| Migratory pests pose a continuous threat to global food security,including China,due to their sudden,explosive,and destructive nature.The surrounding nations and territories of the South China Sea,situated within the tropics,serve as the year-round breeding region for numerous migratory insects and as the origin of northward migrations in spring and summer,as well as the overwintering areas for returning populations in autumn.Thus,clarifying the patterns of migratory insect occurrences in this area is extremely important for understanding the disaster mechanism of crop pests and improving the ability of early-warning system.Here,we employs multiple technological approaches and methods,such as searchlight trap,DNA barcoding,isotopic tracing,pollen labeling,and trajectory simulation,to systematically monitor and investigate the transoceanic migration behavior of six critical noctuid agricultural pests(Spodoptera frugiperda,Spodoptera exigua,Spodoptera litura,Ctenoplusia albostriata,Trichoplusia ni,and Ctenoplusia agnata),as well as two rice planthoppers(Nilaparvata lugens and Sogatella furcifera)and their natural enemy,Cyrtorhinus lividipennis,on the small island Yongxing Island in the northern region of the South China Sea during 2017–2022.The main results were summarized as follows:1.For fall armyworm(FAW)Spodoptera frugiperda,from 2019 up till 2022,molecular and morphology-based identification confirmed that FAW made its first appearance on Yongxing Island on 11 April 2019.A total of 132 moths were captured during the four-year period,with the outbreak in 2020.Migration of FAW occurred the whole year,and the mean time from the earliest trapping to the latest trapping within a year was 194.00 ± 22.08 d.An obvious migration wave was found from July to September.During the monitoring period,the proportion of females was less than males.And DNA barcoding showed most trapped moths belonging to “corn-strain”.Meanwhile,the result of carbon isotope showed FAW moths mainly originated from C4 host plants,which carried 18 plant species of pollens inferred from carrier pollens.Trajectory analyses revealed that migratory S.frugiperda adults are able to cross the South China Sea and enter South China in spring(summer)and ruturn back to Indo-China Penisula in autumn.2.For beet armyworm(BAW)Spodoptera exigua,a total of 294 individuals were captured during 2017–2021,with the outbreak in 2020.Migration of BAW occurred the whole year,and the mean time from the earliest trapping to the latest trapping within a year was 195.40 ± 57.34 d,with peak of migration in July–October.During the monitoring period,the proportion of females was less than males.Pollen analysis revealed that 82.69%(172 / 208)of individuals were contaminated with 22 species of pollens,and further showed that pollen was found primarily on the proboscis and compound eyes.Trajectory analysis unveiled that beet armyworm adults were from Indo-China Penisula and the Philippines and migrated into South China and North Vietnam in spring and summer,then flew back from China and northern Luzon Island of the Philippines to Indochina Peninsula for wintering in autumn.3.For common cutworm(CCW)Spodoptera litura,a total of 646 moths were captured.The vast majority of CCW moths were trapped in 2020 and 2021.Migration of CCW occurred the whole year,mainly occurred from July to November,with the mean time from the earliest capture to the latest capture within a given year was 326.00 ± 41.65 d.During the monitoring period,the proportion of females was less than males.Trajectories showed CCW migrated from Luzon Island of the Philippines to Hainan Province in China and North Central Coast of Vietnam in spring,from Indochina Peninsula to South China(Hainan,Fujian and Taiwan Provinces)in summer,and from China and Luzon Island of the Philippines to Indochina Peninsula in autumn.4.For Ctenoplusia albostriata,a total of 59 catches were captured from 2017 to 2021,with most individuals in 2018.C.albostriata undertook the long distance migration during January–November,with the mean time from the earliest capture to the latest capture within a given year was123.00 ± 45.38 d,and the vast majority of moths were trapped in September and October.As for sex ratio,the proportion of females was less than males.Pollen analysis revealed that 98.18%(54 /55)of adults were contaminated with 19 species of pollens.Simulation of trajectories revealed that C.albostriata migrated from Guangxi Province of China and Indochina Peninsula to Luzon Island in spring,from Indochina Peninsula to Hainan Province in China and Luzon Island of the Philippines in summer,and from China and the Philippines to Indochina Peninsula for overwintering in autumn and winter.5.For cabbage looper(CL)Trichoplusia ni,a total of 173 individuals were captured within2017–2021,with the outbreak in 2018.Migration of CL occurred the whole year,mainly occurred from September to October,with the mean time from the earliest capture to the latest capture within a given year was 251.00 ± 41.97 d.As for sex ratio,the proportion of females was less than males.95.59%(130 / 136)of T.ni individuals were contaminated with 31 species of pollens.Trajectory simulations showed that cabbage looper took northward flight from south part of Indochina Peninsula to North Central Coast of Vietnam in summer,then took migrations which originated from China and the Philippines to Indochina Peninsula for overwintering in autumn and winter.6.For Ctenoplusia agnata,from 2017 to 2021,a total of 2,201 moths were captured with the outbreak in 2018.Migration of C.agnata occurred the whole year,mainly occurred from October to February the next year,with the mean time from the earliest capture to the latest capture within a given year was 333.40 ± 19.41 d.As for sex ratio,the proportion of females was less than males.7.For brown planthopper(BPH)Nilaparvata lugens,white-backed planthopper(WBPH)Sogatella furcifera and Cyrtorhinus lividipennis(Mirid),results provided direct evidence that C.lividipennis and its host RPH must have been migrants,and regularly migrated together across the sea from March to October every year.And significantly positive correlation was observed in the biomass between C.lividipennis and two types of RPH in all migration periods,which showed following phenomena and time lag phenomena.Moreover,the percentage of migrants was strongly female biased,and the results of ovarian dissection indicated that almost all trapped females were virgins with little or no ovarian development,as suggests that the onset of migration is initiated mainly by sexually immature individuals and the development of ovary teminate,which conformed to the ‘oogenesis–flight syndrome’ theory.In addition,simulation of possible migration trajectories demonstrated that all of three species mainly migrates between Indochina Peninsula and China(or the Philippines)in both directions.Overall,these findings determined overseas migration patterns of several agricultural insect pests between Southeast Asia and South China,as well as contributed to better understand the accompanying migration by natural enemies with their host insects,which will help advance pest forecasting,risk assessment and area-wide pest management. |
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