- Open Access
A cold high-pressure system over North China hinders the southward migration of Mythimna separata in autumn
Movement Ecology volume 10, Article number: 54 (2022)
In warm regions or seasons of the year, the planetary boundary layer is occupied by a huge variety and quantity of insects, but the southward migration of insects (in East Asia) in autumn is still poorly understood.
We collated daily catches of the oriental armyworm (Mythimna separata) moth from 20 searchlight traps from 2014 to 2017 in China. In order to explore the autumn migratory connectivity of M. separata in East China, we analyzed the autumn climate and simulated the autumn migration process of moths.
The results confirmed that northward moth migration in spring and summer under the East Asian monsoon system can bring rapid population growth. However, slow southerly wind (blowing towards the north) prevailed over the major summer breeding area in North China (33°–40° N) due to a cold high-pressure system located there, and this severely disrupts the autumn ‘return’ migration of this pest. Less than 8% of moths from the summer breeding area successfully migrated back to their winter-breeding region, resulting in a sharp decline of the population abundance in autumn. As northerly winds (blowing towards the south) predominate at the eastern periphery of a high-pressure system, the westward movement of the high-pressure system leads to more northerlies over North China, increasing the numbers of moths migrating southward successfully. Therefore, an outbreak year of M. separata larvae was associated with a more westward position of the high-pressure system during the previous autumn.
These results indicate that the southward migration in autumn is crucial for sustaining pest populations of M. separata, and the position of the cold high-pressure system in September is a key environmental driver of the population size in the next year. This study indicates that the autumn migration of insects in East China is more complex than previously recognized, and that the meteorological conditions in autumn are an important driver of migratory insects’ seasonal and interannual population dynamics.
Migration is an important part of the life history of many animals, and it is a successful survival strategy when confronted by spatial and temporal heterogeneity of natural resources . Among the insects, many species use wind-assisted migration within the planetary boundary layer  and so, under warm conditions, this layer of the atmosphere may contain huge numbers of migrants . Mass insect migration is an important long-distance transport process, with implications for predator–prey interactions, essential nutrient cycles in different ecosystems, pollination in natural and agricultural landscapes, and damage to agriculture and public health from pest insects [3,4,5,6]. In the Northern Hemisphere, many insects migrate northwards during spring and summer, and there is no doubt that some species can reap huge benefits by poleward movement into newly-available habitats at high latitudes. Benefits include increasing population abundance due to producing more generations per year, and leaving parasite- and natural enemy-infected habitats [7, 8]. The gradual deterioration of the environment during autumn requires many high-latitude migrants to move equatorward to avoid being stranded as winter approaches, and aspects of this return journey remain poorly known for most species.
Southward migration (in the Northern Hemisphere) in autumn was more rarely observed than northward migration, which puzzled early workers . Some researchers believed seasonal poleward shifts to exploit temperate ecosystems represent a population sink from which progeny seldom return . But more recently, increasingly more species have been confirmed to show return migrations in autumn [11,12,13,14,15,16]. For example, using the mark-release-recapture method, Showers et al.  proved that black cutworms (Agrotis ipsilon) in America can complete a southward migration of nearly 2000 km in autumn. Chapman et al.  estimated through insect trajectory analyses that 80% of the Silver Y moths (Autographa gamma) from the UK can reach warmer latitudes around the northern fringe of the Mediterranean, from approximately 50° N to 40° N. But several independent observations of different migratory insects have shown that the cumulative year-round observations of populations at low latitudes are significantly lower than those at mid-high latitudes [7, 17,18,19]. So, the question arises as to what's affecting the southward migration of insects in the autumn?
Weather systems with strong northerly winds (i.e., winds towards the south), such as cold fronts, provide a small ‘window’ for southward migration [16, 20, 21]. The seasonally reversed prevailing airflow in the monsoon climate zone  can also provide favorable airflow for the meridional migration of insects in autumn. However, due to the different temperature characteristics between maritime and monsoonal climates, migratory insects in East Asia require a lower latitude winter breeding area to survive in winter compared to Europe, and this may make their autumn migrations more challenging and riskier. In North China, spectacular migrations of many insect species have been observed over an island in the Bohai Strait in autumn, and the number of these autumn migrants was much greater than that of spring migrants [12,13,14]. However, most of these studies were from one site and did not assess how many migrating insects finally reached their destination, so the connectivity of autumn migration  and its implications for insect populations in East Asia remains unclear.
Here, we study the oriental armyworm (Mythimna separata Walker), one of the main agricultural pests that undertakes seasonal, long-distance, multigeneration roundtrip and meridional migration in East Asia [24, 25]. Its migration route in China has been clarified through many large-scale mark-release-recapture experiments in the 1960s and 1980s. Each spring, M. separata moths from their overwintering area (south of 0° C isotherm in January) migrate northward into North China, and further into Northeast China (Fig. 1). From mid to late July onwards, part of the M. separata populations from Northeast China have the opportunity to migrate south . The autumn migration of this pest has been observed in August and September by a scanning radar located on an island in the Bohai Gulf, and migrating moths were moving to the southwest [12, 13, 26], but their autumn migration has not been studied at other places. Mythimna separata larvae are most destructive to crops in summer , but it is puzzling that larval outbreaks rarely occur in autumn in southern China, despite the huge numbers of the previous generation.
