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 (further than 650 km) from southern to northern range limits, lays eggs and dies. A second generation emerges and returns south (further than 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.

1. Introduction

Insects are the most species-rich and abundant group of macroscopic organisms on the planet, yet our understanding of the biological underpinnings of their annual cycles and migratory behaviours drastically trails behind that of vertebrate taxa [1]. Perhaps the best-known insect migrations are the multi-generational migrations of the monarch butterfly (Danaus plexippus) in eastern North America [2], and the painted lady (Vanessa cardui) in Europe [3], which migrate southbound each autumn from the northern reaches of their distribution into southern North America and Africa, respectively. Monarchs wait out the boreal winter [4] and the same individuals migrate northbound the following spring into the southern parts of their distribution where they lay eggs, beginning the next generation that continues the northbound progression of their lifecycle [2,3,5,6].

Few migratory insects have had their full annual cycles sufficiently described despite their profound cultural [7], ecological [8] and agricultural importance [9]. This disparity is owing largely to the difficulties with tracking small organisms over space and time, as well as their complex life histories that often require both terrestrial and aquatic habitats [10]. Among the migratory insects, almost all are suspected to employ complex multi-generational migrations (but see [7,8]) similar to the monarch butterfly and painted lady, where no single individual completes the entire migratory circuit [2,3,5,6,10,11]. The continental distances travelled [12,13], small body sizes and multi-generational aspect of their life histories pose significant challenges to understanding the fundamental biology of the full annual cycles of migratory insects. Adding to this complexity, movements, timing and life-history strategies are likely linked to photoperiod, weather, temperature and other environmental forces [1,3,11,14–16].

The common green darner (Anax junius) is a ubiquitous, charismatic migratory dragonfly found in North America. Its complex life-history includes both resident and migratory populations. The aquatic nymphs likely give rise to resident and migratory phenotypes because they exhibit two distinct developmental strategies [14]. Nymphs either undergo rapid development and emerge within a single season or develop slowly, enter diapause and emerge the following summer [14]. The developmental strategy is governed by the photoperiod experienced by the egg or early instars [17] and temperature [14]. Therefore, the seasonal timing of oviposition likely determines whether nymphs develop, emerge within a single growing season and migrate, or enter diapause.

Darner migration is considered an adaptive strategy because it spreads reproductive effort across multiple, widely separated water bodies and ameliorates the risk of predation, competition and drought [17,18]. Migratory movements are commonly observed in autumn and have been documented using direct [12] and indirect techniques [19]. Northbound migration in spring is suspected and supported by anecdotal observations but is not well documented [20]. Although it is known that common green darners migrate, no study to date has linked multiple populations over time and space to better understand the phenology of the annual cycle and the number of generations involved.

2. Methods and materials

We combined observational and stable-hydrogen isotope data to identify the timing of migration and emergence origins of darners. Two publicly available datasets were assembled to refine emergence assignments using stable isotopes. First, we acquired citizen science data on the first observed migratory adult common green darners (first flight dates) to determine the minimum temperature necessary for darners to migrate (electronic supplementary material, table S1). Daily mean temperature of the first flight date was 9.17°C, similar to the development threshold (8.7 ± 0.1°C) reported by Trottier [14]. In addition to determining the temperature threshold for migratory flights, we used occurrence records submitted by citizen scientists and museum collections to generate a habitat suitability surface that we included as prior information in the probabilistic isotope-assignments [21] (see §2a).

(a) Emergence assignments

We obtained contemporary and museum specimens from 681 locations from eight countries ranging from the tropics to the northern temperate zone (17.98° N to 51.14° N) that we used to describe migration phenology and the number of generations within the annual cycle for the common green darner. Specimens, which included exuviae, tenerals and mature adults, were collected in every calendar month spanning over 100 years (electronic supplementary material, table S2). Isotope analyses were performed on a small distal segment of the wing to determine where the captured individuals emerged (see electronic supplementary material).

(b) Migratory distance

Determining the exact emergence origin from stable isotopes is not possible given the uncertainty in geographical assignment and lack of longitudinal variation in the underlying isoscape. Therefore, we used a conservative distance estimator as an index of migration distance. We determined the minimum distance (kilometres) between the ‘likely' emergence origin surface and the location where the specimen was collected (i.e. shortest possible distance travelled). Migration distances were calculated using the gDistance function in the rgeos [22] package in program R [23].

