Possible source and migration pathway for early-summer immigrants of the oriental armyworm, Mythimna separata, arriving in northern Japan

Akira OTUKA, Tokumitsu NIIYAMA, JIANG Xing-fu

1 Institute for Plant Protection, National Agriculture and Food Research Organization, Koshi 861-1102, Japan

2 Akita Plant Protection Station, Akita 010-1232, Japan

3 Institute of Plant Protection, Chinese Academy of Agricultural Sciences, Beijing 100193, P.R.China

Abstract The first generation of the oriental armyworm, Mythimna separata (Walker), arrives every year in northern Japan in mainly late May to early June.Analyses of weather maps suggested that this moth’s immigration source could be eastern China, but the accuracy of those analyses was very limited due to the lack of a current standard trajectory analysis.The management of migratory insect pests such as M.separata benefits from the identification of the migration source(s)and pathway(s) of the pests.The present study provides a trajectory analysis for M.separata.Backward trajectories from trap sites in northern Japan were calculated with the HYSPLIT System developed by the U.S.National Oceanic and Atmospheric Administration, taking the flight speed of M. separata and the limitation of low ambient temperature at flight height into account.The ending times of the moth’s short and long trajectories were set at dusk on the day before and two days before the possible arrival date, respectively.The results suggested two types of possible migration pathway: a multi-step pathway from Northeast China, the Korean Peninsula, and eastern Russia, which are destination areas of the first-generation’s migration, and a direct pathway from seasonal main emigration areas in eastern China such as Jiangsu and Shandong provinces.These findings contribute to our understanding of the migration ecology of M.separata and can be used for the development of methods to predict the migration of this insect.

Keywords: migration, backward trajectory, oriental armyworm, first generation, East Asia

1.Introduction

A multi-generational migration pathway in East Asia of the oriental armyworm,Mythimnaseparata(Walker), is well documented (Liet al.1964; Hirai 1995; Lee and Uhm 1995; Jianget al.2011).The East AsianM.separatapopulation overwinters in southern China at latitudes<33°N, where the temperature is above 0°C in January,and most of the overwintering-generation adults migrate northward to an area between 33° and 36°N from March to mid-April (Jianget al.2011).That area, which includes the provinces of Jiangsu, Shandong, Henan, and the northern part of Hubei, is called the “first-generation outbreak region” and its main host plant is winter wheat.In late May to early June, i.e., early summer,most of the first-generation adults emerge and migrate further to northern and northeastern China, extending from northern Hebei, Liaoning, and Jilin provinces to Heilongjiang Province (Jianget al.2011).Mark-andrelease experiments, insect captures on a ship under anM.separatamigration pathway in the Bohai Sea, and monitoring data of the nationwide searchlight trap network in China support this northward and northeastward migration (Hsiaet al.1963; Liet al.1964; Jianget al.2016, 2018).

Depending on the weather conditions, a part of the first generation may arrive in Korea and northern Japan as well (Hirai 1995; Lee and Uhm 1995).Major immigrations ofM.separatainto Tohoku District in northern Japan occurs in late May to early June (Koyama and Matsumura 2019).That period corresponds to the emigration period in China (Jianget al.2011).The numbers of trapped immigrant and densities of descendant larval in fields in northern Japan change markedly year by year.Outbreaks ofM.separata, which indicate a field situation of high larval density and complete crop-leaf loss, have been recorded in Tohoku District recently in 1987, 1988, 1990,1991, 1994, 1997, 2000, 2001, 2007, 2009, and 2017(Koyama and Matsumura 2019).The major host plants forM.separataare Gramineae grass and maize (Koyama and Matsumura 2019).

A migration pathway ofM.separatafrom the Chinese first-generation outbreak region to northern Japan was first suggested in relation to outbreaks in 1971–1972 (Oku and Kobayashi 1974).That study used surface weather maps and identified low-pressure systems moving eastward over the Sea of Japan before theM.separataoutbreaks occurred, and it assumed that southwesterly winds to the south of the low pressure system could have assisted the migration.Outbreak events in northern Japan in 1960, 1971, 1972, 1978 and 1987 were similarly analyzed with weather maps at the surface or at the isobaric level of 850 hPa, and the possibleM.separatasources in early summer were all inferred to be eastern China (Oku and Kobayashi 1977; Oku 1983, 1984; Hirai 1988; Kitamura and Saito 1988).

