Seasonal Characteristics Of Migratory Biology In Oriental Armworm,Mythimna Separata Walker

Oriental armyworm Mythimna separata Walker（Lepidoptera:Noctuidae,Hadenina）widely distributed in most countries of Asia and Oceania,is a very typical long-distance seasonal north-south migration pest,also can migrate between high and low altitudes.In China,the insect mainly affects wheat,maize and rice.Because of its long-distance migratory behavior,it often causes serious damage to the crops in the area of destination.However,most of the studies on the migratory biology of M.separata in the domestic and overseas are limited to overlapping population generations,while the biological characteristics of its migrating population in the field are rarely studied.Therefore,to elucidate the seasonal characteristics of migratory biology of M.separata and to improve the ability and level of regional monitoring and early warning,forecasting and integrated management of M.separata related studies were conducted.In this series of studies,the seasonal population dynamics of migratory M.separata and the seasonal differences of morphological characteristics,physiological characteristics,flight parameters,stable isotopes and directional selection behavior of migratory populations were studied,and the following research results were obtained:（1）The monitoring of the population dynamics of M.separata migrating across the Bohai sea showed seasonal and interannual population dynamics in 2018,2019,2020 and 2021 at Beihuang Island,China.Among these,2019 is a large occurrence year of migration flights across the Bohai sea,while 2018,2020 and 2021are common occurrence years.The migration of M.separata across the Bohai sea starts from early April and lasts until late October every year,with an average migration period of 198 days.The peak of northward migration occurred from the end of May to the beginning of June,and the peak of southward migration occurred from the end of August to the beginning of September.The single day catch of northward migration peaked at 2,212（June 7,2018）,and that of southward migration peaked at31,536（August 31,2019）.According to the population dynamics of M.separata,the migration period can be divided into three periods:P₁（01/April-25/June）was the northward migration period,P₂（26/June-14/August）was the mixed migration period,and P₃（15/August-31/October）was the southward migration period.The number of M.separata during the southern migration period was greater than the northern migration period and the mixed migration period.The sex ratio of M.separata decreased in different seasons,and females outnumber males.（2）The morphological parameters（body weight,abdominal weight,body width and forewing width）of the migrating M.separata were different in different seasons at each site.The morphological parameters of spring migrating population were higher than those of autumn migrating population.The morphological parameters of migrating M.separata were also different among different monitoring sites,the morphological parameters of the migratory populations increased gradually from south to north sites.The body length,body width and forewing length of migrating M.separata moths were significantly higher than those of reared moths,and the forewing width of migrating females was significantly higher than that of reared females.（3）The ovary development of migrating M.separata trapped by searchlight from Beihuang Island in 2018-2021 showed that the proportion of ovarian development levelⅠand the proportion of unmated individuals increased gradually from spring to autumn,while the proportion of ovarian development levelⅡandⅢ,and the proportion of mating 1 and 2 times and the mating rate decreased gradually from spring to autumn.The egg loading of M.separata was more in June and August,but less in April and July in 2021.At the same time,the contents of triglyceride and protein in the thorax and abdomen of migrating M.separata at different monitoring sites in 2018 showed that the contents of triglyceride in the thorax and abdomen of migrating male moth was higher in spring than in summer and autumn at Beihuang Island.The protein content in the thorax and abdomen of female moths in summer was higher than that in autumn at Harbin,just the opposite in male moths;The protein content in the thorax and abdomen of female and male moths in autumn was higher than that in spring at Shenyang;The protein content in the thorax of female and male moths in spring was higher than that in autumn at Beihuang Island;The protein content in the thorax and abdomen of female and male moths in spring was higher than that in autumn at Lancang.Overall,the content of triglyceride and protein in the thorax and abdomen of female moths was higher than that of male moths.（4）The results of different flight capability parameters of the migratory population of M.separata at Beihuang Island in 2021 showed that the flight distance and flight time of both migrating female and male moths in spring were significantly greater than those in summer and autumn,and the flight speed of migrating female moths in summer was significantly greater than those in spring and autumn,but the flight speed in spring was significantly greater than those in summer and autumn in migrating male moths.In conclusion,the flight ability of M.separata migrating population in spring is greater than that in summer and autumn.In addition,the wingbeat frequencies（WBFs）of both cultured and migrating M.separata were tested under laboratory using a stroboscope under different conditions.Age and mating status significantly influenced WBF.One day old adults had the lowest WBF,The WBF increases gradually with the increase of age,and unmated females had a significantly higher WBF than that of mated females.In general,the WBF of males was significantly higher than that of female individuals.The WBF decreased gradually with increase of environmental humidity,and WBF had a significant negative binomial regression relationship with temperature change.The WBF of the moths fed on hydromel was much higher than those of the controls fed on water or without diet.However,wind speed and air pressure had no significant effects on the moth WBF in the test environments.（5）The results of the stable isotopes ratio of carbon and hydrogen in their wings showed that the proportion of migrating M.separata at Beihuang Island and other sites in 2018 fed on C₄plants in their larval stage was higher than that of C₃plants,indicating that most migrating M.separata in their larval stage mainly fed on C₄plants.TheδD values in migrating M.separata wings at Beihuang Island had significant differences in different seasons between 2018 and 2021.TheδD values in migrating M.separata from Beihuang Island decreased gradually with the seasonal changes.In 2018,theδD in the wings of migrating M.separata from each monitoring site in different seasons were also different,and theδD values in the wings of migrating M.separata at Beihuang Island,Xinxiang and Lancang decreased gradually with seasonal changes.TheδD values in the wings of migrating M.separata also had significant differences among different monitoring sites,and theδD values increased gradually with the decrease of latitude.（6）The results of the orientation preference of migrating M.separata across the Bohai sea at Beihuang Island during different seasons in 2019 to 2021 showed that the orientating behavior of migrating M.separata was obvious.The migrating M.separata trapped in spring at the peak of migration tended to fly north and northeast.At this time,M.separata migrated from south to north and northeast,which was the northward migration period.The migrating M.separata trapped in autumn at the peak of migration preferred to fly south and southwest.At this time,M.separata migrated from northeast to south and southwest,which was the southward migration period.Briefly,in this study,seasonal characteristics of the population dynamics,morphological characteristics,flight parameters,reproduction parameters,internal energy materials and of larval host plants of migrating M.separata were determined in 2018 to 2021 at Beihuang Island and other monitoring sites,and the migratory population of M.separata in autumn is larger than that in spring season.The morphology,flight ability,fecundity and energy content of the spring migratory population were significantly higher than those of the autumn population.The migration across the Bohai sea of M.separata is directional selective,northward migration in spring and southward in autumn.The above research results enrich the content of migratory biology of M.separata,and provide a basis for further study on the migratory biology of M.separata in worldwide.Meanwhile,it also provides data support for identification parameters for the research and development of insect radar and monitoring and identification of migrating M.separata.