ZHANG Dan-dan ,XIAO Yu-tao ,XU Peng-jun ,YANG Xian-ming ,WU Qiu-lin ,WU Kong-ming

1 State Key Laboratory for Biology of Plant Diseases and Insect Pests,Institute of Plant Protection,Chinese Academy of Agricultural Sciences,Beijing 100193,P.R.China

2 Agricultural Genomics Institute at Shenzhen,Chinese Academy of Agricultural Sciences,Shenzhen 518120,P.R.China

3 Tobacco Research Institute,Chinese Academy of Agricultural Sciences,Qingdao 266101,P.R.China

Abstract Fall armyworm has invaded China and colonized its populations in tropical and sub-tropical regions of South China since December 2018.Chemical spray has been widely used to control the pest,which shall lead to resistance evolution.In this research,we collected five populations of the pest from Yunnan,Hainan,Tibet,and Fujian of China,and tested their susceptibilities to pyrethroid,organophosphorus,oxadiazine,diamide,antibiotics and other types of insecticides(14 insecticides totally) in the laboratory.Based on the susceptible baseline published from the previous studies,the resistance ratio was 615–1 068-fold to chlorpyrifos,60–388-fold to spinosad,26–317-fold to lambda-cyhalothrin,13–29-fold to malathion,9–33-fold to fenvalerate,8–20-fold to deltamethrin,3–8-fold to emamectin benzoate and 1–2-fold to chlorantraniliprole,respectively.The median lethal concentration (LC50) of other six insecticides without the susceptible baselines was 148.27–220.96 µg mL–1 for beta-cypermethrin,87.03–128.43 µg mL–1 for chlorfenapyr,16.35–99.67 µg mL–1 for indoxacarb,10.55–51.01 µg mL–1 for phoxim,7.08–8.78 µg mL–1 for M-EBI (the mixed insecticide of emamectin benzoate and indoxcarb) and 1.49–4.64 µg mL–1 for cyantraniliprole.This study can be helpful for chemical control as well as for resistance monitoring and management of the pest in China.

Keywords:Spodoptera frugiperda,insecticides,resistance monitoring,China

1.Introduction

Fall armyworm (FAW),Spodopterafrugiperda(J.E.Smith 1797) is native to tropical and sub-tropical areas of American continent,and has rapidly spread through the Old World due to its outstanding migratory capacity.In January 2016,serious damages byS.frugiperdawere found in West Africa and Central Africa (Goergenet al.2016).Just one or several nights are needed to complete the flight from western Africa to southern Africa (Earlyet al.2018).As of 28 September 2017,FAW has been found in 28 sub-Saharan African countries,and its presence was suspected in nine other countries (Dayet al.2017).In May 2018,it was discovered in Karnataka in southwestern India (Sharanabasappaet al.2018),and by late 2018,its outbreak was found in other Asian countries,such as Thailand (FAO 2018),Sri Lanka(FAO 2019b),Bangladesh (FAO 2019a),Myanmar (FAO 2019c) and China (Sunet al.2021).

FAW has become one of the major invasive pests in China since it was first captured on 11 December 2018(Sunet al.2021).As of 8 October 2019,it had invaded 26 provinces (autonomous regions,municipalities) in China.FAW consists of two strains,the “corn-strain” and the “ricestrain”.The former prefers corn,cotton,sorghum,and the latter prefers rice,pasture grasses (Dumaset al.2015).The population invaded China was the “corn-strain” from Florida(Zhanget al.2019) which principally attacks corn,sugarcane and sorghum,but can also damage wheat,peanut and goosegrass (Heet al.2019; Yanget al.2019).FAW feeds on corn leaves,tassel and ears during the larval stage,causing extensive defoliation,damage to the corn husk,and destruction to the growth potential of corn plant.China is one of the largest corn producers in the world,and therefore faces a high risk to food security and agricultural production due to the invasion of FAW,especially when considering corn is one of the crops with the largest planting area.

