Studies On The Arthropod Community Structure Inside Pine Shoot Tunnels Of Dioryctria Rubella And The Population Differentiation Of The Dominant Species

Dioryctria rubella is the most notorious pest boring damage to pine shoot and causes greateconomic losses in China. Unfortunately, controlling the pine shoot moth D. rubella is difficultdue to its concealed habitat doubled with overlapping of generations. Our studies on thearthropod community structure inside pine shoot tunnels have revealed that many arthropodspecies live in the tunnels of D. rubella. A species of Staphylinidae (identified as Placusapinearum) has been found most frequently inhabit in the pine shoot tunnel, we also found that ahigh degree of niche overlap (co-occurrence) between the pine shoot moth D. rubella and thebeetle P. pinearum, which triggered our interest as to whether this beetle species can be used asan effective vector to transport biological control agents for controlling pine shoot moths.To develop biological control programs, selection of suitable source colony from naturalenemy population must be focused firstly, because abundant genetic variation has beenconsidered an important factor for successful colonization and establishment of exotic biologicalcontrol agents. In general speaking, those well-adapted biotypes can expand its geographicranges or match to local climate conditions and colonize successfully in a new habitat. Especially,if the genetic divergence of natural enemy population or biological control agents from differentgeographic region is very weak we can select any source colony in the species gene pool.Conversely, if the genetic divergence is distinct, then, selection of local source colony may beappropriate.Considering the beetle P. pinearum is widely distributed in China, it is essential tounderstand the intraspecific differentiation, here, we examined P. pinearum collected from14different geographical populations in China, and analyzed the genetic diversity, genetic structureand phylogeographical relationship among the samples by using morphological and molecularmarkers. The aim of this study is to provide a reliable foundation for application this beetle as avector insect and to pave a new approach for biological controlling of those pine shoot borer. Themain results are as follows:1) The investigations on the rate of damaged pine tree in four sites with different conditionsand forest types around Nanjing areas were carried out. The results showed that D. rubella couldcause severer damage to the artificial pure plantations than the natural regeneration forest andcould cause severer damage to P. taeda than P. massoniana than P. thunbergii. There existsignificant negative linear relationship between diameter at breast height and the rate of damagedshoot of P. massoniana.2) An investigation was made on the population composition and niche of arthropodcommunity in the pine shoot tunnel bored by D. rubella. The populations of the arthropodcommunity were identified as3classe(sCollembola，Insecta，Arachnida),9order(sCollembola， Aranea, Acarina, Dermaptera, Psocoptera, Hemiptera, Thysanoptera, Coleoptera andHymenoptera)and13families(Poduridae, Acarina, Erigonidae, Staphylinidae, Silvanidae,Curculionidae, Scolytidae, Thripidae, Labiduridae, Formicidae, Braconidae, Pentatomidae,Psocidae). Among the13families, Acarina and Staphylinidae were the dominant populations.The spatiotemporal niche overlap calculated by Levins and Pianka formula was the largestbetween D. rubella and Staphylinidae, indicating their closest relationship. This informationpresented here could shed a light on using the specie in Staphylinidae as an effective vector totransport biological control agents for controlling pine shoot moths.3) The morphological characteristics of individuals of P. pinearum from14sampling sitescovering Jiangsu、Anhui、Hunan、Guangdong、Sichuan、Shannxi and Yunnan province in Chinawere observed with the help of optical microscope and scanning electron microscope (SEM). Thebeetle P. pinearum was identified as a new specie by the following combination of characters:male and female tergite8with one long median projection and two elongate lateral teeth;flagellum coiled about2times in bulbus; and spermatheca with small and spherical capsule,median portion of spermathecal stem twisted and φ-shaped. The morphological characteristics ofP. pinearum are mentioned above are different in form from the other recorded species. Thespecific name is, Placusa pinearum Gao, Ji, Liu, sp. nov, derived from the unique habitat, wherethe type species was collected.4) Seven measurements were obtained from87individuals of P. pinearum with an ocularmicrometer under the stereomicroscope at a magnification of40×or100×. These measurementsincluded head width (HW), head length (HL), pronotum width (PW), pronotum length (PL),elytra width (EW), elytra length (EL), and body length (BL). In addition, we found that the elytracolor of the specimens collected from YNKM and SCLS was black, whereas that of specimenscollected from other sampling sites was unanimously bronze yellow. The dendrogramconstructed with the UPGMA methods using Euclidean distances indicated that there was amorphological divergence of the populations in P. pinearum between southwestern and otherthree geographic regions (central, northwestern and eastern region).5) A portion of the mitochondrial cyochrome oxidase subunit I gene (965-nucleotidesequence) was obtained from56individuals and19haplotypes were identified based on28polymorphic sites in this region, and16S gene sequence was also obtained from37individualsand7haplotypes were identified based on6polymorphic sites in this region. The populationsfrom the southwestern region possessed the lowest nucleotide diversity (0.000622) and haplotypediversity (0.600), whereas those from the southern region possessed the highest nucleotidediversity (0.006390) and those from the northwestern region possessed the highest haplotypediversity (1.000). Our study showed that the significant genetic divergence among the differentpopulations in eastern China, meanwhile, absence of shared haplotypes, coupled with high Fstvalues, demonstrated significant genetic divergence between the populations from southwest andother four main geographical regions (central, southern northwestern and eastern region). Theresults of the genetic character of P. pinearum populations mentioned above can also besupported by the analysis using combining mtDNA16S with COI marker. 6)The relationship between morphological and genetic divergence was compared in this study,morphology-based tree shows that there was a morphological divergence of the populations in P.pinearum between southwestern and other three geographic regions. The median-joining networkanalysis indicated the absence of shared haplotype between the southwestern and the othergeographical regions. In addition, the pairwise Fst value between the southwestern and the otherswas significantly high, indicative of weak gene flow between these regions. It suggests thatmorphological divergence is generally in congruence with genetic divergence in P. pinearumpopulations. These findings also suggest that phenotypic traits could serve as a valid criterion forassessing the degree of genetic divergence in P. pinearum populations, especially whenmolecular data are not available.7) The genetic structure of population subdivision was examined by the analysis of molecularvariance (AMOVA), after the populations were grouped either by geographic locations or by hostspecies. The AMOVA results from our study showed that the majority of molecular variationwas within populations, whereas only a small part was among groups, it was conformed thatthere was no distinct geographical structure of genetic variation among different P. pinearumgroups. Moreover, the mantel test showed that there was no significant correlation between theFst values and geographical distances (km)(r=0.219, P=0.255>0.05).8) The demographic history of P. pinearum was inferred based on the Bayesian skyline plot,without any assumption of a particular demographic model. In the absence of a suitable intrinsiccalibration, we can apply the approximate2%per million year divergence rate for insect mtDNACOI gene. The Bayesian skyline analysis demonstrates that the P. pinearum populations in Chinahad undergone a major sudden population expansion starting from approximately around20,000years ago, which was also confirmed by the Fu’s test. The low genetic diversity of thesouthwestern populations suggested that it was colonized very recently and might becomeisolated either through dispersal from the eastern or south+central regions or by undergoingbottlenecks due to population size fluctuations. We infer that genetic divergence between thesouthwestern and other region populations might be explained by the consequence of topographicbarriers; that is, the southwestern samples sites (SCLS and YNKM) were in the HengduanMountains, the second largest mountain in China, in contrast to the hills or platforms in theeastern, central, or southern regions.