europe
Another reason for the 2025 butterfly boom is the weather in Europe. Hot, dry weather creates drought, and butterflies capable of migration escape by altitudinal and geographical migration. By moving north, Clouded Yellow, Large White, Small White, Red Admiral and Painted Lady butterflies can find succulent foodplants for their caterpillars. We are certainly receiving an increased abundance from migrants arriving and breeding here. Their offspring are benefiting from our excellent moist, warm summer.
There are other factors, some arising from the crashes in 2024. When a butterfly population declines, predator abundance declines. With lower predator populations in 2025, our butterflies are showing higher survival rates. This can lead to a boom-and-bust cycle, as has been demonstrated in the Holly Blue butterfly's relationship with a key predator, the parasitoid wasp Listrodromus nycthemerus.
It is possible that foodplants that received less pressure from butterfly caterpillars in 2023 and 2024 grew more healthily in 2025.
No mow May
Improved management of our private gardens and public green spaces is boosting populations. 'No Mow May’ provides more nectar for spring butterflies, while allowing grassland rich in native flora to grow long in summer, helps butterflies to thrive. Fewer people may apply herbicides to their gardens and hedge banks; these toxins are devastating.
Public authorities are better informed now, and great work has been done to develop lowland hay meadows containing the native flowers and grasses butterflies need. This is evident in public parks such as Oldbridge House Estate (Drogheda), Castletown House (Celbridge) and Tymon North Park, (Tallaght).
Another reason we're seeing more butterflies in 2025 is that the better weather allows us to spend longer outdoors, so we see more butterflies. However, impression is not the sole reason for the increases we are seeing. Systematic recording in 2024 shows the declines clearly. We are getting out more, so we are seeing more, but there is more to see!
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Jesmond Harding is secretary of Butterfly Conservation Ireland. He is the author of The Irish Butterfly Book
The views expressed here are those of the author and do not represent or reflect the views of RTÉ