However, since the methods used in the abovecited analyses investigated only the locations of lowpressure systems and the areas of accompanying winds,the accuracy of the source estimation was very limited compared to that achieved by the current standard method, i.e., trajectory analyses (e.g., Wuet al.2022).A simple trajectory analysis traces only air parcels backward from a trap site, and it interprets a backward trajectory and its terminal point location over the land as a probable migration pathway and a source, respectively(e.g., Otukaet al.2005, 2016).This simple method has been applied to migrations of small-sized species such as the slow-flying rice planthopper, and the oriental fruit fly (Otukaet al.2005, 2016).For larger insect species(e.g., migratory moths), their higher flight speeds may affect estimations of their sources, because the insect’s backward trajectory may extend for a longer distance than a simple air-parcel trajectory.

The ambient air temperature at an insect’s flight altitude can also affect the insect’s possible flight heights,as the insect may stop flying in colder air.The upperair temperature in early summer sometimes drops significantly depending on weather conditions.Since the target species of this study (M.separata) is a relatively large nocturnal migrant (35- to 50-mm wingspan) having a flight speed of about 3 m s–1(Hu and Lin 1983), it is important to take these insect’s flight speed, and ambient temperature into account in a trajectory analysis, as was done by Wuet al.(2022).Obtaining an accurate estimation of a migratory insect’s migration source is important to our understanding of its migration ecology and outbreaks.The migration source information forM.separatais also necessary for predicting its arrival in northern Japan.

This study accurately estimated the immigration sources and pathways to northern Japan ofM.separataby using a trajectory calculation model with the insect’s flight behavior implemented, and basic ecological knowledge of this species that had been lacking was obtained.This is the first advanced trajectory analysis for the overseas immigration events ofM.separataoccurring in Japan.

2.Materials and methods

2.1.Monitoring data

A food-lure trap using syrup or molasses was used to monitorM.separataimmigrant adults (Koyama 1968).The syrup was a mixture of sugar, Japanese sake brewed liquor, vinegar, and water (Koyama 1968).Four traps in two prefectures were installed at Ooma (41.52°N,140.90°E; 1993–2017) and Fukaura (40.67°N, 139.97°E;1993–2017) in Aomori Prefecture, and Noshiro (40.24°N,140.05°E; 1997–2017) and Yurihonjo (39.28°N, 140.08°E;1997–2017) in Akita prefecture in northern Japan (stars in Fig.1).The trapping period differed among the sites and survey years: essentially daily in Aomori and Yurihonjo and at irregular intervals with some variations in Noshiro(Appendices A–D).Since caught insects were collected in the morning, the daily boundary was set at 09.00 h Japan Standard Time (JST) i.e., 00.00 h UTC.To analyze only clear immigration events, dates with a total catch of≥10 individuals per trapping period (daily or multiple-day period) were selected as the analytical target.

2.2.Calculation of the moths’ trajectory

The HYSPLIT ver.5.2.1 trajectory model, an Applebased public version developed by the Air Resources Laboratory of the U.S.National Oceanic and Atmospheric Administration (NOAA), was used to calculateM.separata’s backward trajectories (Draxler and Hess 1997, 1998; Steinet al.2015).Multiple trajectories with different starting times and heights were calculated by assuming the following six conditions.

(1) An insect’s horizontal flight speed of 3.0 m s–1(Hu and Lin 1983; Zhang and Li 1985) was applied by setting a parameter VBUG=3.0 in SETUP.CFG (Draxler 1999;ARL 2022), so that a moth moved at the wind’s horizontal speed plus 3.0 m s–1in the downwind direction at its flight altitude.Although the self-flight speed ofM.separata’s overseas migration is unknown, the above value was assumed based on previous studies’ laboratory flight experiment and radar observations in the field; the flight experiment’s results indicated an averaged maximum flight speed of 2.9 m s–1among 4-day old adults that had consumed water or 10% honey water before the flight (Hu and Lin 1983).The radar observations indicated a speed of (2.8±1.3) m s–1in autumn migration (Fenget al.2008).

(2) Six starting heights, i.e., 100, 300, 600, 900, 1 200,and 1 500 m above the ground at each trap site were selected.No height>1 500 m was used, due to low temperatures at higher altitudes in early summer.For the vertical motion option, “isobaric” was selected in CONTROL.CFG to try to maintain the flight height at each isobaric level (Draxler 1999; ARL 2022).