Currently, insect migrations over large spatial scales are hard to quantify [7, 28]. This study used widely deployed searchlight traps to compare the seasonal population dynamics of M. separata moths at different latitudes, with the aim of elucidating the role of autumn migration in the regulation of the annual migration cycles in this species (and perhaps other migrants in the region).
Monitoring migrant M. separata with searchlight trap data
To monitor the population dynamics of M. separata, the China National Agro-Tech Extension and Service Center has used 19 vertical-pointing searchlights as a monitoring network in different regions of the country since 2014 ; at the same time, we also set up a searchlight trap in Ningjin, Shandong province (Fig. 1). Based on years of understanding of the occurrence pattern of M. separata [24, 25, 27], searchlight traps were mainly placed along the migratory route and in the main outbreak areas of M. separata.
The vertical-pointing searchlight trap uses a 1000 W metal halide lamp, and it can effectively trap phototactic insects flying overhead up to about 500 m above, thus it can sample moths migrating at high altitude [13, 14]. Each trap was set in an open space without extensive maize-growing areas around it. These searchlight traps were run just in the major occurrence season of M. separata in 2014. Specifically, the searchlight traps in the Yangtze River Valley were run from February to May and from August to October, those in North China and Northeast China were run from May to September, and those in South China were run from September to next March. However, all searchlight traps were run all year round in the other three years (2015–2017). During the monitoring period, the searchlight traps were switched on each sunset (~ 19:00) and switched off the next dawn (~ 07:00), and catches were counted once a day.
The searchlight trap data of M. separata was split into four separate seasonal periods (Generation 1–Generation 4) based on our knowledge of migration patterns and local phenology [24, 25, 27] (Fig. 2). Generation 1 (G1) period was defined from 1 March to 30 April, G2 was from 1 May to 30 June, G3 period was from 1 July to 15 August, and G4 was from 16 August to 30 September.
Occurrence levels of M. separata larvae
The historical data on the annual occurrence areas of M. separata larvae from 1959 to 2018 were obtained from the China National Agro-Tech Extension and Service Center [27, 30]. Based on this occurrence area dataset and knowledge from previous studies by Jiang et al.  and Liu et al. , the occurrence of M. separata larvae were classified into three levels: ‘outbreak’, ‘normal’ and ‘light’.
Outbreak years were defined as follows: (1) Before 1970, M. separata larvae outbroke frequently, but statistical omissions may have occurred due to technical reasons. Years in which the occurrence area was equal to or greater than that in 1960 were regarded as severe outbreak years, including 1960, 1966, and 1967; (2) Years (after 1970) with an occurrence area equal to or greater than that in 1987, including 1972, 1976, 1977, 1987, 1990, 2012 and 2013; (3) The occurrence degree of M. separata larvae during the 1990s was relatively light, but the occurrence area in 1998 was significantly higher than in other years, only slightly lower than in 1987.
The occurrence level of this pest in twenty-two years with the smallest occurrence area was defined as ‘light’, including 1961–1965, 1968–1969, 1988, 1992–1996, 2001, 2002, 2009–2011, and 2016–2018. The rest of the years were termed ‘normal’ years.
Meteorological reanalysis data and climate conditions in autumn
In this study, we focused on the southward migration of M. separata moths in autumn. After late August, the summer populations of M. separata moths emerged in large numbers , and most of the moths were observed migrating at the height of 600–800 m in previous radar observations . In general, most moths migrating at night move in approximately the same direction as the wind , so the wind direction in autumn will greatly affect the southward migration of mid to high latitude migrants. Therefore, the wind pattern at the 925 hPa level (about 800 m above sea level) from 16 August to 30 September in East China was explored. Examination of atmospheric circulation features during this period revealed that a cold high-pressure system at the 925 hPa level over North China was the major factor influencing the wind pattern.
In pursuit of higher resolution data, we used the ERA-interim (2002–2017) and ERA-40 (1959–2001) sets from the ECMWF (European Centre for Medium-Range Weather Forecasts) reanalysis data, with a resolution of 0.125° × 0.125°. These autumn climate analyses used the Monthly Means of Daily Means data under the ERA-interim data set, and the wind direction analysis was based on the daily data. All these data were downloaded from the Copernicus Climate Change Service (C3S) Climate Data Store (https://cds.climate.copernicus.eu).
Generally, northerly winds (blowing towards the south) mostly appear on the east side of cold high-pressure areas in the northern hemisphere, and thus the location of the cold high-pressure center (CPH) would influence the wind pattern greatly. We used the Grid Analysis and Display System (GrADS, version 2.0.1, http://cola.gmu.edu/grads/) to extract the coordinates of the maximum air pressure in the region of 115° E–125° E and 30° N–38° N at the 925 hPa height field in September each year. Two special cases indicate that the CHP may be outside the latitudes and longitudes defined above, and we need to expand the area appropriately: (1) If the longitude of the maximum pressure is at 115° E, set the range of the current year to 110° E–125° E, 30° N–38° N; (2) If the latitude of maximum pressure is at 38° N, set the range to 115° E–125° E, 30° N–42° N. Only the CHP in the southeast of the designated areas has biological significance for the autumn insect migration.