(c) Annual cycle

We summarized the emergence origins within the hardiness zones (see electronic supplementary material, delineating populations) for each calendar month to describe the phenology of the annual cycle. Isotope-based emergence origins provide spatio-temporal information of adults but the phenology of nymph development and emergence timing is also needed to fully understand the annual cycle of darners.

3. Results

We combined data on stable-hydrogen isotopes of 852 common green darner museum and contemporary specimens spanning 140 years (1874–2013) from eight countries with citizen science observations to determine the emergence origin, continental migrations and annual cycle of the common green darner. Using citizen science observations, we found that northbound darner migration closely tracks a thermocline of 9.17°C (figure 1). We then used this temperature-dependent flight phenology to refine stable-isotope assignments, and found that adults originating from the southern range margin migrate a minimum of 659.46 ± 49.11 km to the northern range edge in March and April (n = 50; electronic supplementary material, figure S1a). Previous estimates of darner daily migration rates indicate this journey would take 55.42 ± 4.13 days to complete [12]. Therefore, the first migratory individuals arriving in the north likely emerge in the south in January and February. However, prevailing wind patterns in spring favour northbound migration, potentially allowing individuals to migrate faster [24]. If darners use high-altitude migratory flights to take advantage of fast-moving winds, the distances reported above could be covered more rapidly [11,13,25]. Figure 1. First flight date of adult common green darner and the latitude of the 9.17°C thermocline. The mean latitude of the thermocline was extracted along the observation longitude from 1994 to 2014. Error bars represent the standard error in the latitude of the thermocline on the first flight date. Data represented in figure 1 can be found in electronic supplementary material, table S1. Graphic was drawn by Matthew Dodder (https://neornithes.wordpress.com) and used with permission. (Online version in colour.)

Combining stable-isotope assignments and temperature-related nymph development [14], we found the annual cycle of common green darners is comprised of at least three generations (figure 2). The first generation emerges between January and May in the southern part of the distribution and migrates north into the northern reaches of the distribution. The first generation is complete by August, the last month when darners with southern stable isotopic origins are captured in the north (figure 2, August). The first cohort of the second generation—young from the previous year that overwinter as late-stage instars and eclose between May and July [14]—begin flying in May with 4% (2 of 53; figure 2, May) of individuals assigned to where they were captured. Spatio-temporal overlap exists between the first and second generations as northbound migration continues through July. The number of locally emerged darners in the north increases throughout the season, peaking in September when offspring of the first generation in April and early May eclose (May: 4%, 2 of 53; June: 22%, 16 of 74; July: 29%, 15 of 52; August: 50%, 5 of 10; September: 86%, 6 of 7; October: 67%, 2 of 3; electronic supplementary material, figure S3). This latter cohort of the second generation then migrates south. Figure 2. The full annual cycle of the common green darner is comprised of three generations. The first generation is migratory, emerges in the south between February and May and dies in the north. The second generation emerges in the north, migrates south and dies. The third generation, offspring of migratory individuals in the fall, is non-migratory and emerges in the south in November. The emergence origins of adults (colour ramp; grey = zero, red = many) captured at the sampling locations (black dots) in each month are shown around the perimeter. Graphic was drawn by Matthew Dodder (https://neornithes.wordpress.com) and used with permission.

Second generation darners with northern isotopic signatures are first detected in the south during August and September (figure 2, September), suggesting southbound migration begins in July and continues into October [12]. These southbound migrants are composed of darners that overwinter as nymphs and emerge in June and July and darners who develop within a single growing season that eclose in September. Southbound migration to the southern range edge peaks in September and October, with 64% (7 of 11) of green darners captured at least 680.97 ± 178.84 km from where they emerge in the north (electronic supplementary material, figure S1b). The second generation composed of individuals that emerge in the northern portion of the distribution ends in late October (figure 2, October). After this time, no individuals with northern isotopic signatures were captured in the south (figure 2, November), suggesting they die shortly after they arrive in the south. However, southbound migrants from mid-latitudes were captured in November (43%, 3 of 7; figure 2, November) and December (57%, 4 of 7), emerging a minimum of 226.13 ± 108.50 and 232.03 ± 93.28 km away from where they were captured, respectively. A third, non-migratory generation occurs in the south between November and January (figure 2, November–January). This generation must be resident because adults that emerge in November likely die before April and May when the first migratory individuals are captured in the north, based on the adult longevity reported for the species (five to seven weeks [16]). Thus, third generation adults that emerge in November likely give rise to the first generation of migratory individuals that are captured in the north. Stable isotopes, however, are unable to differentiate between the first and proposed third generation.

