Regularity of population occurrence and migration in the oriental armyworm, Mythimna separata (Walker)

The oriental armyworm, Mythimna separata (Walker) (Lepidoptera: Noctuidae), is a major migrant pest of grain in China and other Asian countries, causing huge crop production and economic losses nationwide annually. Much of the uniquely difficult aspects of managing M. separata arises from its long-distance migratory behavior. For example, direction and timing of winds from its overwintering regions in the south largely determine where and when large influxes of migrants arrive in the north to oviposit, making prediction of infestations difficult. Since the 1960s, the migration routes and infestation patterns of M. separata in China have been resolved (Li et al. 1964; Jiang et al. 2011), allowing development of effective forecasting and early warning technology to help manage this insect. Over the past two decades,large-scale outbreaks of M. separata have rarely occurred.However, in 2012, M. separata infested much of northern and northeastern China.

In 2014, a National Research and Demonstration Project on Integrated Pest Management Technology for the oriental armyworm was funded by the Agro-scientific Research in the Public Interest of China (201403031). As the project holder, the Institute of Plant Protection, Chinese Academy of Agricultural Sciences, cooperated with seven other research units and agriculture universities to focus research on the occurrence, regularity of migration,monitoring and forecasting, and pest management of the armyworm. This special focus in the Journal of Integrative Agriculture reports some new aspects of occurrence and damage of the armyworm discovered during this project.We are also honored to present a review by Dr. Thomas W. Sappington from USDA on migratory flights occurring within a year-round distribution by some insect pests like European corn borer and probably Asian corn borer, which contrasts with the more spectacular migratory flights of pests like the oriental armyworm across long distances between overwintering and breeding ranges (Sappington 2018). It is important to remember that migratory flight behavior is not defined by the distance traveled, but by the flight being non-appetitive. Migratory flight differs fundamentally from local or station-keeping flight behavior, which is appetitive,i.e., the individual is searching for a resource.

During the investigations and monitoring of population dynamics of the oriental armyworm, we found more and more individuals of closely related species appearing in the field, including Mythimna loreyi and Mythimna roseilinea.They frequently co-occur with M. separata, but little is known about their biology, making it difficult to predict their population dynamics and to develop management strategies.Previous work demonstrated the influence of temperature on biology and flight performance of M. loreyi (Qin et al.2017, 2018b). Here, we report the effects of temperature on development and reproduction of M. roseilinea (Qin et al.2018a). The results provide insight into the phenology of flight activity and the population ecology of this insect pest,and will make advances in population prediction and management possible in the field.

Long distance migration of M. separata consumes much flight energy substrate. Nectar plants provide important supplemental nutrition, thus contributing to this insect’s migration and outbreak potential. The presence of adequate nectar sources along the migratory pathway is a prerequisite for moths of M. separata, to reach target habitats and lay eggs. Therefore, identification of plant species and assessment of their suitability as wild nectar sources will help disclose their migratory pathways and perhaps improve effective early warning. Guo et al. (2018) has identified several summer nectar plants contributing to outbreaks of M. separata in North China. Their results suggest that T. sinensis is an important, suitable summer nectar source that potentially influences migration and reproduction of M. separata moths, thus contributing to outbreaks of this pest.

Migratory species, including M. separata, usually take advantage of strong winds high in the atmosphere to make long-distance displacement possible (Chapman et al.2015). Population trajectory analysis reconstructs migratory pathways based on wind speed and direction during the migratory period, helping to identify probable source areas of an immigrant pest population (Kim et al. 2010). This kind of information can be used to develop a fine-scale method of forecasting outbreaks. Zhang Z et al. (2018) collected migrant M. separata in northern China using searchlight traps and ground light traps. Using background weather pattern analysis and trajectory analysis, they demonstrated that immigrants of M. separata in 2013 originated in Anhui,Jiangsu and Hubei provinces, and concluded that most of them could continue to fly into the northeastern regions of China. These results are consist with our previous migratory route forecasting (Jiang et al. 2011).

When immigrant M. separata arrive in a new habitat in a region of suitable environmental conditions, they search for appropriate hostplants for oviposition. Understanding the relationship between host plant growth and pest damage level is very important for elucidating key factors to consider when developing novel agronomic methods to manage the target insect. Zhang K P et al. (2018) found that armyworm damage tended to be heavy in the presence of abundant weeds and when the corn was stunted. In contrast, when corn was growing well and weed density was low, armyworm harm was minimal. The results imply that promoting corn growth and ensuring timely removal of weeds can help minimize armyworm infestation.

Temperature is the most important factor influencing insect pest population growth and damage, and temperatures experienced during development can also affect later migratory capacity and behavior. In M. separata, for example, low temperatures experienced by adults during the first day after eclosion from the pupa can switch migrants into residents(Zhang et al. 2008). Li et al. (2018) explored the effects of three constant and three stage-specific alternating temperature treatments on survival and reproduction of M. separata,using age-stage, two-sex life table analysis. Both low and high temperatures negatively affected reproduction and population growth, with variability observed across developmental duration and stage under non-optimal temperatures.The results expand knowledge of the relationship between changing temperature and armyworm population development, and adaptive strategies of armyworm in a complex ambient environment.