Migratory trajectory analyses
We used the HYSPLIT model (Hybrid Single-Particle Lagrangian Integrated Trajectory Model, https://www.arl.noaa.gov/hysplit) to simulate the autumn migration of moths. This model was developed to analyze the transport and diffusion of atmospheric pollutants but it has also been used to simulate the migration trajectories of insects such as fall armyworm (Spodoptera frugiperda) and mosquitoes in recent years [18, 32]. Its principle is to regard the insect as a particle that moves with the airflow.
The searchlight trap located in the major food crop growing areas in summer and with the highest catches of M. separata moths was selected as the starting point for insect trajectory analysis, namely, Luanxian, longitude 118° 44′ 24″ E, latitude 39° 45′ 00″ N, located in the summer maize area in North China. (A total of 27,970 moths were captured in 171 days here from 2014 to 2017). The searchlight trap at Ningjin (longitude 116° 48′ 00″ E, latitude 37° 38′ 24″ N, where a total of 477 moths were captured in 51 days from 2014 to 2017), located at the south edge of North China, was also selected for insect trajectory analysis. We regarded Luanxian and Ningjin as representative of the summer breeding area population and we ran trajectory simulations on days when at least one moth was captured.
Radar observations show that M. separata moths, migrating at night, moved in the downwind direction, and that most of their flight altitudes in autumn were below 500 m, and the flight duration was less than 10 h . Some results from tethered flight mill studies indicated that the flight ability of M. separata moths decreases significantly on the 6th day after emergence . So, in order to explore the potential of M. separata moths to return to the south within a reasonable time range, we set the parameters of trajectory simulation based on the above research. We set the start time of the trajectory simulation to the local hour after sunset (Beijing time, UTC + 8 h), the time of each flight to 10 h, the trajectory height to 500 m, and we simulated trajectories for five consecutive nights, using the endpoint of the previous trajectory as the start point of the second. The self-powered flight speed of the moths was not considered in the trajectory analyses.
Estimation of 5-night success rate in autumn migration
The northern boundary of the overwintering area of M. separata is around 33° N (the 0° C isotherm of average January temperature) , and moths must return to south of this geographic line in autumn in order to survive winter temperatures. However, reaching this line may not be enough to maintain the population size because prolonged exposure to temperatures over 0° C may also be deadly. For example, after 21 days at a constant temperature of 5° C, the mortality rate of the pupae exceeds 98%, and the 35-day mortality rate at 10° C exceeds 96% . To return to winter breeding areas where the population can be maintained or increased, the fifth-generation M. separata in southern China must continue to migrate southward in mid-November . Lin and Cheng (1958) found the effective accumulated temperature of the adult stage to be 685.2 degree days above 9.6 ± 1.0° C and 111.0 degree days above 9.0 ± 0.8° C, respectively . Based on the effective accumulated temperature, the fourth-generation moths need to return to areas south of the 16° C isotherms in September–November (around 30° N). We use the ability to cross these two geographic lines (33° N and 30° N) as the basis for judging whether the simulated migration was successful.
To express the success rate of the southward migration objectively, we estimated two success rates based on the results of trajectory simulation. (1) From the population point of view, we regard the daily captured moths as an emigrating population, and estimated the success rate of this population (i.e., the proportion of endpoints south of 33° N/30° N to the total endpoints from 2014 to 2017). This method may reflect the long-term trend of the success rate more accurately. (2) From the individual point of view, the number of daily catches in the searchlight trap varies greatly during the autumn migration, so each endpoint represents a different number of moths. We estimated the proportion of individuals that successfully returned back to South China in the total catch from the north; this method can reflect the size of the return population more exactly, and there may be large inter-annual differences.
We performed a Gaussian fit to the total catches and their latitudes at northern searchlight traps while describing the main summer breeding areas of armyworm. To test whether the longitude or latitude of the CHP in September affects the interannual variation in M. separata population abundance, we used the one-sample t-test (both longitude and latitude conform to normal distributions) and t-test for Pearson’s correlation coefficient. All Gaussian fit, t-test and correlation analyses were carried out in R (version 3.6, https://www.r-project.org/).
Population increased along with northward migration
Based on the seasonal characteristics of the searchlight trap catches of M. separata moths in different regions from 2014 to 2017, the seasonal movement in China can be characterized by four waves of migration (Fig. 2A). In the first and second waves, M. separata moths migrated northward and expanded the distribution of the species from its overwintering areas (to the south of 33° N) in South China and the Yangtze River Valley into North China and even Northeast China. Then, the third migration wave occurred between Northeast China and North China in July and August. In the fourth migration wave (from 16 August onwards), moths migrated southward, as shown by the increased catches in the Yangtze Valley and South China (Fig. 2A).
During these four migration waves, the moth catches increased from generation to generation (Fig. 2B, Additional file 1: Table S1). Compared with the population in March and April (G1), the population size of G4 (from 16 August to 30 September) increased by 6.8 times on average in 2014–2017, with a growth rate (G4/G1) of 23.22 times in 2017, but only 0.72 times in 2014 (Additional file 1: Table S1). This result indicated that the population of migratory M. separata has huge annual fluctuations.
