| Fall armyworm is a worldwide and invasive major agricultural pest,which invaded southern Yunnan at the end of 2018 and rapidly migrated along the southwest-northeast direction,and has caused serious damage to a variety of crops,such as maize,wheat and sorghum,in China,threatening the agricultural production and food security in China.Southern Yunnan and Indo-China Peninsula are located in the tropics and the south subtropics,which is the year-round breeding area of fall armyworm.And southern Yunnan is the southern gate for the north-migrating population of fall armyworm population form Indo-China Peninsula into China.It is necessary to clarify the population occurrence and migration pattern of fall armyworm in southern Yunnan to support the national prevention and control work against fall armyworm.Based on the field monitoring station of Jiangcheng in southern Yunnan border areas,this study systematically monitored and analyzed the population dynamics of fall armyworm in maize fields and cross-border migration dynamics,and evaluated the efficacies of pesticides and Bt maize against fall armyworm population occurrences from 2019 to 2021.The main findings are summarized as follows:1.The cross-border migratory activity of fall armyworm in southern Yunnan has three main northward migration periods throughout the year,namely the spring northward migration period（March1 to May 20）,the summer northward migration period（May 20 to July 19）,and the autumn northward migration period（September 7 to October 27）,and has a distinct annual cycle（53.2 weeks）,quarterly cycle（18 weeks）,and generation cycle（4.3 weeks）.In terms of sex ratio,there is an apparent male bias phenomenon among trapped migrant population.The seasonal fluctuations of ovarian development level,mating frequency,sexual maturity rate and mating rate of females were not obvious,indicating that there is no substantial geographical difference in the source of the trapped fall armyworm populations in Jiangcheng.Basing on the cross-border migration pattern of fall armyworm in southern Yunnan Province,the peak migration period from spring to autumn is a critical to intercept the migration of northward-migrating populations above key corridors by vertical searching lithg trap belt.2.The vertical flight speed of airborne fall armywrom in the four typical migration events in 2020and 2021 were distributed in the range of 0.17-0.60 m/s,showing an upward trend of take-off;the flight direction was northeast,and always kept rightward angle of inclusion with the wind direction,ranged from 21.37°to 50.74°,that is,fall armyworms showed a strong orientation behavior and a strong wind adaptation when flying in the air;the aggregation of flight direction becomes more concentrated as the flight height rises;the vertical flight speed increases with the increasing of air temperature at high altitude,which is a direct manifestation of the migration process in the annual breeding area;the distribution of the self-power flight speed in the air is concentrated in the range of 10m/s to 20m/s,and accelerates as the flight height rises.This indicates that fall armyworm has strong adaptability to high altitude airflow and can dynamically adjust its orientation to the northeast by increasing its wingbeat frequency to counteract the change of wind direction.The application of insect radar helps to analyze the aerial migration mechanism of fall armyworm,especially torientation behavior and adaptation to meteorological conditions,and the accurate detection of flight parameters helps to improve the prediction accuracy of population tracing and immigration area,which can better serve the national management of fall armyworm.3.The analysis of the meteorological background showed that the air temperature field at night over southern Yunnan from March to October was suitable for the migration activities of fall armyworm,and the prevailing southwest monsoon from March to August were favorable for the continuous migration of adult populations in southern Yunnan and Indo-China Peninsula,while the prevailing southeast monsoon from September to October affected the northward migration activities to a certain extent;the high altitude air temperature over southern Yunnan from November to February was near or below the flight temperature threshold of fall armyworm,and the northward migration activities were weakened.The high-altitude meteorological background was the exogenous driving force for fall armyworm migration,and the intensity of the southwest monsoon above southern Yunnan should be closely monitored to cope with the regular changes of the northern migration of fall armyworm.HYSPLIT trajectory analysis showed that the northward migrating populations of fall armyworm via Jiangcheng mainly originated from northeastern Myanmar（40.04±7.18%）,southern Yunnan（33.77±4.60%）and northern Laos（16.73±4.11%）.However,central and northeastern Yunnan（60.11±10.29%）,southern Guizhou（17.63±5.27%）,western Guangxi（17.63±5.27%）,and southern Guizhou（17.63±5.27%）.5.27%),and western Guangxi（15.64±5.44%）were the main immigrant areas.The continued invasion of north-migrant populations from abroad has increased the control pressure and disaster risk of fall armyworm in Yunnan,Guizhou and Guangxi,therefore,source population management and interception of north-migrant populations in southern Yunnan and Indo-China Peninsula are key to the national management of fall armyworm.4.Fall armyworm can infest maize year-round in all planting season in southern Yunnan.Larval population densities varied significantly among planting seasons and different growth stages.In terms of instar structure dynamics,there were 2-3 peak infestation periods for the younger larvae at each planting stage,mainly at the V3-V5,V7-V10 and R1,indicating that fall armyworm could occur in 2-3 generations accompanied by serious overlapping generations on maize at different planting seasons in southern Yunnan.Year-round infestation of fall armyworm in southern Yunnan directly causes crop yield losses on the one hand,and forms a constant stream of northward migrant populations on the other.The population accumulation should be suppressed in time by using integrated control measures such as light/food/sex traps,biological control and chemical control,to prevent large-scale and regional emigration.5.Scientific and rational use of pesticides is an important measure to avoid economic losses and reduce the occurrence of pests in the population.New chemical insecticides,such as emamectin benzoate,chlorantraniliprole,and spinetoram,showed high control efficiency and very strong insecticidal activity against fal armyworm,and can be used as the preferred single agent to carry out chemical control measures.Spinosad and abamectin,Bacillus thuringiensis and Beauveria bassiana have high control efficiency against 2nd instar larvae of fal armyworm,and could be applied to the biological control given condition of lower instar and lower density populations.6.Fall armyworm showed an obvious oviposition bias for Bt maize after infestation on non-Bt maize,and the use of Bt maize as a trap-killing plant could continuously attract females to lay egg-masses and kill the offspring concentratedly,which providing continuous protection for other major host crops and reducing the population size and also level of infestation in the whole crop system.Field trials showed that DBN3601T maize could control the occurrence of fall armyworm,and continuously protect Bt maize from fall armyworm feeding during critical growth stages.At the same time,fall armyworm showed a more obivious oviposition bias for DBN3601T under serious infestation with high larvae density than mild infestation with low larvae density.Higher incidience of damaged plant in the R stage of Bt maize were existed,and the damaged proportion of different ear tissues were ranked as follow:silk>kernel>husk,which is related to the spatial and temporal expression of Bt proteins in Bt maiz.Overall,the weighted control efficiencies of DBN3601T maize against fall armyworm reached more than 95%,which is a direct indication of an excellent insect-resistance maize.The transgenic insect-resistant maize is sustainable and efficient in the control of fall armyworm infestation,and it can be used as a trap-killing plant to provide protection for multiple cropping systems,which is the core of the future management for fall armyworm infestation in China.Synthesizing the above research results,nationwide integrated management of fall armyworm in China must be based on the seasonal migration patterns,and adopt partition management strategy to build a green and sustainable integrated control technology system with accurate population monitoring and early warning,efficient migration interception and Bt maize cultivation as the core.Firstly,in order to the control regional migration activitity of fall armyworm,the establishment of ground and aerial population dynamic monitoring network,insect radar network-trajectory analysis collaborative monitoring and early warning platform are necessary to serve the implementation of accurate and efficient migrant population interception and ground population management.Secondly,with the aim of efficient management for field larval population and occurrence,a sustainable management system combined with pesticide emergency control and transgenic insect-resistant maize planting should be implemented in year-round breeding area. |
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