Under the increasing pressure of insecticide use in China in recent years, most pest species have evolved some level of insecticide resistance. Although long-distance migrant populations of M. separata usually exhibit no significant pesticide resistance, it is getting more difficult to control field populations with common insecticides in China. Therefore,Zhao et al. (2018) evaluated resistance of five field populations of M. separata collected from Shaanxi and Shanxi provinces to six different insecticides. Compared with a relatively susceptible strain of M. separata, all populations showed variable levels of resistance to six insecticides except that all were still susceptible to Emamectin benzoate. Based on these and pair-wise correlation analyses of resistance level among different insecticides, the authors suggest alternating rotation of chlorantraniliprole with either emamectin benzoate or chlorpyrifos to slow development of insecticide resistance in M. separata.

We hope that the readers will find the articles in this special focus informative and useful for oriental armyworm pest management. We are grateful to the authors and anonymous reviewers. We thank Editorial Board and Editor-in-Chief of the Journal of Integrative Agriculture for accepting our proposal to publish these papers. We also thank the special fund for Agro-scientific Research in the Public Interest of China (201403031) for support of this special focus.

Prof. JIANG Xing-fu

Guest Editor

State Key Laboratory for Biology of Plant Diseases and

Insect Pests, Institute of Plant Protection,

Chinese Academyof Agricultural Sciences,

Beijing 100193, P.R.China

Chapman J W, Reynolds D R, Wilson K. 2015. Long-range seasonal migration in insects: mechanisms, evolutionary drivers and ecological consequences. Ecology Letters,18, 287–302.

Guo P, Wang G P, Jin L J, Fan X Q, He H L, Zhou P W, Guo X R, Li W Z, Yuan G H. 2018. Identification of summer nectar plants contributing to outbreaks of Mythimna separata(Walker) (Lepidoptera: Noctuidae) in North China. Journal of Integrative Agriculture, 7, 1516–1526.

Jiang X F, Luo L Z, Zhang L, Sappington T W, Hu Y. 2011.Regulation of migration in the oriental armyworm,Mythimna separata (Walker) in China: A review integrating environmental, physiological, hormonal, genetic, and molecular factors. Environmental Entomology, 40, 516–533.

Kim K S, Jones G D, Westbrook J K, Sappington T W. 2010.Multidisciplinary fingerprints: Forensic reconstruction of an insect reinvasion. Journal of the Royal Society Interface,7, 677–686.

Li B L, Xu X L, Ji J Y, Wu J X. 2018. Effects of constant and stage-specific-alternating temperature on the survival,development and reproduction of the oriental armyworm,Mythimna separata (Walker) (Lepidoptera: Noctuidae).Journal of Integrative Agriculture, 7, 1545–1555.

Li G B, Wang H X, Hu W X. 1964. Route of the seasonal migration of the oriental armyworm moth in the eastern part of China as indicated by a three-year result of releasing and recapturing of marked moths. Acta Phytophylacica Sinica,3, 101–110. (in Chinese)

Qin J Y, Liu Y Q, Zhang L, Cheng Y X, Luo L Z, Jiang X F.2018a. Effects of temperatures on the development and reproduction of the armyworm, Mythimna roseilinea:Analysis using an age-stage, two-sex life table. Journal of Integrative Agriculture, 7, 1506–1515.

Qin J Y, Liu Y Q, Zhang L, Cheng Y X, Sappington T W, Jiang X F. 2018b. Effects of moth age and rearing temperature on the flight performance of the loreyi leafworm, Mythimna loreyi (Lepidoptera: Noctuidae), in tethered and free flight.Journal of Economic Entomology, 111, 1243–1248.

Qin J Y, Zhang L, Liu Y Q, Sappington T W, Cheng Y X, Luo L Z,Jiang X F. 2017. Population projection and development of the Mythimna loreyi as affected by temperature: Application of an age-stage, two-sex life table. Journal of Economic Entomology, 110, 1583–1591.

Sappington T W. 2018. Migratory flight of insect pests within a year-round distribution: European corn borer as a case study. Journal of Integrative Agriculture, 7, 1485–1505.

Zhang L, Jiang X F, Luo L Z. 2008. Determination of sensitive stage for switching migrant oriental armyworms into residents. Environmental Entomology, 37, 1389–1395.

Zhang K P, Yu Z H, Jiang S X, Sun D W, Hui J T, Zheng Y L,Li X Z, Wang X Y, Wu J X. 2018. Associations of host plant growth and weed occurrence with armyworm (Mythimna separata) damage in corn fields. Journal of Integrative Agriculture, 7, 1538–1544.

Zhang Z, Zhang Y Y, Wang J, Liu J, Tang Q B, Li X R, Cheng D F, Zhu X. 2018. Analysis on the migration of first-generation Mythimna separata (Walker) in China in 2013. Journal of Integrative Agriculture, 7, 1527–1537.

Zhao Y Y, Su L, Li Y P, Xu X L, Cheng W N, Wang Y, Wu J X. 2018. Insecticide resistance of the field populations of oriental armyworm, Mythimna separata (Walker) in Shaanxi and Shanxi provinces of China. Journal of Integrative Agriculture, 7, 1556–1562.