Citizen science reveals a shift in the commonness and rarity of Trichoptera in the Netherlands

Understanding aquatic insect species responses to environmental change necessitates robust data on species occurrences. Using over 140,000 citizen-science records from the Netherlands, we analysed trends in adult caddisflies (Trichoptera) to explore temporal changes in 146 species with differing life-history traits, particularly summer diapause. Diapause, a state of suspended development, allows certain Trichoptera to withstand unfavorable environmental conditions. Our analyses indicate a disproportionate rise in observations of species with summer diapause, which is independent of ease of detection (i.e. a species' body size and rarity). This is exemplified by the increased prevalence of the caddisfly, Glyphotaelius pellucidus (Retzius, 1783), a species with an adult summer diapause. We interpret these results from the perspective of climate change. Summer droughts and high water temperatures have become more prevalent in recent decades, representing harsh conditions for aquatic larvae. Having an adult summer diapause helps species to avoid these harsh conditions, which is consistent with the observed changes. Conversely, species lacking summer diapause, such as Mystacides longicornis (Linnaeus, 1758), may be more vulnerable to warming. Our results underscore the value of citizen-science data in elucidating shifts in insect populations and offer insight into the adaptive significance of life-history traits.