(3) Since there was no information about the actualM.separataimmigrants’ arrival time at each trapping site,all possible backward trajectories with different hourly starting times were calculated for a 72-h (3-day) period before the above selected catch date (of ≥10 individuals).For example, for a catch of 42 moths on 10 June 2017 at the Fukaura site (Table 1), hourly trajectories starting from 00.00 h UTC on 7 June to 23.00 h UTC on 9 June were calculated.In total, 432 trajectories (=72 different hours×6 heights) were calculated for a single catch date.This study used 72 h as the search period because a migration study for the common cutworm (Spodoptera lituraFabricius) arriving in western Japan showed that airstreams from southern China favorable for overseas migrations were observed mostly within 3 days before each catch peak date, suggesting that those moths were likely to be trapped within 3 days after their arrival (Tojoet al.2013).

When no terminal point of any backward trajectory for the 72-h period reached over a possible source area,the examination was extended to another previous day in some cases.When a trap monitoring interval(n) was more than 2 days, (n-1) days between sample collection dates were also considered possible arrival dates, because the actual arrival date was unknown.Therefore, trajectories within a period of (n+2) days were investigated.For example, trajectories in a 7-day period were investigated for the 5-day interval trapping data (for an example, see the catch of 12 moths on 31 May 1995 at Fukaura and the terminal point area in Appendix B.

(4) SinceM.separata, a nocturnal moth, starts actively flying at dusk (Hill and Hirai 1986), the terminal time (or ending time) of a short backward trajectory and that of a long backward trajectory was set at dusk hour, 19.00 h China Standard Time (11.00 h UTC) of the day before or 2 days before the starting date (arrival date), respectively(Appendix E).As the starting times were set hourly, the flight durations of the short and long trajectories ranged from 13 to 36 h and from 37 to 60 h, respectively.

(5) Trajectories were calculated with thehyts_stdprogram of the HYSPLIT System (Draxler 1999; ARL 2022).The meteorological data used (wind speed and air temperature, etc.) were the U.S.National Center for Environmental Prediction (NCEP) Global Data Assimilation System (GDAS) one-degree three-hourly archive data available from 2004 to the present (archive information: https://www.ready.noaa.gov/gdas1.php)and the NCEP/NCAR Reanalysis 2.5-degree and sixhourly archive data available from 1948 to the present(archive information: https://www.ready.noaa.gov/gbl_reanalysis.php).The former data were used for making trajectories in and after 2004, and the latter data were used for making trajectories before 2004.The hourly node position of each backward trajectory (its latitude,longitude, and height) and the ambient air temperatures at the node positions were saved in an output file,tdump.

(6) Each trajectory was checked to determine whether its ambient air temperature at any node dropped to <9°C.If the ambient air temperatures of all of the nodes of a trajectory were ≥9°C, the trajectory was considered valid;otherwise, it was considered invalid and was removed from the output file.The value of 9°C was determined as the temperature at the half maximum of the flight duration by referring to the relationship between flight duration and air temperature in the flight experiment reported by Zhang and Li (1985).This condition was applied because the moth was able to keep flying along the estimated pathway (trajectory) at sufficiently high air temperatures.This temperature filtering against the output filetdumpwas conducted with an in-house C Program,tempf.Only valid trajectories were plotted on a map with thetrajplotProgram of HYSPLIT (Draxler 1999; ARL 2022) for the examination of migration pathways.

2.3.Source estimation

The terminal point of each valid backward trajectory (or the starting point of a migration pathway) was further limited to a point with an ambient air temperature≥11°C as a valid terminal point.This additional condition was applied not only becauseM.separataflies best at the temperatures of 11–32°C (Zhang and Li 1985), but also because the ambient air temperature at a flight height should be high enough when the insect is taking off.Only valid terminal points for catch dates of ≥10-individual catches in each year were counted in each grid cell of 0.25°×0.25° in latitude and longitude, and the terminal points’ frequency distribution was made with the HYSPLIT System’strajfreqProgram (ARL 2022).Finally, frequency plots over the sea were removed.The possible source area was estimated by finding a land area with a non-zero terminal-point frequency.