Most moths failed to reach overwintering areas during southward migration
The major southward migration of M. separata moths during the G4 period (16 August-30 September) was the focus of this study. In total, 4060 moths were caught in the overwintering area (i.e., south of 33° N) in 2014–2017, but this was only 4.13% of the 98,293 moths caught to the north of 33° N (Fig. 2C). Moreover, M. separata adults were present to the south of 33° N all year round (Fig. 2A), and thus part of the catches here might be from the local population, not from migrants from the north; this was especially the case in years where the population size in the G4 period was quite small (Additional file 1: Table S2). For example, the ratio of catches between south and north of 33° N was up to 59.9% in 2016, but only 1026 moths were caught to the north of 33° N in this year. Therefore, it is inferred that mortality is high during the southward migration period, and a comparatively small proportion reached their overwintering area successfully. Moreover, the moth catches to the south of 33° N in the G4 period were even fewer than that in the G1 period (Fig. 2C, Additional file 1: Table S2), and this suggests that: (1) the large losses of moths during southward migration counteracted the population increase during the spring and summer migration period, and (2) the population must recover in size in its winter-breeding area.
Wind pattern in September is often unsuitable for the southward migration of M. separata moths
As mentioned previously, the major southward migration of M. separata moths occurred from 16 August to 30 September (Fig. 2A). To locate the latitude range of the source area of this migration wave, the variation of population size along latitudes was checked. Both the largest population size of M. separata in the G3 and G4 periods (1 July–30 September) were mostly near 39° N (Fig. 3A, B) and located in the summer maize area in North China. Here, the moth catches in the G3 period were immigrants from further afield and/or the locally-produced progeny of the G2 period. Thus, we infer that the large numbers of moths in North China mean that this is the major breeding area for G4 individuals, that the migrants during G4 periods are mostly from North China, but that most individuals migrate unsuccessfully over a short distance.
The weather conditions during the southward migration in the G4 period was explored. In late-August, the East Asian summer monsoon gradually weakens and moves southward, and at the same time, the East Asian winter monsoon began to affect North China. From the 925 hPa average height field in September, there was a high-pressure system over North China (Fig. 3C). High-pressure systems are usually associated with clear skies and calm weather, and thus the wind speed at 925 hPa over North China was quite slow (mean wind speed: 3.68 ± 0.03 m/s, n = 3540) and therefore not very suitable for the long-distance windborne transport of insects. From north to south in East China, the probability of experiencing a north wind fluctuated, first falling, then rising and falling again (see Fig. 3D); the minimum value appears around latitude 40° N, where the probability of a north wind is less than 40% (Fig. 3D). The wind direction in North China was mostly towards the northeast (Rayleigh test: n = 3540 mean direction = 51.92°, r = 0.345, p < 0.0001) (Fig. 3E). Therefore, most M. separata moths faced unfavourable airflow conditions during September’s southward migration.
Modelling the success rate of M. separata moths during southward migration
To analyze the effect of autumn prevailing winds on migration, we quantified the southward migration success rates of M. separata in the G4 period using a trajectory analytical approach, with the Luanxian and Ningjin monitoring sites as the origin of the trajectories (Fig. 4). During the G4 period in 2014–2017, there were 171 nights in Luanxian and 51 nights in Ningjin when at least one moth was caught by searchlight traps, and thus in total 222 trajectories were calculated. After five days of migration, 55.4% (123/222) out of these trajectories eventually moved northward (Fig. 4A, B). This suggests that a large proportion of M. separata populations from North China were moving in an unfavourable direction during the G4 period.
The southernmost endpoints of the trajectories indicated the greatest potential for southward migratory distance, and we hypothesized that moths actively stopped migrating when these endpoint locations were reached. Among these 222 trajectories, 64.9% (144/222) of their southernmost endpoints were located to the south of their origins (Luanxian and Ningjin), and the distance of these southernmost endpoints from their origins were about 2.25° of latitude on average, and no more than 7° of latitude in maximum (Fig. 4C, D). Finally, only 6 trajectories (2.70%) reached the overwintering area (south of 33° N), and none of them reached south of 30° N where moths can breed in winter (Fig. 4C, D). If the daily catches are taken as an indication of emigrant population size and used as a weighting factor, the success rates of M. separata populations returning to the overwintering area were 3.52% and 5.03% in Luanxian and Ningjin, respectively. Not only that, the success rates of return migration estimated from trajectory simulations are close to those from the catches at searchlight traps (Figs. 2, 4). This agreement of the two results, i.e., trajectory simulations and the trap catches, confirms that migrating M. separata moths from North China in the G4 period failed to reach their overwintering area due to unsuitable wind patterns.
Influence of the high-pressure system over North China on interannual variation of M. separata population abundance
Most M. separata moths cannot migrate southward successfully to reach their overwintering area under the influence of the high-pressure weather system over North China. Therefore, we speculated that the southward migration in the G4 period is crucial for the M. separata population, and its annual population fluctuation would be driven by the characteristics of this high-pressure system. The centers of the high-pressure system (CHP) at 925 hPa from 1959 to 2017 were defined and extracted, and their mean coordinate in September was 117.17° E, 36.80° N. In the eleven years with an outbreak population of M. separata, the mean longitude of CHPs in the previous September was 115.18 ± 2.66° E, but it was at 117.54 ± 1.82° E for the 22 years when the occurrence level of this pest was light. The longitude values of CHPs between these two types of years were significantly different (Two-sample t-test: t = 2.647, df = 14.82, p = 0.018). In other words, CHPs in the previous September of the ‘outbreak’ years were located to the west of the mean CHP, and vice versa (Fig. 5A). Generally, a northerly wind (blowing towards the south) predominates at the eastern periphery of a high-pressure system, and thus the intensity of southerly winds (toward the north) over North and Northeast China was significantly correlated with the longitude of CHPs (Fig. 5B). Therefore, a westward position of the CHP results in more insects migrating southward successfully. Following from the above results, we suggest that the longitude of the CHP in September is a key factor determining the success rate of insect migration in autumn, and further influences the population size of northward migrants in the next spring.
