Responses And Adaptation Of The Beet Webworm, Loxostege Sticticalis(Lepidoptera: Carambidiae) To The Variations In Temperature And Humidity

The beet webworm, Loxostege sticticalis L.（Lepidoptera: Crambidae） is one of the most destructive insect pests of crops and fodder plants in northern, northeast and northwest areas in China. It has been outbreak for 26 years during the 3 outbreak periods and significant yield and economic losses have resulted in since 1949. To demonstrate the population dynamics and mechanisms for the outbreaks of larval population, improve the forecasting and controlling technology, and minimize the yield and economic losses caused the development, survival, migration and reproductive potential and capacity of L. sticticalis were investigated at different combinations of temperature and relative humidity（RH） by utilization of combination of physiological and ecological principles for the wild and lab populations. Additionally, the population tendency of this species in the coming decades was forecasted based on the results obtained. The major results obtained in this dissertation were showed as follows:1. The effects of temperature and relative humidity on egg hatch, survival and developments of larvae, the flight and reproductive potentials of adults, and major parameters of life table was investigated at the combination of four temperature（18, 22, 26 and 30℃） and five relative humidity（20%, 40%, 60%, 40% and 100% RH）. The major results obtained were listed as follows. 1) The survival rates of egg, 1st-2nd instar larvae and the total larval stage were all significantly affected by temperature and RH, while the survival rates of 3rd- 4th instar larvae were not. The survival rate for 5th instar was only significantly affected by RH, and not by the temperature. The survival rate under the combination of 22°C and 60% – 80%（RH） were the highest, while the lowest were under 30°C and 20%（RH）. In addition, duration of the larval stage was significantly affected by temperature, but not by relative humidity, which was shortened with the increasing of temperature. 2) The development time of pre-pupal and pupal period and bodyweight of newly emerged moth differed significantly among the moth from the larva experienced different combination of temperature and RH. The developmental duration was shortened while the body weight of the moth was decreased as the temperature or RH increased. Significant differences were found in flight and reproductive capacity of the moths developed from larvae experienced different temperature and RH conditions. The 22°C and 60% – 80% combination resulted in the most fertile, for a shorter duration of preoviposition and a larger amount of fecundity. While it showed the worst under 30°C condition about that. However, the flight capacity was the strongest for those whose larva developed under the 18°C and 20 – 40% RH conditions indicating that the temperature and RH experienced during larval period affect not only the development and survival but also the reproduction and migration behavior of L. sticticalis. 3) The development time, survival rate and reproductive capacity, and life table parameters were measured for populations reared at different combination of temperature and relative humidity. The net reproductive rate（R0）, intrinsic rate of increase（rm） and the finite rate of increase（λ） were greater, and the mean generation time（GT） and mean doubling time（DT） at 22- 26℃, 60%- 80%RH were shorter than those at the other temperature and RH conditions, suggesting that the population growth was the best at 22- 26℃,60%- 80%RH.2. The temperature and RH conditions surrounding the adults influence the migration and reproduction of L. sticticalis. By treating the moth emerged from the larva experienced the same temperature and RH conditions with different combination of temperature and RH referred above, the flight and reproductive capacity of the moths showed that: 1) the flight capacity was significantly affected by the ambient temperature and RH. The moth tethered under the temperature 18- 22°C, or RH 60% – 80% exhibited the strongest flight capacity. And furthermore, the moth tethered under suitable temperature was detected to have fewer energy substitute and weight loss, but with a higher utilization efficiency. Interestingly, the reproductive capacity was not brought down by the flight compared to the CK. However, that is not the case for the free flight activity tested by the using of an insect autonomous flight activity monitoring system, which turned out that fewer flight activities occurred under the ambient of 22°C and 60% – 80% RH but more for other temperatures. 2) The ambient temperature and RH significantly affected the preoviposition period, mating frequency, life time fecundity, longevity, and egg hatch of L. sticticalis. The life time fecundity and egg hatch rate were relatively higher under the ambient temperature 22°C and 60% – 80% RH, while lower for other conditions. In addition, the longevity was longer under lower temperature, and vice versa.3. Temperature and RH were the key factors that affect field population dynamics and outbreak of L. sticticalis. 