2.4.Meteorological factors

East Asian surface weather maps at 6-h intervals were used to search for meteorological factors particularly for migrations from eastern China to northern Japan.The weather maps were made by the Japan Meteorological Agency and were accessed at the 100-year Weather Map Database (http://agora.ex.nii.ac.jp/digital-typhoon/weather-chart/).The positions of low- and high-pressure systems affecting the target migrations were investigated,and their weather map patterns were categorized based on the position of each pressure system affecting a migration.

3.Results

3.1.Trap data and typical backward trajectories

Fig.2 Seasonal and annual population dynamics of the first generation in northern Japan.A, seasonal pattern of averaged total catch number/year from 4th pentad of May to 6th pentad of June.The catch data (2017–1993) were obtained from two sites, Ooma and Fukaura, in Aomori prefecture.B, annual change of the total catch number from 4th pentad of May to 6th pentad of June at the two sites.Akita prefecture’s data were not used due to their irregular survey intervals and very small catch numbers.

Seasonal and annual population dynamics showed that immigrant’s peak of the first-generation in northern Japan occurred in early June (Fig.2-A), and the annual total catch number from 15 May to 30 June changed largely year by year, indicating peaks at 2- to 3-year intervals (Fig.2-B).A gradually decreasing trend in Fukaura’s catch number is visible.Trap data of the four sites indicated 55% of the total catch in northern Japan were female (Appendix F).Female ratio for catch peaks of ≥10 individuals was also found 55%.Specifically,examples ofM.separatatrap data in Fukaura, Aomori Prefecture in 2017, 2001, and 2000 (i.e., peak years)are shown in Table 1.Trap data for other survey years and sites are shown in Appendices A–D.These years have records of ≥10-individual catches.For example,total catches of 42 and 30M.separatawere recorded on 10 and 12 June 2017, respectively.For the former catch, trajectories during the three-day period of 7–9 June (UTC) were calculated, and for the latter catch,trajectories from 9–11 June (UTC) were calculated.Examples of backward trajectories on 9 June 2017 are provided in Fig.3-A and B.In this study, an inland emigration area (EAi) is defined as an area of the firstgeneration outbreak region (Jianget al.2011) excluding Jiangsu and Shandong provinces (i.e., northern Anhui,Henan and northern Hubei).The provinces and the EAi were separated in this study to highlight the eastern provinces as migration sources as described below.Some of the terminal points reached over Shandong Province (SD) as well as EAi in China at dusk on 7 and 8 June (Fig.3-A and B).

Trajectories arriving at three different hours on 2 June 2001 are depicted in Fig.3-C–E.The terminal points were distributed over SD, northeastern China(NE), the Korean Peninsula, i.e., North and South Korea (K), and Russia (RU).Similarly, the terminal points of trajectories arriving on 28 and 29 May 2000 for the catch of 257 individual moths on 30 May were distributed over JS, K, and NE (Fig.3-F–H).Some terminal points were distributed over western Japan(WJ) as shown in Fig.3-I.

3.2.Distribution of terminal points

The terminal points of the short trajectories ending at dusk 1 day before each catch date or possible arrival date are mapped in Figs.4–6.Appendices G, H and I map the terminal points of the long trajectories.Fukaura’s terminal points (short) in 2017, 2001, and 2000 (Table 1) are shown in Fig.5-A, G, and H, respectively, distributing over the Chinese continent and western Japan.Terminal point distributions for the long-duration case also extended to eastern China regions such as Shandong and Jiangsu provinces more often than those for the short-duration case due to longer flight durations.The estimated flight durations of the short and long trajectories from Jiangsu and Shandong provinces to northern Japan ranged from 27 to 60 h.Particularly, the trap locations and survey years whose terminal points never reached over eastern China but did reach Korea, Northeast China, and eastern Russia only were Ooma in 2007, 2001, 1994 (Fig.4-E, H and K; Appendix G-e and k), Fukaura in 2016 (Fig.5-B),and Noshiro in 2007 (Fig.6-A; Appendix I-a).Overall,the terminal points were distributed over eastern China(Shandong, Jiangsu and EAi), northern and northeastern China (frequently Jilin, Liaoning, Heilongjiang, and sometimes Hebei), the Korean Peninsula, eastern Russia,and western Japan (Figs.4–6; Appendices G–I; Table 2).The Korean Peninsula (K) was most frequently found as a terminal-point (TP) area by the short trajectory (30.2%),and followingly, WJ (24.6%), NE (22.8%), RU (17.4%),SD (2.1%), JS (1.1%), around Bohai Sea (ABH; 1.1%)and the others (<1%) came (Table 2).In total, 71.9%of the valid short trajectories came from the parts of the Chinese continent except the first-generation outbreak region: K, NE, RU, ABH, and Zhejiang Province (ZJ).The long trajectory analysis found TP areas of K (25.6%), NE(21.9%), WJ (14.1%), SD (11%), RU (9.1%), ABH (7.7%),JS (6%), EAi (4%) and ZJ (0.7%) (Table 2).