Populations of M. separata emerging in late-summer or autumn at high latitudes need to migrate a linear distance of at least 600 km to return to the south of 33° N (Fig. 3B), which is undoubtedly a considerable challenge. Migratory insects have evolved a range of behavioral and physiological strategies to maximize their chances of successfully completing long-range movements to more favorable climes as local conditions deteriorate . For example, numerous migratory insects select favorably-directed tailwinds for southward autumn migrations [3, 6, 7, 28], and newly-emerged rice leaf roller moths (Cnaphalocrocis medinalis), a major pest of rice in southern China, respond to deteriorating conditions such as food shortage by increasing their likelihood of migration in the next few nights compared to well-fed adult moths . By contrast, however, M. separata moths respond to low temperatures and starvation within the first 24 h post-eclosion by shortening their pre-oviposition period (i.e., effectively reducing their window for migration, as the period of greatest flight activity is restricted to the pre-oviposition period) and increasing their fecundity . This suggests that when autumn emigrant M. separata are carried in the wrong direction by prevailing winds, they may terminate their migration and bet-hedge against the risk of harsh environments by increasing the level of reproductive investment. This kind of strategy may explain why so few of the autumn generation of M. separata emigrants reach the permanent winter-breeding region.
Long-range migration of insects and birds is generally considered to be a costly strategy, with a high risk of mortality, especially during autumn migrations when conditions (weather, habitat suitability, food availability) tend to be deteriorating [38,39,40,41,42,43,44,45]. Severe weather does at times impact the migration of insects , and is also a major cause of mortality during bird migration , but such isolated and comparatively short-lived events are unlikely to explain seasonal and interannual population dynamics of insects such as M. separata. Extensive ecological barriers that must be crossed, such as deserts and seas, can also be a major mortality factor, especially for birds [41, 43]. However, this is unlikely to be an issue in East China as there are no large water bodies or deserts to cross. Food shortages along the migratory route, in the guise of availability of nectar plants, are important drivers of migration-related mortality in monarch butterflies (Danaus plexippus) [44, 45] and perhaps other migratory insects. The interannual population dynamics of M. separata in China are affected by the area of nectar sources along the migration route in spring , but the relationship between availability of nectar sources and M. separata migration mortality during autumn is unknown, and worthy of further study. But the most important driver of mortality and uncompleted autumn ‘return’ migrations in flying animals is likely to be the availability of suitable transport opportunities on winds blowing towards the south. Periods of unfavorable winds are known to affect mortality of migrant songbirds , but the impact will be even greater for insects given their comparatively slower speeds compared to typical winds. Previous studies have suggested that unfavorable autumn winds will hinder southward migrations of M. separata [26, 48], and our study provides strong support for this idea.
Southward autumn migrations are challenging for all insects, relying as they do on favorable tailwinds being frequent enough during the relatively short migration windows of insect migrants to enable several days or nights of transport in the seasonally appropriate direction. Some migratory insects achieve successful southward migration by the mechanisms of: (1) restricting migration to occasions with favorable winds, (2) selecting flight altitudes coincident with the optimal wind conditions, and (3) taking up headings that partially correct for drift while maximizing speed of transport [3, 6, 7, 28, 49]. Radar studies in East China indicate that M. separata moths tend to orientate downwind [14, 50], and our results indicate that favorably directed winds in the autumn are too scarce to allow many moths to reach the southern winter-breeding regions. In migratory species with longer adult lifespans and migration windows, such as birds [51, 52] and a few long-lived insects such as monarch butterflies and common green darner dragonflies (Anax junius) [53, 54], stopping over until winds and other weather conditions improve is a common strategy. This is unlikely to be feasible for migrant moths such as M. separata, as they have a limited window of opportunity in which to migrate—typically just the 4–5 nights immediately following adult eclosion . Thus, in situations where winds are not frequently favorable for southward autumn migration, as in our study in East China, migratory moths presumably have to ‘make the best of a bad job’ and often fly on unfavorable winds, leading to the low success rates we observed.
Our results therefore indicate that during the autumn migration period the population undergoes an annual crash, as most moths will never reach a location suitable for producing the next generation. The small proportion of the migrants that reach the winter-breeding region are clearly crucial to the persistence of the population, and consequently the winter generations are the primary drivers of population growth in this species. This situation makes an interesting comparison to migratory insects that have been studied in Western Europe, where many insects have been observed to carry out successful mass migrations back to Mediterranean breeding grounds [3, 6, 7, 17, 28]. In this case, mortality along the autumn migration route is predicted to be comparatively small , and the populations appear to go through a ‘bottleneck’ during the winter in most (typical) years [7, 19]. This is because the winter-breeding regions (Maghreb and Sahel regions of north-west Africa) are usually rather arid, which restricts the growth of larval host plants , and it is only in atypically wet winters that large winter populations emigrate into Europe in the spring [19, 55]. Winter-breeding regions in South China by contrast do not go through such an extreme dry period, and as such the combination of moist, warm conditions provide perfect opportunities for population growth over the winter.
