| Marasmia exigua and Cnaphalocrocis medinalis are two important rice leaffolders in Sichuan and Chongqing rice cultivation areas,and caused serious losses to rice production.Due to the similar habits and morphological characteristics,M.exigua is often mis-identified as an another species of rice leaffolder C.medinalis by researchers in many countries.However,there are few literatures on the comparative study of these two types of rice leaffolders.This dissertation mainly compares the biological characteristics and ecological characteristics of the two rice leaffolders through a combination of field surveys and laboratory experiments.The main results are as follows:1Comparative study on the biological characteristics of M.exigua and C.medinalisThe morphological and biological characteristics of M.exigua and C.medinalis were compared by field investigation and laboratory observation.Each stages(egg,larva,pupa,and adult)is distinguishable easily,although the two species are similar species.C.medinalis can complete its life history on 8 species of gramineous crops and weeds,including rice,corn,Echinochloa crusgalli,and Setaria viridis and so on,but M.exigua can only be found in rice,Leersia hexandra and Leersia sayanuka.The food range of C.medinalis was more extensive than that of M.exigua.The symptoms and feeding habits of the two rice leaffolders were almost the similar,but the food intake of C.medinalis(90.9 cm~2/worm)was greater than that of M.exigua(50.9 cm~2/worm),and caused greater damage to rice.M.exigua could overwinter as larvae and appears 4-5 generations in Sichuan and Chongqing,however C.medinalis cannot overwinter there,and the number of generation varies with the time of migration,3-5 generations occurred generally.2Effect of temperature on the growth,development and reproduction of M.exiguaTen temperature gradients(10,15,18,20,22,25,27,30,32,35℃)were set up to establish the life table of M.exigua,and the results showed that M.exigua could complete the life history at 18-30℃,and it~’s development period is shortened as the temperature increases.It’s survival rate,intrinsic rate of increase,finite rate of increase,net reproductive rate,and fertility were the highest at 25℃and27℃.And there was a significant decrease trend for the parameters as the temperature changed from both sides.The hatchability of the eggs was extremely low and the mortality rate was extremely high at 15℃and 35℃.The linear model was used to fit the development rate of M.exigua,and the lower developmental threshold and thermal contanst of the egg,larval,pupal and immature stage of M.exigua were estimated as 14.47℃,47.62 degree days;14.6℃,200.00 degree days;11.56℃,111.11degree days;13.67℃,333.33 degree days.In addition,the developmental rate of M.exigua were used for fitting Brier-1 model,the developmental termination temperature(T_M)of egg,larval,pupal and immature stage was 35.13℃,38.49℃,40.10℃,38.13℃respectively.3Comparison of growth and reproduction of two rice leaffolders under fluctuating and constant temperatureThe age-stage,two-sex life tables of M.exigua and C.medinalis were established under fluctuating and constant temperature conditions respectively.The results showed that the influence of temperature on the growth and reproduction of C.medinalis was greater than that of M.exigua.Compared with the constant temperature condition,the development period of C.medinalis was shorter at the fluctuating temperature,and the fecundity ability was stronger,the intrinsic rate of increase and net reproductive rate were higher,the mean generation time was shorter;however,there was no significant difference in developmental duration,fecundity,and population parameters of M.exigua.Under the same temperature condition,the fecundity of C.medinalis was significantly weaker than that of M.exigua,the intrinsic rate of increase and net reproductive rate of C.medinalis were lower than those of M.exigua.4The intraspecific and interspecific relationship between C.medinalis and M.exiguaField trapping experiments were conducted.It was proved that there was a mixture occurrence of M.exigua and C.medinalis in the field.But there was a significant difference in the proportion of the two,which indicating that there may be a competitive relationship between them.Density gradient experiment showed that there were some intraspecific competitions in M.exigua and C.medinalis.As the density increased,the larval duration was prolonged and the eclosion rate was decreased.The intraspecific competition of C.medinalis is more intense than that of M.exigua,which reflected in its lower pupation rate and eclosion rate.The two rice leaffolders were mixed feeding at 22℃,26℃and 30℃,with high densities and scarce food.The larval development time of the two rice leaffolders were both prolonged and the pupation rates were very low.However,the larval development time of C.medinalis was shorter,the pupation rate and eclosion rate were both higher,C.medinalis showed a stronger competitiveness.5Analysis of the causes of C.medinalis prone to outbreaks in ChongqingIn the rice fields in Chongqing,C.medinalis has occurred all year round and outbreaks intermittently,causing serious economic losses to rice.As a closely related species,M.exigua occurred in Chongqing is relatively light,and is not susceptible to outbreaks.According to the conclusions of this dissertation and combing analysis of previous research results,we conclude the reasons for the outbreak of C.medinalis in Chongqing.The following are main points:First,C.medinalis has a stronger adaptability to the environment.Compared with M.exigua,C.medinalis has a wider temperature tolerance range,a wider range of feeding habits,a stronger feeding ability,and a higher rate of transformation,and a stronger interspecific competition;Second,C.medinalis has a large number of insect source.A large number of C.medinalis migrated from Guangxi,Guizhou,and Yunnan pass by Chongqing.The adequate insect source is the basis for the outbreak of the disaster.Third,the control of natural enemies was poor,and the natural parasitism rate of C.medinalis in Chongqing was low.The abuse of the insectiside caused natural enemies to be harmed,which led to the resistance and resurgence of C.medinalis.In addition,there were many rice varieties in Chongqing,the planting time was different,and the growth period was prolonged which provide a good conditions for the population of C. medinalis. |
