The Afrotropical breeding grounds of the Palearctic-African migratory painted lady butterflies (Vanessa cardui)

Migratory insects are key players in ecosystem functioning and services, but their spatiotemporal distributions are typically poorly known. Ecological niche modeling (ENM) may be used to predict species seasonal distributions, but the resulting hypotheses should eventually be validated by field data. The painted lady butterfly (Vanessa cardui) performs multigenerational migrations between Europe and Africa and has become a model species for insect movement ecology. While the annual migration cycle of this species is well understood for Europe and northernmost Africa, it is still unknown where most individuals spend the winter. Through ENM, we previously predicted suitable breeding grounds in the subhumid regions near the tropics between November and February. In this work, we assess the suitability of these predictions through i) extensive field surveys and ii) two-year monitoring in six countries: a large-scale monitoring scheme to study butterfly migration in Africa. We document new breeding locations, year-round phenological information, and hostplant use. Field observations were nearly always predicted with high probability by the previous ENM, and monitoring demonstrated the influence of the precipitation seasonality regime on migratory phenology. Using the updated dataset, we built a refined ENM for the Palearctic-African range of V. cardui. We confirm the relevance of the Afrotropical region and document the missing natural history pieces of the longest migratory cycle described in butterflies.This work was funded by the National Geographic Society (grant WW1-300R-18); by the British Ecological Society (grant LRB16/1015); by the Research and Conservation Projects of the Fundació Barcelona Zoo; by the grant PID2020-117739GA-I00/MCIN/AEI/10.13039/501100011033 of the Spanish Ministry of Science and Innovation and the Spanish State Research Agency to G.T.; by the grant LINKA20399 from the Spanish National Research Council iLink program to G.T., C.P.B., N.E.P., and R.V.; by fellowship FPU19/01593 of the program Formación de Profesorado Universitario (FPU) to A.G.-B.; by the Turkana Basin Institute, National Geographic Society, and Whitley Fund for Nature to D.J.M.; and by grant 2018-00738 of the New Frontiers in Research Fund (Government of Canada) to G.T. and C.P.B.Significance Abstract Results Field Surveys, Larval Hostplants, and Field-Based Model Validation Monitoring Results and Population Dynamics across Regions A Refined Model for the Afrotropical Region Discussion The Afrotropical Breeding Grounds of V. cardui: Multiple Generations Shift South Toward the Tropics Diversity and Phenology of Larval Hostplants in the Afrotropics The Ecological Relevance of Delimiting Spatiotemporal Distributions in Migratory Insects Conclusion Methods December-January Field Surveys and Year-Round Monitoring Spatiotemporal Ecological Niche Modeling Data, Materials, and Software Availability Acknowledgments Supporting Information Reference

A hydrogen isoscape for tracing the migration of herbivorous lepidopterans across the Afro-Palearctic range

Rationale Many insect species undertake multigenerational migrations in the Afro-tropical and Palearctic ranges, and understanding their migratory connectivity remains challenging due to their small size, short life span and large population sizes. Hydrogen isotopes (δ2H) can be used to reconstruct the movement of dispersing or migrating insects, but applying δ2H for provenance requires a robust isotope baseline map (i.e. isoscape) for the Afro-Palearctic. Methods We analyzed the δ2H in the wings (δ2Hwing) of 142 resident butterflies from 56 sites across the Afro-Palearctic. The δ2Hwing values were compared to the predicted local growing-season precipitation δ2H values (δ2HGSP) using a linear regression model to develop an insect wing δ2H isoscape. We used multivariate linear mixed models and high-resolution and time-specific remote sensing climate and environmental data to explore the controls of the residual δ2Hwing variability. Results A strong linear relationship was found between δ2Hwing and δ2HGSP values (r2 = 0.53). The resulting isoscape showed strong patterns across the Palearctic but limited variation and high uncertainty for the Afro-tropics. Positive residuals of this relationship were correlated with dry conditions for the month preceding sampling whereas negative residuals were correlated with more wet days for the month preceding sampling. High intra-site δ2Hwing variance was associated with lower relative humidity for the month preceding sampling and higher elevation. Conclusion The δ2Hwing isoscape is applicable for tracing herbivorous lepidopteran insects that migrate across the Afro-Palearctic range but has limited geolocation potential in the Afro-tropics. The spatial analysis of uncertainty using high-resolution climatic data demonstrated that many African regions with highly variable evaporation rates and relative humidity have δ2Hwing values that are less related to δ2HGSP values. Increasing geolocation precision will require new modeling approaches using more time-specific environmental data and/or independent geolocation tools.1 INTRODUCTION 2 MATERIALS AND METHODS 2.1 Known-origin sample set 2.2 Hydrogen isotope analysis 2.3 Development of hydrogen isoscape 2.4 Statistical analysis of uncertainty 2.4.1 Covariates 2.4.2 Multivariate regression model for intra-site variance 2.4.3 Mixed effect model for predicting residuals and for predicting hydrogen isotope in wings 3 RESULTS 3.1 Known-origin dataset and isoscape 3.2 Factors of hydrogen isoscape uncertainty 3.2.1 Intra-site δ2Hwing variance 3.2.2 Model residuals from δ2H wing–precipitation relationship 3.2.3 Multivariate linear mixed model predicting δ2Hwing values 4 DISCUSSION 4.1 Known-origin dataset and hydrogen isoscape 4.1.1 Isoscape for the whole Afro-tropics Palearctic range 4.1.2 Isoscape for Palearctic 4.1.3 Isoscape for Afro-tropics 4.2 Controls of hydrogen isoscape uncertainty and future improvements 4.2.1 Intra-site variance 4.2.2 Residuals in tissue–precipitation relationship 4.2.3 Implications for building insect isoscapes 5 CONCLUSION AUTHOR CONTRIBUTIONS ACKNOWLEDGMENT