Tracking dragons: stable isotopes reveal the annual cycle of a long-distance migratory insect

Insect migration is globally ubiquitous and can involve continental-scale movements and complex life-histories. Apart from select species of migratory moths and butterflies, little is known about the structure of the annual cycle for migratory insects. Using stable-hydrogen isotope analysis of 852 wing samples from eight countries spanning 140 years, combined with 21 years of citizen science data, we determined the full annual cycle of a large migratory dragonfly, the common green darner (Anax junius). We demonstrate that darners undertake complex long-distance annual migrations governed largely by temperature that involve at least three generations. In spring, the first generation makes a long-distance northbound movement (>650 km) from southern to northern range limits, lays eggs and dies. A second generation emerges and returns south (>680 km), where they lay eggs and die. Finally, a third resident generation emerges, reproducing locally and giving rise to the cohort that migrates north the following spring. Since migration timing and nymph development are highly dependent on temperature, continued climate change could lead to fundamental changes in the biology for this and similar migratory insects.

昆虫迁徙在全球范围内广泛存在，其迁徙规模可覆盖大陆尺度，且伴随复杂的生活史。除少数迁徙性蛾类与蝶类外，学界对迁徙昆虫的年度生活周期结构仍知之甚少。本研究通过对覆盖140年、源自8个国家的852份翅部样本开展稳定氢同位素分析（stable-hydrogen isotope analysis），并结合21年的公民科学观测数据，明确了大型迁徙性蜻蜓——普通绿晏蜓（Anax junius）的完整年度生活周期。研究表明，该晏蜓会进行主要受温度调控的复杂长距离年度迁徙，其生命周期至少涵盖三代。春季时，第一代个体从分布范围南界向北长距离迁徙（航程超650公里）至北界，产卵后死亡。第二代个体羽化后向南回迁（航程超680公里），抵达南界后产卵死亡。最终第三代定居型个体羽化，在本地完成繁殖，其后代将在次年春季开启北迁旅程。鉴于迁徙时间与若虫发育均高度依赖温度，持续的气候变化可能会对该物种及其他同类迁徙昆虫的生物学特性带来根本性改变。