FAW is a long-distance migratory pest,with regional and sudden outbreaks.Chemical control is the major method of emergency prevention and control due to its efficient and quick effect and convenient usage (Gao 2010; Wu 2018).In Americas,chemical insecticide is still the main method especially when FAW developed resistance to some Bt maize varieties (Storeret al.2010; Fatorettoet al.2017).In China,chemical control plays an even more important role in the absence of Bt maize.However,the insecticide resistance evolution of FAW is driven by an intensive crop production system and bioecological characteristics of the species,such as high population density and overlapping generations in the agroecosystem.In American countries,the FAW has evolved resistance to at least 29 insecticidal active ingredients in six modes of action group insecticides because of the strong selection pressure caused by intense and frequent application of insecticides (Gutierrez-Morenoet al.2019).

Resistance monitoring of FAW is essential and important for its effective integrated pest management (IPM) and insecticide resistance management (IRM).In this study,we measured the resistance level of five FAW populations in China to different kinds of insecticides.It can be used to inform integrated pest management strategies for monitoring and controlling FAW,and provide a susceptible baseline to insecticides of FAW in China.

2.Materials and methods

2.1.Fall armyworm populations

We collected FAW populations from Linzhi (LZ2019),Quanzhou (QZ2019),Xundian (XD2019),Yuanmou (YM2019)and Haikou (HK2019),China.The collect information of FAW populations was listed in Table 1.The populations were reared on artificial diet without any Bt proteins and insecticides.The diet mainly contained wheat bran and soybean powders,and the operation method was referred to the artificial diet of cotton bollworm (Lianget al.1999).The diet was mixed thoroughly with a rod,allowed to cool and harden,then cut into small pieces.The larvae of FAW were put into the 24-well plates that contained pieces of diet and were reared to the pupal stage,then pupae were kept separately in a glass tube for adult emergence.The adults were placed in cages (40 cm×25 cm×25 cm) and fed with 5% sugar solution.The top of each cage was covered with white medical gauze for oviposition,which was changed every 24 h.Then the gauze was put into a zip lock bag with eggs.The bioassays were conducted on the resulting 3rd larvae.All of them were placed in an insect chamber with a controlled environment of (26±1)°C,(60±10)% RH and a photoperiod of 16 h:8 h (L:D).

2.2.Insecticide

We measured the median lethal concentrations (LC50) values to pyrethroid,organophosphorus,oxadiazine,diamide,antibiotics and other types of insecticides,the insecticide information is presented in Table 2.

2.3.Bioassay procedures

Bioassay was conducted by a topical application procedure(Armeset al.1992).Drops of 1.0 µL serial dilution oftechnical insecticide in acetone solution were applied with a micropipette to the thoracic dorsum of the 3rd instars.And the control larvae were treated with 1.0 µL acetone.After treatment,the larvae were individually put into 24-well plates containing adequate fresh normal artificial diet(without any Bt proteins and insecticides).All of them were placed in an insect chamber with a controlled environment of (26±1)°C,(60±10)% RH and a photoperiod of 16 h:8 h(L:D).Mortalities were assessed after 72 h treatment.Larvae were considered dead if they were unable to move in a coordinated manner when prodded with a small soft brush.

Table 1 The population information of fall armyworm in China

Table 2 General information on insecticides used against fall armyworm populations in topical bioassays

2.4.Data analysis

We used LC50to evaluate the insecticide resistance level of different FAW populations.The LC50values were estimated by probit analysis using the software package POLO-PC(LeOra Software,Berkeley,CA,USA) (Russellet al.1977).The significant difference was considered by whether the 95% fiducial limits (95% FL) have overlap.

3.Results

3.1.Pyrethroid

The LC50values of different FAW populations to pyrethroid insecticides are presented in Table 3.The LC50value ranges to beta-cypermethrin,fenvalerate,deltamethrin and lambdacyhalothrin were 148.27 µg mL–1(QZ2019)–220.96 µg mL–1(HK2019),301.32 µg mL–1(XD2019)–1058.58 µg mL–1(QZ2019),32.03 µg mL–1(LZ2019)–78.49 µg mL–1(HK2019)and 13.54 µg mL–1(YM2019)–37.34 µg mL–1(XD2019),respectively.There was no significant difference among the LC50values to beta-cypermethrin and lambda-cyhalothrin of different populations,the LC50value to fenvalerate of HK2019 was significantly higher than that of XD2019,and the LC50value to deltamethrin of HK2019 was significantly higher than that of LZ2019 and XD2019.

