Trans-Saharan migratory patterns in Vanessa cardui and evidence for a southward leapfrog migration

Some insects, such as the painted lady butterfly Vanessa cardui, exhibit complex annual migratory cycles spanning multiple generations. Traversing extensive seas or deserts is often a required segment of these migratory journeys. We develop a bioavailable strontium isoscape for Europe and Africa, and then use isotope geolocation combining hydrogen and strontium isotopes to estimate the natal origins of painted ladies captured north and south of the Sahara during spring and autumn, respectively. Our findings reveal moderate migratory connectivity across the Sahara characterised by a broad-front, parallel migration. We also report evidence of a leapfrog migration, wherein early autumn migrants from higher latitudes cover greater distances southward than their late autumn counterparts. This work represents a major advancement in understanding insect migratory patterns and connectivity, particularly across extensive barriers, which is essential for understanding population dynamics and predicting the impacts of global change on insect-mediated ecosystem services.