4. Discussion

The annual cycle of green darners is composed of at least two migratory and one non-migratory generations. In spring, the first generation makes a long-distance northbound movement (659.46 ± 49.11 km) from southern to northern range limits, lays eggs and dies. A second generation emerges and returns south (680.97 ± 178.84 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.

During spring, individuals with southern emergence origins arrive at northern locations prior to those with origins from mid-latitudes (electronic supplementary material, figure S2). A similar pattern is observed in autumn, where individuals with northern origins are captured in the southern portion of their distribution before green darners that emerge at mid-latitudes. This migratory phenomenon could arise from several alternative life-history strategies. First, such patterns could result if individuals spread reproductive risk across multiple, widely separated water bodies and reproduce during their migratory journey north [18]. Their offspring could take advantage of warm surface waters, mature rapidly within the same growing season and migrate northbound (electronic supplementary material, figure S4). Such a strategy is akin to the multi-generational migration of the monarch and painted lady butterflies [1,2], however, the longevity and strong direct flights of darners allow populations to be connected over larger spatial scales compared to the northbound multi-generational progression of migratory butterflies. The observed pattern of shortened migratory distances as the season progresses also could result if individuals reproduce during southbound migration. If darners reproduce during the journey south, their eggs would hatch but the nymphs would enter a state of diapause as temperatures drop, emerge and migrate north the following spring when suitable thermal conditions allow [14,18]. A combination of these strategies is likely. Plasticity in nymph development further complicates discerning between the two strategies. Eggs laid during migratory periods could either (i) undergo rapid development after hatching and emerge within the same season or (ii) hatch, enter diapause as nymphs and emerge the following spring, depending on the photoperiod experienced by the egg or early instar [17]. Stable-hydrogen isotope data cannot differentiate between these competing hypotheses. Regardless of the strategy, the timing of migratory movements allows for spatial and temporal overlap of both migratory and non-migratory populations, explaining the lack of genetic structure in this species [18,26].

Common green darner migration appears to be governed in part by temperature. Northbound adult migration during spring coincides with a daily mean thermocline of approximately 9°C (9.17°C) and the last observed adults in autumn coincide with a similar thermocline (9.54°C). These findings are consistent with the short migratory movements of tagged individuals following synoptic cold fronts [12]. In addition, the 9°C thermocline observed here is similar to the development threshold of the final instar (8.7 ± 0.1°C) [14]. Taken together, these findings suggest that temperature is a key limiting factor to the northbound migration of dragonflies in spring, the emergence of nymphs and southbound migration in autumn. Given the critical role of temperature, a warming climate could induce earlier spring flights, trigger later autumn flights and potentially shorten migratory distances as the 9°C thermocline moves northbound, allowing adults to reside at higher latitudes during the winter months. Further research is needed to determine how a changing climate may alter the migratory schedules and annual cycle of dragonflies and other migratory insects.

Data accessibility

Museum holdings and specimen numbers and data are available from the Dryad Digital Repository: http://dx.doi.org/10.5061/dryad.17hf071 [27]. R code used to analyse data can be found at https://github.com/MTHallworth/Hallworth_et_al_Anaxjunius.

Authors' contributions

M.T.H., P.P.M., K.P.M. and C.E.S. conceived and designed the study. K.P.M. and S.Z. coordinated and collected samples from field and museum specimens. M.T.H. and C.E.S. analysed the data. M.T.H., P.P.M. and C.E.S. wrote the manuscript. All authors read, edited, approved the final version of the manuscript and agree to be held accountable for the content therein.

Competing interests

The authors declare no competing interests exist.

Funding

Major funding was provided by the U.S. Forest Service International Programs and the Xerces Society for Invertebrate Conservation.

Acknowledgements This study was part of the Migratory Dragonfly Partnership coordinated by The Xerces Society for Invertebrate Conservation. Michael May, Desiree Narango and Dara Satterfield provided comments on a previous version of the manuscript. We thank Desiree Narango and Tim Guida for preparing samples, and citizen scientists who contributed observations and samples. We thank the many natural history, public and private collections that allowed us to sample from priceless specimens.

Footnotes

Electronic supplementary material is available online at https://dx.doi.org/10.6084/m9.figshare.c.4320911.