Research on the invasive pest of fall armyworm (Spodoptera frugiperda)in China

The fall armyworm (Spodopterafrugiperda) is a major agricultural pest native to the Americas.It is widely distributed throughout the Western Hemisphere.It damages crops such as maize,wheat,rice,sorghum,cotton,herbage and various weeds,and reportedly feeds on more than 300 species (Montezanoet al.2018).Fall armyworm is a typical lepidoptera pest and goes through four life stages:egg,larva,pupa and adult.Adults can lay eggs between 1 000 and 2 000 and complete a life cycle within about 30 days,reproducing at a fast speed.There are two biotypes of fall armyworm:Corn-biotype (C-strain) and Rice-biotype(R-strain).Both biotypes are widely distributed throughout the Americas.C-strain mainly feeds on maize,sorghum,sugarcane,wheat,and cotton plants,while R-strain mainly feeds on grass and rice plants (Murúaet al.2015).They occasionally graze on plants in each other’s host ranges.The two biotypes can be accurately identified using eitherTPI(Triose-PhosphateIsomerase) orCOI(Cytochrome OxidaseSubunitI) molecular markers (Nagoshiet al.2012).In the Americas,fall armyworm is controlled mainly by chemical pesticides and Bt crops.Long-term use of chemical pesticides has resulted in widespread resistance,which has been reported throughout the Americas.Resistance to Bt crops has also recently been found,especially in South American countries.

Fall armyworm is unable to cross the Atlantic and Pacific oceans despite its strong migratory ability.Besides,the direction of monsoon is not conducive to the movement from the Western Hemisphere to the Eastern Hemisphere.Hence,fall armyworm did not invade the Eastern Hemisphere until it was discovered in Nigeria in 2016.A probable reason was that trade and transport of goods between the two hemispheres brought the pest to Africa.By the end of 2017,it had spread to all parts of Africa except the Sahara Desert,thanks to the continent’s hot and humid climate.The loss of maize yield caused by fall armyworm in various regions was estimated to be between 30 and 70% (Dayet al.2017),and some regions even suffered from complete losses of maize yield,which poses a great threat to food security.In 2017,the pest invaded Yemen and other places in West Asia.In 2018,it continued to invade India across the Arabian Sea and rapidly spread to all parts of Southeast Asia (Wuet al.2019).Academician Wu Kongming’s team at the Institute of Plant Protection,Chinese Academy of Agricultural Sciences(CAAS) accurately detected the invasion of fall armyworm on the China–Myanmar border at the end of 2018 through high-altitude light monitoring and molecular identification(Sunet al.2021).

Fall armyworm invasion may cause great harm to corn,rice and wheat,the three major staple crops in China,and thus threatens the nation’s food security.Due to its wide range of diets,it may also have an important effect on other crops.The challenges loom even larger in China as Chinese researchers and practitioners have limited knowledge of the population characteristics,occurrence regularity,biological characteristics and control methods of fall armyworm.It is quite likely that the pest can cause more serious damages in the new environment than in their origin,because the control factors such as natural enemies have not yet formed.

To address a series of challenging problems regarding fall armyworm,Dr.Sun Lujuan,the managing editor ofJournal ofIntegrativeAgriculture(JIA),invited me to organize a special issue,that aims to publish studies on the prevention and control of fall armyworm in China.I believe this is a meaningful and pressing mission and I am very much honored that many outstanding researchers contributed to this special issue.The experts include Academician Wu Kongming and Prof.Wan Fanghao from the Institute of Plant Protection,CAAS,Prof.Liu Kaiyu from Central China Normal University,Prof.Xu Pengjun from the Tobacco Research Institute of CAAS,Associate Prof.Hu Chaoxing from Guizhou University.I look forward to a speedy official publication of the special issue that will make considerable contributions to the prevention and control of fall armyworm in China.

The special issue collected 21 articles and divided them into the following four sections.First,Academician Wu Kongming and Prof.Wan Fanghao gave an overview and reflection on the biological characteristics of fall armyworm and its prevention strategies.The second,third and fourth sections collected research articles focusing on the pest’s invasion and migration,biological and ecological characteristics,and integrated pest management,respectively.

Section 1:Review of current research on fall armyworm

In this section,Zhou Yet al.(2021) introduced the invasion of fall armyworm into China since 2018.Based on the study about the biological habits,occurrence regularity and migration patterns of fall armyworm,the Chinese government implemented a regional control strategy and divided the areas infested with FAW into the annual breeding grounds in Southwest and South China,the transitional migration area in Jiangnan and Jianghuai and the key preventive area in the Huang-Huai-Hai region and North China (Zhou Yet al.2021).Tailored regional management strategies were implemented accordingly.The national monitoring and early warning technology platform built by the National Agricultural Technology Extension Center,China,can report pest occurrence at county level in real time.According to the information,local governments organized plant protection experts and farmers to carry out prevention and treatment work promptly by undertaking chemical,physicochemical induced and biological ecological control measures.The infestation of fall armyworm has been effectively controlled through the approach of joint prevention and control,which resulted in less than 5% loss of the overall yield in the affected area of fall armyworm from 2019 to 2020.This ensured the production safety of corn and other crops in China.Furthermore,Zhou Yet al.(2021) discussed the problems existed in the present and the future development directions of prevention and control strategies.Wanet al.(2021) summarized the invasion and rapid spread of fall armyworm in the Eastern Hemisphere since 2016.They also analyzed a series of superior biological characteristics of fall armyworm that contributed to its fast invasion,including broad host range (at least 353 species of host plants),innate ability to survive in a variety of environments,strong migration ability,fast reproduction rate,rapid development of resistance to insecticides/viruses,and gluttony.Therefore,comprehensive and effective prevention and treatment of fall armyworm must rely on monitoring and reconnaissance,agricultural prevention,chemical pesticides,virus insecticides,sex attractants,biological control agents (parasites,predators,and insect pathogens) and botanical preparations.Besides,its intrusion mechanism should be further studied in order to provide reference for future management.

