The Migration, Overwinter Regulation And Outbreak Mechanism Of Meadow Moth (Loxostege Sticticalis L.)

Meadow moth, Loxostege sticticalis L., is a worldwide pest, mainly distributed in the drought areas of northern temperate regions. It is an important migratory pest in agro-pastoral areas of northern China, including North, Northeast and Northwest China. Its larvae can seriously damage crops, such as soybean, sugar beet, alfalfa, sunflower, and pasture. The outbreak of meadow moth is intermittent. Since the foundation of the People’s Republic of China in 1949, meadow moth had three outbreaks and caused significant economic loss to agro-pastoral production. To date, the comprehensive understanding of the occurrence of meadow moth is still limited. The objective of this study is to explore the regulation of migration, overwintering and mechanisms underlying outbreak of meadow moth by combining atmospheric numerical simulation, trajectory analysis, weather analysis and correlation analysis and to provide a scientific basis for improving the forecast accuracy of meadow moth. The major findings are as follows:1. Migration of meadow moth.(1) The majority of the spring population of meadow moths in Northeast China are from local overwintering areas or neighboring countries, that is, from the north of East Asia desert. The "permanent breeding area" in North China has important effect on the outbreak in Northeast China, but is not the key source. In some years, overwintering generation adults in neighboring countries can also immigrate into North China on a large scale. This is a new phenomenon in the third outbreak period. Based on analysis of the migration process, a map showing the major migration routes of overwintering generation adults of meadow moth is drawn.(2) During the occurrence period of the first generation adults of meadow moth in Northeast China, southwest winds are prevailing, and north and northeast winds seldom occur. This situation goes against meadow moth migrating to North China. In the period of the second generation, frequencies of northeast and north winds increase, southwest wind is still predominant, and wind direction changes frequently. Under these conditions, adult migration to the "permanent breeding area" in North China is unfavorable, but conducive for migration to the Bohai Bay area. Among the 20-year occurrence of meadow moth since 1979, the events that the summer population of meadow moth in Northeast China migrated to North China only occurred 4 times, including 1 time to the "permanent breeding area" and 3 times to the Bohai Bay area, and migration to eastern Inner Mongolia and neighboring countries occurred 3 times. Therefore, the summer population of meadow moth in Northeast China seldom migrates to North China, and can’t be the source of the first generation adults in North China. The first generation adults emerged in Songnen Plain are usually depressed due to high summer temperatures.(3) The summer population of meadow moth in North China often disperses over a short distance. The first generation adults emerged in the southern plain region during the end of June and early July are usually spread northward to the northern high-altitude region, and the second generation of larvae occurred there. If the first or second generation of adults emerged in northern high-altitude region during the end of July and first half of August spread to the south region, the second or third generation of larvae will occur there. Under abnormal climatic conditions, the summer population of meadow moth in North China may take long-distance migration.(4) Meadow moth staying in North China has atypical migration cycle between generations. For the meadow moth taking long-distance migration, their offspring can not migrate back to their native source area, thus no migration cycle occurs between generations.2. Outbreak mechanism of the immigration population of the first generation adults of meadow moth in China.Outbreak of the immigration population occurs when (1) there was a broad overwintering area in the neighboring countries in last autumn; (2) the precipitation in the East Asia desert or steppe was abnormally high in May and June, leading to the large scale propagation of the first generation larva in neighbor countries; (3) the adverse weather conditions after emergence of the first generation adults promoted the moth immigration into China massively.3. Formation of overwintering area in neighboring country and Northeast China.(1) The precipitation in steppe is good. But before 1990s, the accumulated temperature in steppe is not high enough for meadow moth to complete two generations, and overwintering areas are rare. Since 1990s, the accumulated temperature has been increasing along with the global warming, which was beneficial to the occurrence of second-generation larvae and formation of overwintering areas. (2) The accumulated temperature in East Asia desert and south part of steppe is high enough for meadow moth to complete two generations, but the severe drought has limited the occurrence of meadow moth. Only when the precipitation is abnormally abundant, second generation larvae may occur, and form overwintering areas.(3) The high summer temperature of the Northeast China Plain is a key constraint factor for the occurrence of first generation adults. Highest temperature usually emerges in the late July, and whether the adults could avoid it is a key for the continuity of the population. Local first generation adults emerge in mid-July. Only when they migrate westward to the high altitude areas of eastern Inner Mongolia, a larger number of second generation larvae could be produced. If immigrating first generation adults emerge after the end of July, they can produce the second generation larvae in Northeast China Plain and some of them may overwinter.4. The relationship between long-distance migration of meadow moth and climate.(1) Long-distance migration of overwintering generation adults of meadow moth in North China towards Northeast China is restricted by the location of source areas, climate factors, wind field and so on.