3.2.Organophosphorus insecticide

The LC50values of different FAW populations to organophosphorus insecticides are presented in Table 4.The LC50value ranges to malathion,chlorpyrifos and phoxim were 1 934.04 µg mL–1(YM2019)–4 303.74 µg mL–1(XD2019),7 606.25 µg mL–1(YM2019)–1 3215.18 µg mL–1(LZ2019)and 10.55 µg mL–1(LZ2019)–51.01 µg mL–1(XD2019),respectively.The LC50values to these three insecticides among different populations were not significantly different.

3.3.Oxadiazine,diamide,antibiotics and other types of insecticides

The LC50values of different FAW populations to oxadiazine,diamide,antibiotics and other types of insecticides are presented in Table 5.The LC50value ranges to indoxacarb,chlorantraniliprole,cyantraniliprole,spinosad,emamectin benzoate,M-EBI and chlorfenapyr were 16.35 µg mL–1(XD2019)–99.67 µg mL–1(LZ2019),1.28 µg mL–1(LZ2019)–2.34 µg mL–1,1.49 µg mL–1(LZ2019)–4.64 µg mL–1(XD2019),598.57 µg mL–1(LZ2019)–3 878.74 µg mL–1(QZ2019),1.94 µg mL–1(YM2019)–4.59 µg mL–1(QZ2019),7.08 µg mL–1(XD2019)–8.78 µg mL–1(QZ2019 and YM2019) and 87.03 µg mL–1(YM2019)–128.43 µg mL–1(QZ2019).

The LC50value to indoxacarb of XD2019 was significantly lower than that of LZ2019,HK2019 and YM2019,the LC50value to cyantraniliprole of LZ2019 was significantly lower than that of HK2019 and XD2019,and there was no significant difference among the LC50values to chlorantraniliprole,spinosad,emamectin benzoate,M-EBI,and chlorfenapyr of different populations.

4.Discussion

Insecticide resistance of FAW has been reported in many American countries.In Mexico,about 3 000 tons of synthetic insecticides were used for controlling FAW every year (Blancoet al.2014).The long-term and extensive usage of chemicals prompted resistance development to chlorpyrifos and permethrin (Gutierrez-Morenoet al.2019).In Puerto Rico,synthetic insecticide is the major control strategy of FAW,and it has evolved remarkable resistance to a wide range of insecticides in the field (Gutierrez-Morenoet al.2019).In Brazil,FAW is one of the most serious pests in maize and cotton fields,and the intensive application of chemical insecticides to control it was up to three sprays per season (Burtetet al.2017).As a result,FAW has developed resistance to synthetic insecticides,addressing different modes of action,such as lambda-cyhalothrin,chlorpyrifos (Carvalhoet al.2013),lufenuron(Nascimentoet al.2016) and spinosad(Okumaet al.2018).

We calculated the resistance ratio to eight insecticides of these five invaded FAW populations in China based on the susceptible baseline to fenvalerate,lambdacyhalothrin,malathion and chlorpyrifos from Yu (1991) (the susceptible stain was originally obtained from the USDA in Tifton,Georgia in 1975) and susceptible baseline to deltamethrin,chlorantraniliprole,spinosad and emamectin benzoate from Gutierrez-Morenoet al.(2019) (the susceptible stain was provided by Monsanto Company,USA).

FAW is a new invasive pest in China,we did not have any susceptible population,and can only use the susceptible value of insecticides reported in the references.However,the topical application method adopted in our research,existed some different points with Yu (1991) and Gutierrez-Morenoet al.(2019).The 3rd instar larvae were tested in our study,but 4th instar larvae in Yu (1991) and 2nd instar larvae in Gutierrez-Morenoet al.(2019).The treatment time was 72 h in our research and Gutierrez-Morenoet al.(2019),but 48 h in Yu (1991).In consideration of different units for the insect dose responses in different paper,we recalculated and standardized them as µg mL–1in our paper.Although the resistance ratios here were calculated based on the data from previous reports,we thought it was still valuable and can serve as a reference for indicating resistance level of the invaded populations.

