Migration pathway of Spodoptera frugiperda in Northwestern China.
Background: As a major migratory insect pest putting the whole world on alert, the fall armyworm Spodoptera frugiperda warned by Food and Agriculture Organization of the United Nations (FAO) poses a serious threat to the agriculture production (including maize) of China from the end of 2018. Taking advantages of the long-distance transport of seasonal monsoons and its self-powered migratory capacity, S. frugiperda performs two migration pathways to fly across the eastern and western China separately, which has caused regional dispersal and severe infestation. According to 2-year systematic investigation in China, the west migration route of S. frugiperda ends in Northwestern China, especially in Ningxia and Alxa Left Banner of Inner Mongolia. However, little is known about the source areas of S. frugiperda population invading Northwestern China, and few reports explore the migration route of this devastating pest through the whole western China. Objective: The objective of this study is to accurately analyze on the key atmospheric factors driving the immigration of S. frugiperda into the Northwestern China, source regions of the first populations to arrive, and Asian monsoon-induced migration pathways of S. frugiperda, which can provide fundamental evidence for the early warning and regional management and control of this invasive pest in China. Method: Based on the invasion dynamics of S. frugiperda in Ningxia of the Northwestern China and meteorological data, a meso-scale numerical model, insect's flight trajectory calculating program, and Geographic Information System (GIS) were used to identify the atmospheric transport backgrounds, simulate the succussive 1-3 night migration routes and trace their source regions of S. frugiperda in the Northwestern China. Result: The southerly summer monsoon from July to September each year was the key factor for the successive and successful immigration of S. frugiperda into Ningxia and other regions of Northwestern China, of which their major source populations of S. frugiperda were located in southeastern Gansu and eastern Sichuan, while some were from western Shaanxi. In addition, southwestern Chongqing, northeastern Yunnan and part of western Shanxi could possibly provide population source of S. frugiperda. Conclusion: Under the influence of southerly Asian summer monsoons, S. frugiperda can fly towards the north for 1-3 successive nights via its west migration pathway "Yunnan-Sichuan and Chongqing-Shaanxi and Gansu-Ningxia" in China, which originates from Myanmar and ends in Inner Mongolia, China. In particular, the government should be vigilant against the occurrence and damage of this devastating pest in the maize-cropping regions of Northwestern China while the preponderance of early southerly wind is advanced and wind speed gets strong during July to September.