Orientation tests and long-term movement phenology establish the red admiral Vanessa atalanta as an applicable model for navigation research in migratory butterflies

Abstract

Animal migrations are disappearing globally, while insect populations are on alarming declines. Both ecosystem degradations, influenced by unpredictable impacts of climate change, are also exacerbated by human activities such as intensified land use and various forms of environmental pollution. Butterfly migrations may serve as sensitive indicator phenomena of these broader environmental changes. While the transcontinental journeys of one of the most famous Lepidopteran species, the North American monarch butterfly, Danaus plexippus, are documented in depth, they are a geographically restricted phenomenon. Comprehensive studies from other areas and on other migratory butterflies like the European red admiral, Vanessa atalanta, are notably sparse. In addition, the details of their navigational capacities and how they might be affected by the aforementioned changes remain largely enigmatic. Against this backdrop, we seek to establish the red admiral as a model for insect movement phenology and navigation behaviour which both might be impacted by environmental changes. Employing a combination of orientation tests, utilizing flight-simulators and free-flight trials during late summer, together with a 23-year study on movement phenology at a coastal migration flyway, the Baltic Sea coast, we offer broad insights into red admiral migration. In our experiments, butterflies exhibited a southwestern orientation on the Courish Spit and chose a south-southeastern trajectory in free-flight trials after translocation at the Latvian Baltic Sea coast. Directional records from decades-long trapping data, based on more than 16,000 individuals, match these findings. Nevertheless, we also found reverse movements to occur under some circumstances. At the same time, the observed estimated median dates of red admiral passages did change by one day only between decades, however, generally more butterflies were recorded in recent years. Our data thus suggest a certain degree of adaptability in the butterflies movement behaviour, indicating an innate migration schedule, possibly supported by a flexible navigational capacity. As the world is facing biodiversity loss at a high rate, long-term monitorings of indicator species become important. By establishing the red admirals as a model for butterfly migration, we expect insights into broader movement patterns and navigational strategies in Lepidoptera negotiating human-dominated environments, filling a crucial gap in our current understanding of these interdependent aspects of insect biology.