1) By comparing the temperature and RH conditions between the 60 cases of immigration and the 136 cases of emigration incidents, it indicated that only 5% of the temperature of the emigration day ranged between 21- 23°C, while that percentage was 35.4% for the immigration day, which was significant different between these two cases. Meanwhile unfavorable environmental conditions for the immigration were less than that of the emigration suggesting that seeking a suitable habitat for reproduction is the cause for L. sticticalis migration. 2) The survival rate of the overwintering larvae of L. sticticalis were also significantly affected by comparing the temperature condition between outbreak and intermittent period during the overwintering period for Datong city in Shanxi Province, Siziwangqi county in Inner Mongolia, and Kangbao county in Hebei Province. It was revealed that there were more beneficial conditions for overwinter of this pest during outbreak period than intermittent period. 3) by studying the relationship between environmental conditions such as temperature, RH and precipitation during the moths peak period of overwintering generation, and the degree of occurrence of first generation larvae, it was made clear that the degree of first generation larvae occurrence was significantly correlated to the thermal and humid conditions; the more the days meet the temperature of 21-23°C, RH 40%- 80%, the more serious the larval damages. 4) Exploring the courses for the outbreak of second generation larvae during the August of 2008 found that first generation adults and second generation larvae were both majorly located in areas with a temperature range of 18- 26°C and RH > 40%, furthermore the RH in the most intensive region was 60%- 80%. The distribution of adults and larvae were aggregated. 5) The damage areas of first generation larvae in 2009（4 million hm2） was significantly lower than that of the overwintering area（16 million hm2）, which was serious imbalance.The causes for this phenomenon is that the temperature during the development of overwintering larvae（April） was abnormally increases, advancing emergence of adults; however temperature and humidity conditions during the peak periods were significantly decreased, which prevented the incidence of reproductive and migratory activities by adults.4. The shift of overwintering area of this pest in the future was predicted by the Maxent model base on the results gained above and the global climate change data. In order to discover shift of overwintering area under climate change conditions, this dissertation adopted the Maxent model and combined with the overwintering data of this pest since 1949 and 4 scenarios（RCP 2.6, 4.5, 6.0 and 8.5） of climate change forecasting data（BCC-CSM1.1 model data） released by IPCC（Intergovernmental Panel on Climate Change） the fifth assessment report and predicted the distribution shift of overwintering area in the 2050 s and 2070 s. The main results are: 1) the AUC value for the training and testing data were 0.989 and 0.987 respectively, which indicated to be a good fit of the prediction. 2) Precipitation seasonality（Bio15） was the most important variables, and precipitation of wettest month（Bio13） was the secondary and followed by mean precipitation of October（prec₁0） and temperature annual range（Bio7）, determined by the Jackknife test. The variables were all important for the model construction for all more than 10% of contributions. 3) The area and location of overwintering area would enlarge and northward shift in 2050 s and 2070 s compared to the current ones under all climate change scenarios tested. The highly suitable overwintering area would be 1.41- 2.94 times of the size of the current one, and 78.79- 226.97 km northern off the location of the current. These results indicated that the overwintering ground of this pest would enlarge and northward shift.The major innovative discoveries in this dissertation were as follows:1) Studied the effect of temperature and RH on survive of eggs and larva of different instars and larval development. Discovered the influence of temperature and RH experienced during the larval period on the reproductivity and flight.2) Revealed the influence of temperature and RH on reproductivity, and found the most suitable condition for flight and reproductivity, and distinguished the most suitable condition for free flight and tethered flight.3) Confirm that the temperature and RH were the key factors for field population dynamics and outbreak of this pest, by studying the relationship of field temperature and RH with survive of overwinter, emigration and immigration, the degree of larvae or adults occurrence.4) Predicted the overwintering area shifts in 2050 s and 2070 s by Maxent model. All these results above provided important experimental basis for further research of migrant behavior and population dynamics, and for improving and enhancing monitoring and forecasting technology of this pest.