Many latitudinal insect migrants including agricultural pests, disease vectors, and beneficial species show huge fluctuations in the year-to-year abundance of spring immigrants reaching temperate zones. It is widely believed that this variation is driven by climatic conditions in the winter-breeding regions, but evidence is lacking. We identified the environmental drivers of the annual population dynamics of a cosmopolitan migrant butterfly (the painted lady Vanessa cardui) using a combination of long-term monitoring and climate and atmospheric data within the western part of its Afro-Palearctic migratory range. Our population models show that a combination of high winter NDVI (normalized difference vegetation index) in the Savanna/Sahel of sub-Saharan Africa, high spring NDVI in the Maghreb of North Africa, and frequent favorably directed tailwinds during migration periods are the three most important drivers of the size of the immigration to western Europe, while our atmospheric trajectory simulations demonstrate regular opportunities for wind-borne trans-Saharan movements. The effects of sub-Saharan vegetative productivity and wind conditions confirm that painted lady populations on either side of the Sahara are linked by regular mass migrations, making this the longest annual insect migration circuit so far known. Our results provide a quantification of the environmental drivers of large annual population fluctuations of an insect migrant and hold much promise for predicting invasions of migrant insect pests, disease vectors, and beneficial species.

包括农业害虫、疾病媒介和有益物种在内的许多纬度昆虫迁徙在到达温带的春季迁徙者的逐年丰度中表现出巨大的波动。人们普遍认为，这种变化是由冬季繁殖区的气候条件驱动的，但缺乏证据。我们确定了国际化移民蝴蝶（彩绘女士Vanessa cardui) 在其非洲-古北迁徙范围西部使用长期监测以及气候和大气数据的组合。我们的人口模型表明，撒哈拉以南非洲大草原/萨赫勒地区的高冬季 NDVI（标准化差异植被指数）、北非马格里布的高春季 NDVI 以及迁徙期间频繁的有利顺风是三个最重要的因素。西欧移民规模的重要驱动因素，而我们的大气轨迹模拟显示了风载跨撒哈拉运动的定期机会。撒哈拉以南植物生产力和风力条件的影响证实，撒哈拉两侧的彩绘女士人口与定期的大规模迁徙有关，使其成为迄今为止已知的最长的年度昆虫迁徙路线。我们的研究结果量化了昆虫迁徙每年大量人口波动的环境驱动因素，并为预测迁徙害虫、疾病媒介和有益物种的入侵提供了很大希望。