The meteorological factor analysis identified four pressure patterns that induced migrations from eastern China (SD and JS) to northern Japan (Table 1; Appendices A–D; Fig.7).The first pattern was a low-pressure system moving eastward from the Yellow Sea to the Sea of Japan(e.g., Lys to Lsj, Lsj; Fig.7-A).The second pattern was a low-pressure system moving over the Chinese continent(Lc), mainly Northeast China.Southwesterly winds provided by the low-pressure systems assisted insects’flights in these two cases.The third pattern was a high-inwest with low-in-east pattern (e.g., H-Lsj), and the fourth pattern was a low-in-north with high-in-south pattern (Lc/H).Westerly or northwesterly winds around high-pressure systems affect eastward or south-eastward migrations.A frequency analysis indicated the Lc pattern appeared most frequently (41%).Combinations of a low-pressure system and a high-pressure system (H-Lc, H-Lsj, or Lc/H) and lows over the sea (Lys, Lsj, and Lso) appeared 30 and 29%,respectively (Table 3).All the patterns showed a common feature: a low located to the north of northern Japan and probable source areas, indicating a main migrationinducing factor.

Fig.3 Typical backward trajectories from Fukaura, Aomori Prefecture, Japan.A, long trajectories starting at 12.00 UTC on 9 June 2017 and ending at 11.00 UTC on 7 June 2017.B, short trajectories reaching over Shandong Province at dusk on 8 June 2017.C–E, short and long trajectories starting on 2 June 2001.F–H, short and long trajectories starting on 28 and 29 May 2000.I, short trajectories reaching over western Japan and the Korean Peninsula.Different path colors indicate different starting heights.EAi,inland emigration area; SD, Shandong Province, China; RU, Primorsky Krai of Russia; NE, northeastern China including Liaoning, Jilin and Heilongjiang provinces; K, Korean Peninsula (South and North Korea); JS, Jiangsu Province, China; WJ, western Japan<138°E.

Fig.4 Terminal-point (TP) frequency distribution of short backward trajectories from Ooma, Aomori Prefecture, Japan.Collection dates with ≥10-individual catches were selectively investigated.Backward trajectories were terminated at dusk 1 day before each possible arrival date (long trajectories are shown in Appendix G).The size of the grid cell is 0.25°×0.25°.

Fig.5 Terminal-point (TP) frequency distribution of short backward trajectories from Fukaura, Aomori Prefecture, Japan.

Fig.6 Terminal-point (TP) frequency distribution of short backward trajectories from Noshiro and Yurihonjo, Akita Prefecture,Japan.The location name Yurihonjo is shortened as Yuri.

4.Discussion

4.1.Source estimation

Shandong and Jiangsu provinces are located in the major emigration area of the first generation ofM.separatain China (Jianget al.2011).Searchlight trap monitoring demonstrated that the overwintering generation arrives on winter-spring wheat in late March to early April in Dongtai,Jiangsu Province (A black triangle in Fig.1), which is a part of the emigration area (Jianget al.2016).Its descendant first generation emigrates from there in late May to early June (Jianget al.2016).These emigrants arrive in northern sites such as Laizhou, northern Shandong; Luanxian,Hebei; Zhangwu, Liaoning; and Changling, Jilin (Black triangles in Fig.1; trapping data by Jianget al.2016).The terminal point areas in Shandong and Jiangsu (Figs.5 and 6; Appendices G–I) can therefore be considered migration sources.This result suggests that first-generationM.separataadults might migrate overseas directly from eastern China to northern Japan in the early summer.

