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Studies On The Cold Hardening And Overwintering Regions Of Spodoptera Exigua In China

Posted on:2013-02-26Degree:DoctorType:Dissertation
Country:ChinaCandidate:X L ZhengFull Text:PDF
GTID:1113330374979094Subject:Agricultural Entomology and Pest Control
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The beet armyworm, Spodoptera exigua Hiibner (Lepidoptera:Noctuidae) is an important and cosmopolitan agricultural crops pest, whose populations often explode intermittently resulting in serious economic losses in many areas of the world. S. exigua is a species of freezing intolerance, has known no diapause, and escape the cold winter through migration. However, a few larvae and pupae had been observed to overwinter in high latitude regions so that the primary question that the population source of outbreak regions migrated from southern provinces, or arose from local or both haven't still a conclusion. So, the issue is always paid attention to as one of focus of the species in the world. In this study, overwintering regions of S. exigua was studied as the basis for analyzing the population source, monitoring and forecasting of this species. Main results are summarized as following:1. Cold hardening of S. exiguaCold hardening is one of the important indicators for evaluating the potential capability of overwintering. The effects of seasonal and diel change of air temperature, photoperiod (PP), host plant and soil moisture on cold hardening were studied. The results showed that S. exigua eggs, larvae and pupae could endure the low temperature above their supercooling point (SCP) and these factors could influence their cold hardening except for PP. The process of seasonal change of air temperature (cold acclimation, CA) decreased significantly their SCPs, and incresasing their survival rate when they were exposed to extreme low temperature after CA. Interestingly, the survival rate decreased rapidly when the process of CA was discontinued. Diel change of air temperature, such as rapid cold hardening (RCH) and rapid heat hardening (RHH), could affect the survival rate of these stages to some extend. During the thermoperiod (TP), survival rate increased significantly after encountering the RCH/RHH (TP+RCH/RHH) of these stages. However, the survival rate at low temperature decreased rapidly when these unexpected temperatures (TP+RCH/RHH) return to RHH/RCH (TP+RCH+RHH or TP+RHH+RCH). Photoperiod didn't affect the SCPs and BWC of the species, and the interaction between PP and instars could not also influence them. Host plant could influence the cold hardening of larvae and pupae and the levels of cryoprotectant. Host plant could influence the SCP of3rd and5th instar larvae, and pupae, but the cryoprotectants of these stages are different. Trehalose was the cryoprotectant of3rd instar larvae, and glycerol was the cryoprotectant of5th instar larvae and pupae. Soil moisture couldn't influence the cold hardening of pupae, but affected the larval developmental progress before pupation and larval and pupal mortalities. Soil moisture was higher, the developmental progress of larvae before pupation was slower, and the larval and pupal mortalities were higher. We presumed that the cause of intermittence outbreak over a wide region in S. exigua could be partially attributed to a comprehensive effect of warm winter, little rain and adaptable host plant during winter based on these information mentioned above.2. Projecting overwintering regions of S. exigua in China, using Climex and ArcGis modelOur attempt is the first to explicitly predict the over-wintering range of an insect. In this study, the overwintering regions of S. exigua in China was projected by the method of Compare Locations in Climex model based on the information of overwintering sites consistent with the literature and databases and combined with the meteorological database of758stations in China. The final Ecological Index (EI) for S. exigua in China was output and integrated the output files into ArcGIS. Then, the interpolation method of Kriging within ArcGIS was used to obtain the El surfaces. The results exhibited that the southern and northern overwintering boundaries of S. exigua in China located at the Tropic of Cancer (about23.5°N) and the Yangtze River valley (about30°N).3. Steps toward model validationThree methods, including caged experiment, fieldwork during winter and monitoring the population dynamic of adult by sex pheromone in the field, were adopted toward model validation. Wuhan City located at the Yangtze River valley (the northern overwintering boundary) was chosed as representative point to conduct the caged experiment, and the results indicated that S. exigua pupae could overwinter in Wuhan regions. Furthermore, the living larvae were found from January to March in the fieldwork during winter, and the adult were also captured by sex pheromone from January to March in the region. Fourteen sites were chosed throughout China from southern to northern provinces, and the results showed that all stages of this insect were found throughout the year in Sanya and Guangzhou Cities, so these regions can be considered as perennial damage regions. Living larvae or pupae were found during winter in Longnan and Yongxiu Counties, and Nanchang, Yibin and Wuhan Cities, so these regions can be considered as overwintering regions. However, no individuals were found in Jurong and Nanjing Cities of Jiangsu Province, Xi'an City of Shanxi Province, Tai'an, Anqiu and Zhangqiu Cities of Shandong Province, and Beijing City, so these regions can be considered as non-overwintering regions. All of these data were consistent with the results projected by Climex and ArcGis model. Therefore, we considered that the overwintering regions of S. exigua were accurately projected by Climex and ArcGis model.
Keywords/Search Tags:Spodoptera exigua, overwintering, cold hardening, low temperature, photoperiod, host plant, soil moisture, Climex, ArcGis, overwintering region, fieldworkduring winter, pupation behavior, excavating pupae, monitoring by sex pheromone
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