The resistance ratio of these five FAW populations in China to chlorpyrifos,spinosad,lambda-cyhalothrin,malathion,fenvalerate,deltameththerin,emamectin benzoate and chlorantraniliprole ranged from 615-to 1 068-fold,60-to 388-fold,26-to 312-fold,13-to 29-fold,9-to 33-fold,8-to 20-fold,3-to 8-fold and 1-to 2-fold,respectively (Tables 3–5).We showed that the resistance of FAW populations to pyrethroid,organophosphorus insecticides and spinosad in China was all at moderate to high level,while the resistance of them to chlorantraniliprole and emamectin benzoate were still at low level.

Zhanget al.(2019) reported that there is a high similarity in theCOIsequence between Chinese FAW samples and those from Florida.Yu (1992) reported that FAW populations collected from central and southern Florida in 1990 showed resistance to pyrethroid (3-to 264-fold),organophosphorus (11-to 517-fold) and oxadiazine (10-to 507-fold) insecticides.In 2002,a FAW population collected from Citra,Florida showed high resistance to carbaryl(562-fold) and parathion-methyl (354-fold) (Yuet al.2003),and two field FAW strains collected in northern Florida showed high resistance to carbaryl (626-and 1 159-fold)and moderate resistance to parathion-methyl (30-and 39-fold) (Yu and McCord 2007).Altogether it showed that the insecticide resistance of FAW populations in China are similar to that of the populations from Florida,both having moderate to high level resistance to pyrethroid and organophosphorus insecticides.

In this study,we found that there is no significant difference in the resistance level of different FAW populations in China to the same insecticides.However,the spraying frequencies of insecticides for controlling FAW would be increased significantly in the annual breeding area for FAW in tropical and subtropical areas of Yunnan,Guangxi,Guangdong,Hainan,Fujian,Sichuan,Tibet and Guizhou (Wu 2020),and would pose a high risk to the evolution of resistance.Song and Wuet al.(2020) reported that the spraying frequency of emamectin benzoate,which had a high efficiency on FAW,increased to 6.83 times in summer 2019 in west Yunnan since FAW invaded China.Continued resistance monitoring of insecticide,especially chlorantraniliprole and emamectin benzoate,is important for efficient prevention and control of FAW in the annual breeding areas and the provinces(regions) with large areas of corn cultivation.

There are many factors can influence the development of insecticide resistance,including the selective pressure,the mode of inheritance and stability of resistance,the fitness costs of resistance,the long-distance migration behavior and population dynamics of pest,the temporal and spatial distribution on different host plants of pest (Wuet al.1995,1996; Lianget al.1996; Wu and Guo 2000).In American countries,the main strategies for resistance delay include monitoring the presence and damage of FAW on Bt and non-Bt maize,carrying on proactive management strategies to maintain the resistance at low level,reducing selection pressure,and use of insecticides,with different modes of action (Muraroet al.2019).In African countries,an integrated management approach was proposed for the control and prevention of FAW,including host plant resistance,biological control,use of bio-pesticides or appropriate chemical pesticides at reasonable doses and early resistance monitoring (Togolaet al.2018).

In China,the Ministry of Agriculture and Rural Affairs has proposed an effective and regional control guideline,including different strategies in the annual breeding area,immigration area and important prevention area.Also,a comprehensive control system of chemical control,physical control,biological control and agricultural control is being implemented in recent years,aiming to solve the emergency problem caused by invasion of FAW (Wu 2020).Meanwhile,in Asian countries,most growers lack the basic knowledge about FAW,and very few crop advisors or growers have heard of maize IPM,IRM or similar programs.Consequently,these programs must be established in order to prevent control failures of FAW in the future.

5.Conclusion

In this study,we tested the insecticide susceptibility of the five FAW populations collected from Yunnan,Hainan,Tibet and Fujian of China.It showed that there is no significant difference in the resistance level of different FAW populations in China to the same insecticides.The resistances to pyrethroid,organophosphorus insecticides and spinosad of invaded FAW populations in China were all at moderate to high level,while resistances to chlorantraniliprole and emamectin benzoate were still at low level.Our results provide a foundation for designing management strategies to control and monitor this serious pest in China.

Acknowledgements

This work was supported by the National Key Research and Development Program of China (2019YFD0300101) and the Central Public-interest Scientific Institution Basal Research Fund,China (CAAS-ZDRW202007).

Declaration of competing interest

The authors declare that they have no conflict of interest.