(2) The climates of massive immigration for the adults from neighboring countries into China have significant difference. If the source area locates in the East Asia desert, due to the harsh environmental conditions, both the overwintering generation and the first generation adults will emigrate after eclosion. If the source area locates in steppe where precipitation is a major factor for the propagation of meadow moth, adults will emigrate when severe drought occurs in the spring, or stay when the spring has abundant waterfall. When the abnormal summer high temperature and drought comes, first generation adults will emigrate massively. If the source area locates in forest-steppe, since the low temperature in June and August is a major constraint for meadow moth, temperature fluctuations will promote emigration.5. The relationship between migration process of meadow moth and weather.(1) The responding mechanism of meadow moth in air to wind and temperature is analyzed based on the radar observations of meadow moth migration by Jilin Plant Protection Institute at Zhenlai County in 1999. The results show that the layer formation of meadow moth in the air is a response to the combination of effects of temperature and wind speed. In hot weather, the temperature is not a constraint factor to meadow moth flight, and moth layer form in the maximum wind due to the accumulation of low-level jet; in cold weather, low temperature is a constraint factor to meadow moth flight, moth layer form in the area of temperatures above the threshold for fight. The low-temperature threshold for meadow moth flight may be between 12.0-12.5℃. During the nights with favorable temperature conditions and without low-level jet, the moth in the air will be distributed by higher altitude and the layer may be wider.(2) A migration process that occurred in North China during 24-25 May,2004 is simulated. The results showed that the massive takeoff of meadow moth in Datong and other regions in the night of May 24 was closely related with the passage of a cold front. Most of the emigrating population flew northeast and landed involuntarily in Fengning County due to the atmospheric subsidence.(3) The weather background for taking-off, landing, and long-distance migration of meadow moth are summarized. Termination of adverse weather events, passage of cold fronts, and coming of adverse weather will trigger massive emigration of meadow moth. Precipitation, atmospheric subsidence, air convergence, and cold front will lead convergent landing of meadow moth. Northern cyclone is closely related to the long-distance migration of the spring population of meadow moth in China;. Typhoon moving northward is favorable to the migration of the summer population of meadow moth in Northeast China towards North China; anticyclone affects the migration of meadow moth in Europe to west.6. Climates of abnormal fluctuations of spring populations of meadow moth in China.In Northeast China, high temperature and drought in June are necessary for meadow moth outbreak, while extreme cold or wet weather will lead to a sharp drop in population. In North China, excessive precipitation in May and June is conducive to meadow moth outbreak. The coefficient distribution of key climatic factors and early sea surface temperature and geopotential height field were calculated and prediction equations were setup using the significant correlation zone. In addition, the collapse of the meadow moth population in Northeast China is found to be closely related to volcanic eruption.7. Space-time dynamics of meadow moth population in Eurasia of the past 100 years.Since the late of 1920s, meadow moth in Eurasia has undergone four long periods of outbreaks. They are 1928-1935,1948-1960,1970-1989 and 1997 to present. Occurrence areas of meadow moth have intercontinental shift. Meadow moth outbreak in China has a quasi-stable cycle of 20-years. The outbreak cycle of meadow moth in the European part of Russia changes between decades.8. The distribution of meadow moth in China has significant changes between decades, and the occurrence area keeps moving northwards.The cause is that meadow moth in North China seldom migrate northeastwards in 1950s, but they immigrate into Northeast China frequently since 1970s; overwintering areas seldom emerged in Northeast China in 1950s, but frequently after 1990s. Changes of the regulation of migration and overwintering of meadow moth are related with global warming. Warming not only make grasslands in North China severe drought, but also advanced the eclosion of overwintering cocoons and separated the adult stage and rainy season, consequently leading to massive emigrating of adults. Moreover, warming increased the frequency of second-generation larvae in East Mongolia, East Inner Mongolia and Chita of Russia, further leaving a large number of overwintering cocoons.9. Possible mechanisms of meadow moth intermittent outbreaks.(1) The population of meadow moth in China began to rise during the stage with excessive precipitation in July in North China and entered a new outbreak cycle. The high-altitude westerly jet in East Asia has decadal shift in North-South direction, resulting in the decadal fluctuations of precipitation in North China in July, consequently affecting the intermittent outbreaks of meadow moth.(2) Termination of the outbreak cycle of meadow moth in China is an interaction of natural enemies and adverse climate. However, in the current third cycle, serious occurrence areas moved northwards, the population of natural enemies at the "permanent breeding area" in North China can not increase steadily, thus the inhibition of natural enemies to meadow moth became weakened. This is the reason why the third cycle lasts so long.(3) The outbreak cycles of meadow moth in the European part of Russia are coincident with the decadal fluctuations of precipitation in June and July, which is corresponding to the decadal changes of sea level pressure field.