Given the relatively small proportion of the summer generations that return to the south, it is legitimate to pose the question as to why these moths migrate at all. We consider that seasonal changes in temperature drive the migration. Survival over the summer in South China will be severely compromised due to the climate in this region, as the eggs, larvae, and pupae of M. separata cannot survive temperatures > 35° C , and persistent temperatures > 30° C significantly reduce their reproductive abilities . Thus, it is advantageous to depart the southern part of the range during the spring to escape the adversely hot temperatures. Conversely, autumn temperatures in North China rapidly drop, and this will cause the population to crash as the developmental period of M. separata is extended by nearly 100 days when the temperature drops from 20 to 15° C . Thus, a rapid retreat to the south will be highly beneficial for those migrants that can make it. Another benefit that will accrue to migrants relative to individuals that remain in the same area year-round is migratory escape to comparatively enemy-free spaces, and consequently lower rates of parasitism and disease [8, 28]. These benefits presumably favored the evolution of migration in M. separata despite the risks and high levels of mortality associated with the migratory journey. Winter areas not only play a decisive role in regulating summer population abundance [19, 55] but also play an important role in regulating migratory populations (Fig. 2C) . A long-term stable winter breeding area is extremely important for any seasonal migration pattern, and considering that migratory behaviors are currently threatened to varying degrees around the world , the protection of winter-breeding areas of non-pest species will be a key part of human intervention . In the management of migratory pests, it is also essential to carry out comprehensive research in the source regions. We need to raise the focus on winter-breeding areas to a higher level, and these need to be based on more comprehensive and in-depth cross-border cooperation.
The influence of cold high-pressure systems on autumn wind patterns in East China seems to control the success rate of the southward ‘return’ migration of M. separata moths which have bred in North and Northeast China. A greater proportion of moths successfully returned to the south in years when the center of the high-pressure system (CHP) in September was further to the west than typical, and this increased the possibility of larval outbreaks in the next year . Our results indicate that the southward migration in autumn is crucial for sustaining pest-level populations of this species, and the September position of the CHP is a key environmental driver of this process. Our study reveals factors influencing seasonal population fluctuations of a long-distance migratory insect pest, and provides crucial information for predicting outbreak populations.
Availability of data and materials
Excel spreadsheets of population data have been deposited in Dryad, https://doi.org/10.5061/dryad.0vt4b8h38 .
Dingle H. Migration: the biology of life on the move. 2nd ed. Oxford: Oxford University Press; 2014.
Drake VA, Farrow RA. The influence of atmospheric structure and motions on insect migration. Annu Rev Entomol. 1988;33:183–210.
Hu G, Lim KS, Horvitz N, Clark SJ, Reynolds DR, Sapir N, Chapman JW. Mass seasonal bioflows of high-flying insect migrants. Science. 2016;354:1584–7.
Bauer S, Hoye BJ. Migratory animals couple biodiversity and ecosystem functioning worldwide. Science. 2014;344:1242552.
Satterfield DA, Sillett TS, Chapman JW, Altizer S, Marra PP. Seasonal insect migrations: massive, influential, and overlooked. Front Ecol Environ. 2020;18:335–44.
Wotton KR, Gao B, Menz MHM, Morris RKA, Ball SG, Lim KS, et al. Mass seasonal migrations of hoverflies provide extensive pollination and crop protection services. Curr Biol. 2019;29:2167–73.
Chapman JW, Bell JR, Burgin LE, Reynolds DR, Pettersson LB, Hill JK, et al. Seasonal migration to high latitudes results in major reproductive benefits in an insect. Proc Natl Acad Sci USA. 2012;109:14924–9.
Satterfield DA, Villablanca FX, Maerz JC, Altizer S. Migratory monarchs wintering in California experience low infection risk compared to monarchs breeding year-round on non-native milkweed. Integr Comp Biol. 2016;56:343–52.
Williams CB. Insect migration. London: Collins; 1958.
Stinner RE, Barfield CS, Stimac JL, Dohse L. Dispersal and movement of insect pests. Ann Rev Entomol. 1983;28:319–35.
Walker TJ. Migrating lepidoptera: Are butterflies better than moths? Fla Entomol. 1980;63:79.
Guo J, Fu X, Zhao S, Shen X, Wyckhuys KAG, Wu KM. Long-term shifts in abundance of (migratory) crop-feeding and beneficial insect species in northeastern Asia. J Pest Sci. 2020;93:583–94.
Feng HQ, Wu KM, Cheng DF, Guo YY. Radar observations of the autumn migration of the beet armyworm Spodoptera exigua (Lepidoptera: Noctuidae) and other moths in northern China. Bull Entomol Res. 2003;93:115–24.
Feng HQ, Zhao XC, Wu XF, Wu B, Wu KM, Cheng DF, Guo YY. Autumn migration of Mythimna separata (Lepidoptera: Noctuidae) over the Bohai Sea in northern China. Environ Entomol. 2008;37:8.
