The unprecedented heatwave of 2026 has resulted in one of the most successful butterfly seasons in the UK over the past 16 years, as reported by the Big Butterfly Count. This global initiative, which involved over 100,000 volunteers, recorded nearly 1.4 million instances of common butterflies and day-flying moths, reflecting a significant increase in butterfly sightings during the summer’s five heatwaves.

Among the most abundant species were painted ladies, a migratory butterfly that travels from sub-Saharan Africa to the UK each summer. Their population surged by 335% compared to the previous year, marking the second-highest count recorded by Butterfly Conservation since the initiative began in 2010.

Notably, both the holly blue and gatekeeper butterflies achieved their best-ever results in the Big Butterfly Count, while the small copper butterfly also had its third-best performance. Overall, this year’s findings represent the most favorable conditions for butterflies since the summer of 2019.

However, Butterfly Conservation has raised concerns about the potential long-term effects of drought conditions experienced in many regions of Britain. The extreme dryness this summer may have adversely affected the caterpillars that will emerge as adult butterflies next year, as many vital plants, including grasses and wildflowers, were severely impacted by the lack of moisture.

Richard Fox, the head of science at Butterfly Conservation, expressed that while adult butterflies thrive in hot, sunny weather, the conditions favorable for adult butterflies do not necessarily indicate a healthy population trend. He emphasized the significance of the environment for caterpillar survival and the plants they rely on, cautioning that the same weather patterns that benefited adult butterflies could lead to a population decline with long-lasting effects.

Historical patterns show that previous excellent butterfly seasons, such as those in 1976 and 1995, were followed by population crashes due to drought, although the droughts of 2018 and 2022 did not yield similar outcomes. Experts note that while droughts can generally be detrimental to butterfly populations, each drought event is unique.

Independent naturalist Matthew Oates highlighted that the drought of 1976 was worsened by preceding dry conditions, unlike this summer, which followed a wetter winter. He noted that the poor butterfly outcomes of 1977 were influenced not just by drought, but also by adverse weather conditions during spring and one of the wettest June months recorded.

Oates reported that he observed healthy survival rates for key butterfly food plants, such as horseshoe vetch, in areas like the Cotswolds and Wiltshire, aside from south-facing slopes. He expressed cautious optimism that the preceding wet winter and some rainfall in early June could mitigate the risk of a population crash next year.

Despite this being a remarkable summer for migratory species, such as the clouded yellow and non-native Queen of Spain fritillary, which have been spotted in significant numbers, some drought-sensitive species have shown declines in their populations. The speckled wood, green-veined white, and ringlet butterflies were all recorded in lower-than-average numbers, reflecting declines of 44%, 54%, and 39%, respectively, since 2011.

Fox noted that while certain butterfly species may benefit from hotter summers, few can withstand the extreme drought conditions experienced this year across much of the UK. Both Fox and Oates concurred that the nation is entering a new era regarding how climate extremes will affect local wildlife and insect populations.

Fox warned that if future summers bring similar drought or excessive heat, it could signify a significant shift for both wildlife and butterflies. Oates remarked that the current weather patterns are unlike any he has recorded over the past 62 years, indicating a departure from familiar climatic norms and raising questions about the relevance of existing knowledge in the face of changing conditions. He emphasized the need for increased research and expertise in entomology to address these challenges.

Here are some key results from the Big Butterfly Count:

1. Small white: 281,634 (+32% from average)
2. Gatekeeper: 211,029 (+50%)
3. Large white: 190,295 (+19%)
4. Red admiral: 141,178 (+39%)
5. Meadow brown: 134,335 (-15%)
6. Painted lady: 85,508 (+161%)
7. Common blue: 54,430 (-16%)
8. Peacock: 54,414 (+39%)
9. Holly blue: 43,849 (+128%)
10. Small tortoiseshell: 33,123 (-53%)