The meteorological factors inducing overseas migrations showed four pressure patterns on the surface weather map.Lows moving eastward over the Chinese continent,the Yellow Sea, or the Sea of Japan (Lc, Lys, Lsj, Lso) were major factors at 70% (Table 3; Appendices A and D), which is similar to low-pressure-system-driven migrations of rice planthoppers and nocturnal moths over the East China Sea(e.g., Kisimoto 1971; Tojoet al.2013; Wuet al.2022).This result also showed that a high-pressure system (H-Lsj, Lc/H,etc.) could play a certain role in the induction of the direct overseas migration ofM.separatain early summer that was revealed for the first time in the present study.The weather conditions that are present when a direct migration occurs seem diverse, but lows in the north of the migration pathway were found to be the key factor.

In other frequent cases, the terminal points were distributed over Northeast China, Korea, and eastern Russia.Notably, the terminal points for Ooma in 2007,2001, and 1994, for Fukaura in 2016, and for Noshiro in 2007 (Figs.4–6; Appendices G–I) were located only over these areas, where first-generation adults usually arrive from the first-generation outbreak region in China in early summer (Lee and Uhm 1995; Jianget al.2011, for South Korea).The monitoring information ofM.separatain eastern Russia was available at the Russian Agriculture Center’s website (https://rosselhoscenter.ru/index.php/25/12691-signalizatsionnoe-soobshchenie-1),indicating thatM.separataadults are captured by foodlure traps beginning in late May every year in the Primorsky Krai region of Russia facing the Japan Sea.Information from North Korea was not available.However, it is likely thatM.separataadults arrive in North Korea where is surrounded by the major destination areas: South Korea,Northeast China, and the Primorsky Krai region.It is thus reasonable to expect thatM.separataadults arrive at these destination areas during the period late May to early June.The distance between Jiangsu Province and the Primorsky Krai region, for example, is approximately 1 500 km.

In addition,M.separataadults flying over landmonitored by an entomological radar were observed to fly only during the nighttime (Chenet al.1989; Fenget al.2008).A laboratory experiment indicated thatM.separataadults flied best when they were 4–8 days old (Zhang and Li 1985), suggesting that they can fly for several nights.In addition, monitoring of the first generation with ground light traps in northern China indicated that moth catch peaks appeared early (30 May–3 June) in Henan Province and later (5–15 June) in northeastern China (Zhanget al.2018), showing gradual northeastward expanding movements.A trajectory analysis demonstrated nightby-night northeastward flight pathways connecting source areas (Anhui, Jiangsu, Henan and Shandong)viaHebei and Beijing to northeastern destination areas(Inner Mongolia and northeastern China) (Zhanget al.2018).These observations and analyses suggest thatM.separataadults perform multiple nighttime northeastward flights in China, which has been widely accepted.

Table 3 Summary of meteorological patterns

Thus, four major conclusions are indicated by the data:(i)Mythimnaseparatapopulation occurs every year in the first-generatio n outbreak region (Jianget al.2018); (ii)the first-generationM.separataadults arrive at the major destination areas in late May to mid-June; (iii) at an early age, the adults are likely to perform multiple nighttime flights starting at dusk; and (iv)M.separata’s pathways from the destination areas (NE, K and RU) to northern Japan were estimated.The circumstantial evidence here suggests thatM.separataadults might migrate from the first-generation’s destination areas to northern Japan as well.

Some terminal points were distributed over western Japan.It was reported thatM.separataoverwinters and stays throughout the year in Kagoshima Prefecture in the southern part of western Japan (AFFRCS 1989), but the catch number in the emigration season was small,ranging from a few to 20 daily catch numbers at most.The catch number of the other areas of western Japan in the same period is ～1–2 catches per 5 days (AFFRCS 1989).Therefore, even if the emigration ofM.separataadults could occur and they could arrive in northern Japan, the catch number there would be very small.Based on this discussion, western Japan might not be a major source forM.separataimmigrants in northern Japan.However,a further study onM.separata’s domestic migration is necessary to confirm the discussion.A method that can discriminate the natal origin of immigrants in northern Japan will be the key in such a study.Feasibility of another multi-step migration pathway from Chinaviawestern to northern Japan must also be evaluated by observation in western Japan of both the arrival and emigration of immigrants from the continent.