Riley JR, Reynolds DR, Smith AD, Edwards AS, Zhang XX, Cheng XN, et al. Observations of the autumn migration of the rice leaf roller Cnaphalocrocis medinalis (Lepidoptera: Pyralidae) and other moths in eastern China. Bull Entomol Res. 1985;85:397–414.
Showers WB, et al. Mechanism of southward migration of a noctuid moth [Agrotis Ipsilon (Hufnagel)]: a complete migrant. Ecology. 1993;74:2303–14.
Stefanescu C, Páramo F, Åkesson S, Alarcón M, Ávila A, et al. Multi-generational long-distance migration of insects: studying the painted lady butterfly in the Western Palaearctic. Ecography. 2013;36:474–86.
Westbrook JK, Nagoshi RN, Meagher RL, Fleischer SJ, Jairam S. Modeling seasonal migration of fall armyworm moths. Int J Biometeorol. 2016;60:255–67.
Hu G, Stefanescu C, Oliver TH, Roy DB, Brereton T, Swaay CV, et al. Environmental drivers of annual population fluctuations in a trans-Saharan insect migrant. Proc Natl Acad Sci USA. 2021;118:e2102762118.
Beerwinkle KR, Lopez JD, Witz JA, Schleider PG, Eyster RS, Lingren PD. Seasonal radar and meteorological observations associated with nocturnal insect flight at altitudes to 900 meters. Environ Entomol. 1994;23:676–83.
Krauel JJ, Westbrook JK, McCracken GF. Weather-driven dynamics in a dual-migrant system: moths and bats. J Anim Ecol. 2014;84:604–14.
An ZS, et al. Global monsoon dynamics and climate change. Annu Rev Earth Planet Sci. 2015;43:29–77.
Gao BY, Hedlund J, Reynolds DR, Zhai BP, Hu G, Chapman JW. The ‘migratory connectivity’ concept, and its applicability to insect migrants. Move Ecol. 2020;8:48.
Jiang XF, Luo LZ, Zhang L. Regulation of migration in Mythimna separata (Walker) in China: a review integrating environmental, physiological, hormonal, genetic, and molecular factors. Environ Entomol. 2011;40:516–33.
Li KP, Wong HH, Woo WS. Route of the seasonal migration of the oriental armyworm moth in the eastern part of China as indicated by a three-year result of releasing and recapturing of marked moths. Acta Phytophyl Sin. 1964;3:101–10.
Wu KM, Cheng DF, Xu G, Zhai BP, Guo YY. Radar observation of autumn migration of insects in northern China. Acta Ecol Sin. 2001;21:1833–8.
Jiang YY, Li CG, Zeng J, Liu J. Population dynamics of the armyworm in China: A review of the past 60 years’ research. Chin J Appl Entomol. 2014;51:890–8.
Chapman JW, Reynolds DR, Wilson K. Long-range seasonal migration in insects: mechanisms, evolutionary drivers and ecological consequences. Ecol Lett. 2015;18:287–302.
Jiang YY, Liu J, Zeng J. Using a national searchlight trap network to monitor the annual dynamics of the oriental armyworm in China. Chin J Appl Entomol. 2018;55:778–93.
Liu J, Yang JJ, Zhang Z, Wang CR, Song ZY, Jiang YY. Analysis on occurrence characteristics of Mythimna separata (Walker) in 2018. Plant Prot. 2020;46:229–33.
Chapman JW, Klaassen RHG, Drake VA, Fossette S, Hays GC, Metcaflfe JD, et al. Animal orientation strategies for movement in flows. Curr Biol. 2011;21:861–70.
Huestis DL, Dao A, Diallo M, Sanogo ZL, Samake D, Yaro AS, et al. Windborne long-distance migration of malaria mosquitoes in the Sahel. Nature. 2019;574:404–8.
Hu BH, Lin CS. Experiments on the fight activity of the oriental armyworm moths, Mythimna separata (Walker). Acta Ecol Sin. 1983;3:367–75.
Zheng AB, Zhai BP. Preliminary studies on the cold hardiness of Mythimna separata (Walker). Chin J Appl Entomol. 2017;54:831–7.
Lin CS, Cheng CL. Testing the number of generations of the armyworm (Cirphis Unipuncta Haw) occurring in various geographical regions in China by the method of “thermal sums.” Acta Entomol Sin. 1958;8:41–56.
Guo JW, Li P, Zhang J, Liu XD, Zhai BP, Hu G. Cnaphalocrocis medinalis moths decide to migrate when suffering nutrient shortage on the first day after emergence. Insects. 2019;10:364.
Zhang L, Jiang XF, Luo LZ. Determination of sensitive stage for switching migrant oriental armyworms, Mythimna separata (Walker), into residents. Environ Entomol. 2008;37:1389–95.
Rankin MA, Burchsted JCA. The cost of migration in insects. Annu Rev Entomol. 1992;37:533–59.
Newton I. Weather-related mass-mortality events in migrants: Weather-related mass-mortality events in migrants. Ibis. 2007;149:453–67.
Roff DA, Fairbairn DJ. The evolution and genetics of migration in insects. Bioscience. 2007;57:155–64.
Klaassen RHG, Hake M, Strandberg R, Koks BJ, Trierweiler C, Exo KM, et al. When and where does mortality occur in migratory birds? Direct evidence from long-term satellite tracking of raptors. J Anim Ecol. 2014;83:176–84.
