Ninety-year trends reveal sharpest insect declines in the mid-twentieth century.

Most studies on insect trends have focused on the past few decades, but major changes could have occurred earlier. If so, shifted baselines can result in misleading conclusions or poor conservation decisions. Here we analyse nine decades of insect trends in Switzerland and relate them to changes in land use and climate. By reconstructing continuous trends in species richness based on 1.2 million records of 595 saproxylic beetle and 216 butterfly species, we find that both groups declined from the 1930s to the 1960s. While saproxylic beetle richness stabilized and subsequently recovered, butterfly richness continued to decline until the 1980s and has not recovered. The strong mid-century decreases were linked to increases in agricultural mechanization, while the subsequent increases were linked to climate warming. Over the entire 90-year period, declines primarily affected specialist and cold-adapted species, while warm-adapted species have increased since the 1980s. Recent gains in saproxylic beetle richness might also reflect increased deadwood availability from windthrow and 'biodiversity-friendly' forest management. Our findings suggest that reducing adverse land use is key to maintaining and promoting insect diversity, and that shifted baselines and climate-change effects should be considered when setting restoration goals and priorities.