Effects Of Host Plants On Parasitism And Entomopathogen Infection Of Chilo Suppressalis And Studies On The Mechanism

Chilo suppressalis Walker?Lepidoptera,Crambidae?is a major pest of rice and water-oat.At present,chemical insecticide is the main method to control this pest.Although the effect of insecticide is direct and quick,the extensive use of chemical insecticides not only damages the farmland ecological environment,but also causes the increase of development of insecticides resistance.In this study,we investigated the population density of the overwintering C.suppressalis in rice and water-oat?including wild water-oat?,compared the biological characteristics of the two host-associated populations,as well as the differences in entomopathogenic infection and parasitism rates.During this study,it was found that the dominant parasitic wasp of the stem borer was Cotesia chilonis,and the parasitism rate was significantly different between the two host-associated populations.Therefore,we have also carried on the thorough analyzed the causes to the parasitic rate difference.The main results are as follows:1.Comparison of population density and biological characteristics of the overwintering C.suppressalis from rice and water-oatThe population density is significantly different between the overwintering C.suppressalis from rice and water-oat.The population density in rice stem borer ranged from 7.74 m² to 12.42 m².While,the population density in the water-oat ranged from3.34 m^-2 and 4.66 m^-2.The population from water-oat pupated from March 5 to the end of April.The rate of pupation on March 22 was over 50%.The rice population began to pupate at the end of March,and pupation lasted until the end of April.The population of water-oat started to emerge around 10 March and lasted until the middle of April,with an eclosion rate of over 50%at the beginning of April.The population from rice began to emerge on April 3,lasted until early May,and the eclosion rate exceeded 50%on April21.In total,pupation and eclosion occurred 15 to 20 days earlier in water-oat-associated populations than in rice-associated populations.The average weight of female pupae from water-oat was about twice as large as that of female pupae from rice.The average weight of male from water-oat is also significantly heavier?about 1.5 times heavier?than that from rice?p<0.01?.Sex ratio of rice-associated population is about 1?:1?,while it is about 3?:2?in water-oat-associated populations.2.Parasitism rate of C.suppressalis and parasitic wasp identificationIn 2017,312 larvae and 711 larvae were collected from water-oat field and paddy field in Wuhan,Hubei Province,respectively.The parasitic rate of rice-associated population was 12.24%,and that of water-oat-associated population was 0%.In 2018,the average parasitic rate in water-oats field surveyed in Hanchuan,Linxing and Wuhan was1.01%,and that in rice field was 11.30%.The results showed that the parasitism rate in rice field was significantly higher than that in water-oat field?p<0.01?.Six species of parasitic wasps were identified by molecular methods,and 5 of which were monoparasitism wasps;they were Cardiochiles fuscipennis,Campopleginae sp1,Campopleginae sp2 and Microgaster russata.C.chilonis is the only polyparasitism wasp which is the dominant wasp species.Scanning electron microscopy?SEM?of the antennae of C.chilonis showed that there were seven kinds of sensillum in antennae.The coeloconica sensilla was first found on the antennae of the C.chilonis.3.Field infection rate of C.suppressalis and screening of highly pathogenic strains of Beauveria bassiana.Through the investigation of the infection rate of various populations of the rice stem borer in the winter of 2017 and 2018,it was found that the mortality from infection was about 18%,and there was no significant effects of sampling time,location and host plant?p>0.05?on the infection rate.The result of 18S rDNA sequencing showed that Beauveria bassiana was the main pathogen,accounting for more than 90%.A total of 5strains with high spore-producing capacity were screened from died bodies of the rice stem borer,and the spore-producing capacity,average growth rate and disease-causing effect rate of these 5 strains were compared.The results showed that the spore-producing capacity of the other 4 strains was more than 11×107 cm^-2,except that the spore-producing capacity of strain HCR₂ was low(4.97×107 cm^-2).There was no difference in the average growth rate of the five strains,which was about 2.5mm d^-1.HCW₅ strain had the strongest pathogenic effect on the 3rd instar larvae of the second borer,and the mortality was 90%.The mortality of larva caused by HCR₂ strain was68%.LXW₂ had the lowest virulence and the mortality rate was only 48%.In conclusion,the fatality rates of strain HCW₅ and strain LXR₁ were respectively 90%and 86%,and the spore production was higher,indicating a good potential of biological control and application value.4.The causes for the difference in parasitism rate of C.suppressalis between water-oat and cultivated rice fieldWe studied the causes for the differences in parasitism rate by C.chilonis between rice and water-oats field from 3 perspectives:host orientation?plant odor response of C.chilonis?,access to host insects and development of wasp larvae.The Y type olfactory test showed that there was no preference difference to volatiles from rice field soil and water-oat field soil?p>0.05?.There was no significant difference in preference either to volatiles from the two plants?p>0.05?,indicating that the difference of parasitism rate between the two host–associated populations was not caused by plant volatiles.At the same time,we found that there was no difference in the development of parasitoid wasp larvae in C.suppressalis larvae feeding on rice and water-oat plants?p>0.05?.By studying the relationship between the parasitism rate and stem diameter,we found that within a certain stem diameter range,the parasitism rate was significantly negatively correlated with stem diameter?r²=0.832,p<0.001?.We think that the parasitoid wasp is difficult to get in touch with the rice stem borer feeding the water-oat because of the thicker stem compared with rice stem,which is the main reason of the difference of parasitism rate.