Badgett G, Davis AK. Population trends of monarchs at a northern monitoring site: analyses of 19 years of fall migration counts at Peninsula Point. MI Ann Entomol Soc Am. 2015;108:700–6.
Lok T, Overdijk O, Piersma T. The cost of migration: spoonbills suffer higher mortality during trans-Saharan spring migrations only. Biol Lett. 2015;11:20140944.
Ries L, Taron DJ, Rendón-Salinas E. The disconnect between summer and winter monarch trends for the eastern migratory population: possible links to differing drivers. Ann Entomol Soc Am. 2015;108:691–9.
Inamine H, Ellner SP, Springer JP, Agrawal AA. Linking the continental migratory cycle of the monarch butterfly to understand its population decline. Oikos. 2016;125:1081–91.
Wang GP, Zhang QW, Ye ZH, Luo LZ. The role of nectar plants in severe outbreaks of armyworm Mythimna separata (Lepidoptera: Noctuidae) in China. Bull Entomol Res. 2006;96:445–55.
Huang AC, Bishop CA, McKibbin R, Drake VA, Green DJ. Wind conditions on migration influence the annual survival of a neotropical migrant, the western yellow-breasted chat (Icteria virens auricollis). BMC Ecol. 2017;17:29.
Pan L, Wu XW, Chen X, Jiang YY, Zeng J, Zhai BP. Pied piper effect of the migration arena in northeastern China on Mythimna separata (Walker). Chin J Appl Entomol. 2014;51:974–86.
Gao B, Wotton KR, Hawkes WLS, Menz MHM, Reynolds DR, Zhai BP, Hu G, Chapman JW. Adaptive strategies of high-flying migratory hoverflies in response to wind currents. Proc R Soc B. 2020;287:20200406.
Chen RL, Bao XZ, Drake VA, Farrow RA, Wang SS, Sun YJ, Zhai BP. Radar observations of the spring migration into northeastern China of the oriental armyworm moth, Mythimna separata, and other insects. Ecol Entomol. 1989;14:149–62.
O’Neal BJ, Stafford JD, Larkin RP, Michel ES. The effect of weather on the decision to migrate from stopover sites by autumn-migrating ducks. Mov Ecol. 2018;6:23.
Fusani L, Cardinale M, Carere C, Goymann W. Stopover decision during migration: physiological conditions predict nocturnal restlessness in wild passerines. Biol Lett. 2009;5:302–5.
Zylstra ER, Ries L, Neupane N, Saunders SP, Ramírez MI, Rendón-Salinas E, et al. Changes in climate drive recent monarch butterfly dynamics. Nat Ecol Evol. 2021;5:1441–52.
Wikelski M, Moskowitz D, Adelman JS, Cochran J, Wilcove DS, May ML. Simple rules guide dragonfly migration. Biol Lett. 2006;2:325–9.
Stefanescu C, Hu G, Oliver TH, Reynolds DR, Chapman JW. Reply to Lopez-Manas et al.: Spatial population models of migrants should be underpinned by phenology, behavior and ecology. PNAS. 2022;119:e2203349119.
Jiang XF, Liu YQ, Luo LZ, Hu Y. Effects of high temperature on the immature stages of oriental armyworm Mythimna separata Walker. J Beijing Univ Agric. 1998;13:20–6.
Jiang XF, Luo LZ, Hu Y. Influences of rearing temperature on flight and reproductive capacity of adult oriental armyworm, Mythimna separata (Walker). Acta Ecol Sin. 2000;20:288–92.
Dokter AM, Farnsworth A, Fink D, Ruiz-Gutierrez V, Hochachka WM, Sorte FAL, et al. Seasonal abundance, and survival of North America’s migratory avifauna determined by weather radar. Nat Ecol Evol. 2018;2:1603–9.
Vidal O, Rendón-Salinas E. Dynamics and trends of overwintering colonies of the monarch butterfly in Mexico. Biol Conserv. 2014;180:165–75.
Zhu J, Jiang YY, Zhai BP. Autumn migration of Mythimna separata: a case study of the 2012–2013 outbreak. Chin J Appl Entomol. 2018;55:802–9.
Zhu J, Hu G. A cold high-pressure system over North China hinders the southward migration of Mythimna separata in autumn, Dryad, Dataset. https://doi.org/10.5061/dryad.0vt4b8h38.
We thank Huiyu Wei for her assistance in preparing the draft.
This work was supported by the National Key Research and Development Program of China (2021YFE0194800) and the National Natural Science Foundation of China (31822043). Rothamsted Research receives grant-aided support from the United Kingdom Biotechnology and Biological Sciences Research Council (BBSRC).
Ethical approval and consent to participate
Consent for publication
The authors declare no conflict of interest.
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Additional file 1: Table S1
. Cumulative light-trap catches of M. separata in each generation in 2014–2017. Table S2. Cumulative light-trap catches of M. separata in north and south regions.
Rights and permissions
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data.
About this article
Cite this article
Zhu, J., Chen, X., Liu, J. et al. A cold high-pressure system over North China hinders the southward migration of Mythimna separata in autumn. Mov Ecol 10, 54 (2022). https://doi.org/10.1186/s40462-022-00360-3
- Insect migration
- East Asian monsoon
- Wind pattern
- Population dynamics