Ninety-year trends reveal sharpest insect declines mid-20th century

Growing concern about insect declines has spurred increasing research into temporal trends in insect communities. Most studies, however, focus only on the past few decades. Yet many anthropogenic drivers of insect trends – such as land-use change and climate change – have been affecting insect communities for much longer. As a result, major shifts in insect diversity probably occurred earlier. Consequently, the baseline for studies based on time series starting in the late 20th century or even later might have shifted. This risks leading to misleading conclusions or poor conservation decisions.To address this, we extended insect trends back nine decades by analysing 1.2 million records of 595 saproxylic beetle and 216 butterfly species. Using occupancy-detection models, we reconstructed continuous trends in species richness in Switzerland from 1930 to 2021 and related these trends to land-use change, climate change, and to species traits.From the 1930s to the 1960s, the species richness of both studied insect groups declined. While the richness of saproxylic beetles stabilised and subsequently recovered, the richness of butterflies continued to decline until the 1980s and has not recovered since. As a consequence, the average richness of butterflies today is 12% lower than in 1930. The strong mid-century decreases were linked to increases in agricultural mechanisation, while the subsequent increases were linked to climate warming. Across the full 90-year period, declines disproportionately affected specialist and cold-adapted insect species. Since the acceleration of climate warming in the 1980s, warm-adapted species have increased in both saproxylic beetles and butterflies. Recent gains in saproxylic beetle richness may also reflect increased deadwood availability from windthrow and biodiversity-friendly forest management.Our unprecedented long-term analyses reveal the strongest insect declines in the mid-20th century, with only partial recovery since. Our findings highlight the importance of accounting for shifted baselines when interpreting short-term evaluations of insect trends.