Preliminary Studies On Population Outbreak Mechanisms Of Massicus Raddei Blessig(Coleoptera:Cerambycidae)

The oak longhorned beetle（OLB）,Massicus raddei（Blessig,1872）（Coleoptera:Cerambycidae）is known as a serious wood borer mainly attacking Quercus spp.The pest widely distributes in eastern Asia,including China,the Korean Peninsula,Japan,Vietnam and the Russian Far-East.In China,Massicus raddei distributes in most provinces,however,only regional oak forests are severely infested by M.raddei in Jilin and Liaoning provinces,adults of these populations show nearly perfectly synchronized emergences in every three years.It is speculated that regional population outbreak of M.raddei has possible relationships with special periodic emergence pattern of adults and local ecological factors.In order to explore potential ecological factors which result in regional outbreaks of this borer only in northeastern China,this paper mainly focuses on investigations at four aspects:（1）the analysis of genetic diversification and genetic structure among seven spatially separated populations（Kuandian County,Liaoning Province（LN）;Ningcheng County,Inner Mongolia（NMG）;Qixia City,Shandong Province（SD）;Xixia County,Henan Province（HN）;Hefei City,Anhui Province（AH）;Wenshan State,Yunnan Province（YNW）;Pingbian Dawei mountain,Yunnan Province（YNP））based on the combination of COI,COII and Cytb genes;（2）the field survey of variety and richness of natural enemies of M.raddei as well as the determination of cold resistance ability of larvae among periodical populations（LN;Meihekou City,Jilin Province（JL））and non-periodical populations（NMG,SD）.Based on the basis of these practical surveys,the mathematical models were established to explore the possible evolutionary process of initial protoperiodicity and further periodicity of M.raddei;（3）based on climatic factors at the spatial scale of China,current distribution of M.raddei together with host distribution of M.raddei,we evaluated the potential effect of hosts and climate on the local outbreak of this borer and the possiblity of spread and colonization of this borer in distribution-free areas;（4）the principal component analysis of abiotic and biotic factors which are responsible for natural population density of M.raddei.The results are as follows:1.From seven populations,we found a high haplotype and nucleotide diversity.Both haplotype networks and phylogenetic analyses demonstrate apparent genetic diversification between southern China（SC）population groups（AH,HN,YNP and YNW）and northern China（NC）population groups（LN,NMG and SD）.A set of 21 pairwise comparisons for Fst（pairwise fixation indices）and Nm（genetic flow index）showed significant genetic differentiation and limited gene flow among most populations except two pairs,Shandong（SD）and Liaoning（LN）,and Anhui（AH）and Henan（HN）.The two population pairs of SD and LN,AH and LN were found to show frequent genetic flow and lower genetic differentiation.This pattern suggested that the periodic emergence behaviour of the LN population could not be attributed to population isolation resulting from the absence of gene flow with other spatial populations,and that regional environmental factors might be responsible.AMOVA（Analysis of molecular variance）showed that the greater molecular genetic variation was found among populations.Based on neutrality tests and the mismatch distribution test,it is determined that the seven populations were stable and had not experienced any recent population expansion.The fact that all the sampled populations showed only unique haplotypes and lacked shared or ancestral haplotypes,as well as the non-star-like distribution of haplotype network for concatenated genes,collectively provided powerful evidence for the stable and isolated nature of most populations.The high genetic differentiation and spatial genetic structuring among the seven populations are both likely attributed to the beetle’s moderate flight capacity,dispersal by anthropic assistance,microclimate,as well as the geographic distances between sampling sites.2.Both the host–parasitoid interaction and extreme low temperature events were regarded as potential ecological factors which result in the priming of periodical emergence behaviour of M.raddei in northeastern China.Here,we proposed a hypothesis considering the two ecological factors.At the initial period of northeastern forests of China,M.raddei originally appeared due to speciation of other longhorned beetles or spread from other distribution regions,adults emerged every year and adult densities in different years were almost the same.Furthermore,due to hysteretic effects of natural enemies,there would lack in supression effects of natural enemies on different developmental stages of M.raddei for a long time,so accumulated mortality of broods of three emergence years all were very low.Consequently,the density of the 2^nd-3^rdinstar larvae,4^th-5^th instar larvae,mature larvae and pupae were almost the same in tree trunk.Over time,natural enemies of each developmental stage of M.raddei increased in terms of species amount and parasitization or predation rate,the mortality of 4^th-5^th instar larvae were much higher than that of other developmental stages of M.raddei because of the appearance of specialist parasitoids Dastarcus helophoroides and Cerchysiella raddeii Yang.Under the cumulative effect of host-parasitoid interaction,protoperiodicity gradually generated,in which the emergence adult density of one year was significantly higher than that of other two years.In early January 1959,the extreme low temperature event would result in an extinction of mature larvae and the partial survival of the 2^nd-3^rd instar larvae,and there would be significant differences in suppression effects of natural enemies on dominant and small populations of different age-classes with the influence of predator satiation and host-parasitoid interaction.These two ecological events might be the selection factors for the evolutionary of perfect periodicity from protoperiodicity.3.Under the current climatic conditions,the high suitable areas of M.raddei were mainly concentrated in Jilin,Liaoning,Beijing,Hebei,Tianjin and northern Zhejiang,whereas the suitable range did not include some distribution points obtained from the field survey in Yunnan Province.Under the influence of current host distribution,obvious distinction in host preference was observed between southern and northern China.Susceptible hosts Quercus mongolica Fischer and Quercus wutaishansea Mary distribute in the northern provinces,so M.raddei would show high suitability,whereas secondary preferred hosts mainly distribute in the southern provinces,consequently,this borer would show low and moderate suitability.The potential distribution of M.raddei based on the combination of climatic and host factors would be consistent with its actual distribution and infestation conditions,occupying all distribution points obtained from field surveys.Most of China showed climatic suitability for M.raddei except Xinjiang autonomous region and Qinghai Province,total suitable areas were larger than those only based on climatic or host factors.Therein,the range of low and moderate suitable regions became wider and the range of high suitable regions reduced.Host species and distribution are dominant ecological factors which determine the suitability level of M.raddei.Both preferable host plants and high suitability of climate are potential ecological factors which result in regional outbreak of M.raddei.In addition,all non-distribution areas of China are facing risks of dispersal and colonization of M.raddei.4.The first principal component of all factors had the maximum positive load（0.984）at average annual temperature,indicating that temperature is the most important factors associated with natural population density of M.raddei.All biotic factors had the maximum positive load at forest structure（0.819）and host species（0.669）,the results demonstrated that the population density of M.raddei would be higher in monoculture forests of Q.mongolica.Moreover,the first component of abiotic factors also had the maximum positive load（0.982）at annual average temperature.Meanwhile,the effects of host tree species and its DBH（diameter at breast height）on natural population density of M.raddei were examined using variance analysis.The results revealed that host plants and DBH had a comparatively significant effect on natural population density of M.raddei,and there exists interaction effect between host plant and its DBH.In conclusion,annual average temperature,host species and DBH,forest structure are essential factors which determine population development of M.raddei,the low density of this borer in southern China is attributed to possible resistance of local hosts to M.raddei.These results of the study could allow better insights into contribution factors to infestation prevalence of the borer,and also could provide the theoretical basis for ecological management of this borer in outbreak forests of northeastern China and proactive prevention actions in areas where the pest does not occur presently.