Identifying temperature cues driving increased voltinism in a geometrid moth

Identifying the environmental cues that determine the timing of developmental processes is vital to understanding the effects of climate change on populations. However, as developmental processes are inherently difficult to measure directly at the population level, the drivers and potential consequences of change in their timings remain unknown in most species. Here we explore the use of long-term monitoring data for assessments of change in the number of generations per year and its impact on abundance, demonstrating new applications for a rapidly growing data source. Data derived from a light trap in west-central Scotland operated over 56 years (1968 to 2023) showed that the small phoenix moth, Ecliptopera silaceata, switched from a univoltine to bivoltine generation pattern. This voltinism change was predicted by an increased minimum temperature in a critical time window towards the later part of the first generation's flight period. The population shows positive density dependence and the change in voltinism has no significant negative effect on population size, indicating no evidence of a developmental trap that has been postulated for other species. These results identify some of the proximate mechanisms of developmental responses to climate change in general and in voltinism in particular, specifically highlighting the importance of sustained temperature above minimum thresholds for development. These results could also help to make predictions about future changes in population sizes under climate change and increasing voltinism, in addition to how these changes may differ between species.