Section 2:Invasion and migration

One characteristic of fall armyworm is its ability of longdistance migration.Tracing its origin and migration path therefore plays an important role in monitoring,early warning and prevention.Sunet al.(2021) discovered that the fall armyworm invading Yunnan Province was originally from Myanmar.They accurately located its origin in the eastern and northern areas of Myanmar through wind direction simulations,and deduced its beginning time of migratory flight.This study provided guidance for source tracking and prevention of fall armyworm in China.Zhou X Yet al.(2021) conducted migration monitoring on Yongxing Island in the South China Sea,and used wind direction simulations to find that fall armyworm on the island came from Vietnam and surrounding areas.Moreover,Zhou X Yet al.(2021)and Jiaet al.(2021) carried out molecular identification of fall armyworm trapped by searchlight,and the results suggested that they were the C-strain.Wu Q Let al.(2021) deduced the migration path of fall armyworm in Northwest and North China through its monitoring system in Qinling Mountains–Huaihe River region.They also identified that the fall armyworm population in these areas was the C-strain.In addition,Geet al.(2021a) and Heet al.(2021a) optimized the deduction of fall armyworm’s migration path,through conducting a systematic study on its flight characteristics under different environmental conditions that take into consideration temperature and humidity factors.Further,Geet al.(2021b) found that flight activity promotes reproductive processes in the fall armyworm.Through a series of study,researchers have clarified the origin and migration path of the invasion in China,and accurately identified its bio-type.These studies provided important theoretical guidance for the pest’s early warning and prevention in China.

Section 3:Biological and ecological characteristics

As a new invasive species in China,fall armyworm’s host range in the country needs to be identified in a quick and accurately manner.Heet al.(2021b) investigated its possible main host crops,including maize,sorghum,wheat,rice and artificial feed.They found that maize was the best host plant,and sorghum and wheat were also suitable host plants,while rice did not provide a host that fall armyworm can complete its life-cycle.This finding was in agreement with the study about identification of fall armyworm biotypes by Zhanget al.(2019a,b).Heet al.(2021c) and Wu L Het al.(2021) further studied the growth of fall armyworm on potential hosts such as oilseed rapes,soybeans,sunflowers,peppers,tomatoes and eggplants,and showed that fall armyworm can cause serious damage to these plants.These results provided important references for targeted control of fall armyworm in China.Fall armyworm’s cold resistance determines its overwintering area and migration source.Zhanget al.(2021b) conducted a systematic study of fall armyworm at different growth stages under different low temperature conditions.They found that it had a low resistance to cold temperatures,which suggested that its overwintering region was mainly in the southern areas of the subtropical zone in China.Yanget al.(2021) confirmed the results of its low cold resistance through field investigation,whereby the annual breeding areas of fall armyworm in China could be preliminarily delimited.

Section 4:Integrated pest management

The prevention of fall armyworm requires comprehensive pest control approaches,including chemical control,biological control,physical control,transgenic crop planting and other methods.The application of chemical pesticide is one of the most effective methods for fall armyworm prevention at present.It has been widely reported that in the Americas,fall armyworm has developed resistance to various kinds of pesticides,while the resistance in China has not been adequately studied.Zhanget al.(2021a) identified five populations of fall armyworm in four provinces of China including Yunnan and found that all the populations had developed high resistance to pesticides such as organophosphorus,oxadiazine and pyrethroids,except amides.This was consistent with the results of scanning the resistance locus of fall armyworm population in China recently published by Zhanget al.(2020).They also found that the current population of fall armyworm in China was still susceptible to both Cry and Vip toxin,indicating that it was feasible to use Bt toxin for prevention.Lianget al.(2021) evaluated the expression level of Cry1Ab toxin in Chinese transgenic maize with bio-safety certificates and its efficacy against fall armyworm.The result showed that the toxin expression level in related cultivars can provide effective prevention against fall armyworm.Heet al.(2021d) discovered that in the case of mixed planting of Bt maize and conventional maize in the field,there was no difference in oviposition selection of fall armyworm on the two cultivars before they were damaged by the pest.However,fall armyworm was found to prefer Bt maize to the conventional maize.Bt maize therefore played a role in trapping fall armyworm,which may significantly strengthen its prevention.In terms of the potential resistance issue in fall armyworm population due to the planting of Bt maize,Jinet al.(2021) obtained a molecular marker gene with CRISPR/Cas9 system,which ensured subsequent resistance monitoring and control.In terms of physical prevention.Liuet al.(2021) studied the phototaxis of fall armyworm and found the relationship between the expression difference of visual protein genes and the phototaxis level,using cotton bollworm as control.This study provided theoretical guidance for preventing fall armyworm by means of searchlight induction.Liet al.(2021)studied the two-way predation between fall armyworm and the endemic syrphid at immature stages.Chenet al.(2021) investigated fall armyworm’s infection ofJunonia coeniadensovirus.These researches provided practical recommendations for the development and utilization of natural enemy insects and microorganisms.