This study thus unveiled two types of possible migration pathways forM.separata: a multi-step pathway from the first-generation’s destination area and a direct pathway from the emigration area.Earlier studies assumed only a possible direct pathway from eastern China based on weather map analyses (Oku and Kobayashi 1977; Oku 1983, 1984; Kitamura and Saito 1988).The present study suggested that such a direct migration pathway might be feasible by examining theM.separatatrajectories under specific weather conditions in multiple years and at multiple sites.Shandong and Jiangsu provinces were estimated as the probable direct source, although their frequency was found limited (3.2% by the short trajectory,17% by the long trajectory).These provinces can be possible sources because they are relatively close to northern Japan among other provinces in the emigration area.The distance between Jiangsu Province and Aomori Prefecture, for example, is about 1 880 km.The latitude of the provinces is also a factor in being a direct source, as the latitudes of Shandong and Jiangsu provinces are close to but slightly lower than that of northern Japan.Therefore,southwesterly to westerly seasonal winds prevailing in early summer could connect the source and the destination(Fig.3-A, B and F).In other words, northern Japan’s location with the appropriate distance and direction might be the reasons why it has become the major destination area of first-generationM.separatain Japan.

4.2.Flight duration

The flight hours ofM.separataover the mainland in China are believed to be from dusk to dawn, with duration of 10-h (Chenet al.1989; Fenget al.2008;Zhanget al.2018).In the present study, the estimated flight duration over the sea was 27–60 h.This result suggests thatM.separataadults should keep flying even during daytime over the sea.The migration durations over the East China Sea were estimated to be 20–36 h for other nocturnal moths,SpodopterafrugiperdaandSpodopteralitura(Tojoet al.2013; Wuet al.2022), which also suggests the necessity of daytime flights to reach the destination.Long-distance overseas migrations with continuous night-and-daytime flight might thus be common among nocturnal moths.However, the actual maximum flight duration remains unknown since the maximum flight duration of 60 h in this study is an assumed value under an analytical design.The true value might fall in between 27 and 60 h.One of the main controversies can be over this value for the overseas migration ofM.separata.To reveal true range of the value, radar observation and searchlight trapping under a possible direct pathway from eastern China over the Korean Peninsula to northern Japan should be conducted in future studies.Multiple entomological radars and searchlight traps are installed in western and eastern Korea and northern Japan in order to monitorM.separata’s migrations.If timing of the radar echo’s appearance and actual capture ofM.separatamigrants in the countries could match wind data and the insects’ flight speed, those analyses could support the direct migration, presenting an estimate of the maximum flight duration of this species.This approach could be applied to investigation of secondary emigrants from Korea and northeastern China as well.

4.3.Unexpectedly large immigration

Mythimnaseparatasometimes arrives in a destination other than northern Japan, depending on the everchanging weather conditions, and their arrival may cause a severe crop damage.AnM.separataoutbreak in western Japan in 2017 was such a case (TPPS 2017).Farmers and plant protection officers noticed an unusual event when older larvae ate the crop leaves,and it was too late to control this damage.Migration prediction is necessary to anticipate and identify such sudden unexpected immigrations as soon as possible,in order to allow the timely application of proper control measures.The potential sources ofM.separataidentified in the present study can be used to develop a migration prediction method that would calculate the movements ofM.separatataking off from the identified sources.A model that was developed to predict rice planthopper’migration is applicable to this situation (Otukaet al.2005).The findings presented herein provide the basic ecological knowledge that is essential for the future development of migration prediction techniques forM.separata.

5.Conclusion

Backward trajectories of first-generationMythimna separataimmigrants arriving in northern Japan and the flight characteristics of this moth suggest two possible migration pathways: a multi-step pathway from Northeast China, the Korean Peninsula, and eastern Russia, which are destinations of the first-generation migrations, and a direct pathway from a first-generation outbreak region in China such as Shandong and Jiangsu provinces.As this study presented only circumstantial evidence for the possible migration pathways, future studies are needed to support the conclusion with other kinds of evidence as well.

Acknowledgements

Authors thank Mr.Ken Oikawa of Aomori Plant Protection Station, Japan for supplying the trap data.This study was supported by the Strategic International Collaborative Research Project promoted by the Ministry of Agriculture,Forestry and Fisheries, Tokyo, Japan (JPJ008837).

Declaration of competing interest

The authors declare that they have no conflict of interest.

Appendicesassociated with this paper are available on https://doi.org/10.1016/j.jia.2023.06.001