| Hover flies(Syrphidae:Diptera)are an economically important insect group worldwide that provide important ecosystem services including pollination and pest control.The long-range migration of hoverflies is best known in Europe.Howerer,the current evidence for hover fly migration in China is sparse,yet migration is critical critical to the success of pollination and the biological control of aphids,The present article is therefore addressed to investigate the migration of hoverfly Episyrphus balteatus(DeGeer),a widely-distributed aphidophagous hoverfly species in China,using searchlight trapping on a small island in the center of the Bohai Strait during 2003-2018,and several aspects of this species'migration including migration routes,the origin sources of immigrants,larval host plants of migrants,host plant range,population structure and demographic history of the hoverfly were studied by means of the trajectory analysis,natural abundance of stable isotopes(hydrogen and carbon)in wing membranes,the pollen attached on the migrated hoverflies and genetic markers,etc.The main results were summarized as follows1.Firstly confirmed that the existence of long-distance migration of E.balteatus in eastern China through a 16-year-period searchlight trapping data and the population dynamics of this speicies was determined.During 2015-2018,the number of E.balteatus hoverflies caught by searchlight trapping provided direct evidence that the species migrated across the Bohai Gulf waters.There was a large interannual and seasonal fluctuation in number of captured E.balteatus moths,with the highest value in 2009(5068)and lowest in 2018(365);the species undertook the long-distance migration from April to October each year,the peak of migration was generally occurred from late-May to June,late-August to early-September.According to the population dynamics,two different stages,such as,spring-summer migration stage(April to June),autumn migration stage(August to October),can be divided for the migration of E.balteatus.2.The possible sources areas of the immigrants in different migratory stages were explored by backward trajectory analysis.The results revealed that the spring-summer immigrants were mostly from northern China(i.e.,Shandong(61.6%)and Jiangsu(11.7%)provinces),while autumn immigrants were mainly from northeast China(i.e.,Liaoning(57.8%),Inner Mongolia(11.8%)and Jilin(3.6%)provinces).3.Tne significat aifferences in ?D values values between E.balteatus sampled in different migratory stages validated the above trajectory analysis hypothesis.The E.balteatus wings ?D values showed significant seasonal variability and they decreased from May to October.Spring migrants arriving in our study site generally had higher ?D values that indicated a southerly origin,while the ?D values of autumn migrants suggested that came from lower latitudes regions4.stable carbon isotope(?(?)C)was appliea to inter host plants of migrant's larva,the results showed that all of captured hoverflies in first migratory stage displayed the C3 isotope signature,while in the second migratory period the proportion of C4 host plants gradually increased5.The host plants range were clarified preliminarily by identifying the pollens attached on migratory hoverflies,a total of 44 pollen species were detected.Of the 27 pollen species identified from spring-summer northhward migrants,the main pollen species included Taraxacum mongolictum,Ailanthus altissima,Amorpha fruticosa,Chenopodium giganteum,the carrier rate of the four species of pollen up to 72%.Of the 17 pollen species attached on autumn southward migrants,the main pollen species included Artemisia L.,Chrysanthemum zawadskii,Ambrosia trifida,and the carrier rate of these species of pollen reach to 82.2%6.The presences of the pollen species with specific geographical location provide direct evidence for the migration routes of the E.balteatus.In the spring-summer migratory stage,the district distribution of Citrus L.,Sedum japonicum,Neoshirakia japonica,Flueggea virosa,Euonymus myrianthus,mainly in Central and Southern China,while some pollens like Chrysanthemum zawadskii and Adenophora trachelioides were detected in the autumn migratory stage,their district distribution mainly in Northeast China.These results further supported that the seasonal patterns of this migratory hoverfly that moving northwards in spring and return in autumn7.Non-significant genetic differentiation and genetic structure among all populations,implying large-scale migratory movements may drive to significant genetic mixing.The results based on four molecular makers all showed that high haplotype diversity and low nucleotide diversity occurred in the E.balteatus populations,a finding that is typical for migratory species.All the pairwise FST values between different localities were below 0.05.The analysis of molecular variance(AMOVA)based on four markers showed the most of genetic variation was from individuals.Mismatch distribution analysis showed unimodal distribution.The value of Tajima's D and Fu's Fs neutrality tests were both significant negative(P<0.05).The results suggested there might be a population expansion in recent time.Taken together,our study firstly elucidated that annual seasonal migration of E.balteatus in eastern China by means of a variety of technical methods and means,where migrate into northern regions begin around April and are followed by southerly migrations during August-October.The hoverfly migration strategies suggested this speices may have substantial implications for maintaining pollination and biological control ecosystem services in China,and provide a theoretical basis for exploring underlying mechanism in provision of beneficial ecosystem services,as well as formulating reasonable integrated management strategies in this important benifical group. |
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