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Several studies have shown positive responses of parasitism to either host density or landscape complexity.Many semi-natural arable habitats(Grasslands,woodlands,and wetlands) have been shaped by centuries of farming in China and worldwide.Established parasitoid populations have been recognized to provide pest management due to their high rates of consumption and precedence in wheat fields.However,no previous experiments have manipulated host density in agricultural landscapes of various complexity.Here we report the results of a field experiment conducted to determine how agricultural landscape complexity affects cereal aphids and how host density and agricultural landscape complexity jointly affect the parasitism,hyper-parasitism,and species diversity of parasitoids.Parasitism and hyper-parasitism were assessed by experimentally adding cereal aphids at low and high densities to commercial wheat fields located in two different agricultural landscapes in order to detect parasitism therein.Results showed that landscape complexity did not influence population density in cereal aphids.Landscape structure also did not influence parasitism and hyper-parasitism,contrary to our expectations.Increased host densities caused a trend of increasing abundance and species diversity in parasitoids and hyperparasitoids.Aphidius avenae Haliday parasitized a significantly greater proportion of hosts at low-host densities,while the opposite effect occurred for Aphidius gifuensis Ashmead.The discrepancy in responses of these parasitoids and hyper-parasitoids to host density is discussed in relation to differences in morphological traits,body size,and historical characteristics.The specific composition of parasitoid and hyper-parasitoid assemblage could significantly alter parasitism at various host densities at which landscape complexity and habitat composition impart negligible influence.Future studies should both evaluate plants with a more diverse range of traits and behaviors in landscapes with non-crop herbivore densities and span over more years to fully understand the impact of agricultural landscapes on species diversity and biological control service of parasitoids.

Studies have shown that multiple agricultural structures could affect the dynamics and distribution of aphids,parasitoids and hyperparasitoid systems.In this article,we designed two spatial scales of agricultural structure and analyzed the affects of different agricultural landscape factors for wheat aphids,parasitoids and hyper parasitoids in two spatial scales in wheat fields in Ningxia Province,P.R.China.Through stepwise regression methods,we screened key landscape factors in two spatial scales that influenced wheat aphids,parasitoids and hyperparasitoids in different population growth periods.We then stimulated distribution and sequence of wheat aphid,parasitoids and hyperparasitoids with CANOCO software.As a result,we were able to study the influence of the landscape matrix and composition of surrounding landscapes on the proliferation of aphids,parasitoids and hyperparasitoids and analyze the dynamics of wheat aphids,parasitoids,hyper parasitoids and seven landscape factors(wheat fields,non-wheat crop fields,grassland,tree lands,bare area,plastic house,and other lands) in 95 fields over a two year periods(2009—2010).The composition and texture of the surrounding landscape were derived from high altitude photographs.Seven landscape variables,aggregated at two spatial scales ranging from 31400 m2 to 125600 m2,were measured from aerial photographs.Regression models incorporating within-field and seven landscape factors accounted for 37%—58% of the variety of aphids,parasitoids and hyperparasitoids in the wheat fields.Result showed a correlation between landscape factors and dynamics and distribution of aphids,parasitoids and hyperparasitoids.In immigration periods,bare areas greatly enhanced the immigration numbers of two wheat aphids while the employment of a plastic greenhouse sharply decreased the immigration number.In growth periods,grasslands and tree lands accelerated the population growth rate of two aphids species.As plastic greenhouse suppressed the population growth rate of aphids,however,plastic greenhouses,grasslands and tree lands accelerated the population growth rate of parasitoids(aphidiidae),which was greatly restrained by non-wheat crop habitat.Grasslands and tree lands accelerated the population growth rate of hyperparasitoids.Thus,we concluded that,in stable periods,the maximum population density was highest in the agricultural landscapes with the greatest percent of bare area.Both grasslands and tree lands increased the maximum population density of parasitoids and hyperparasitoids.The results in two spatial scales were similar.Overall,parasitoids and hyperparasitoids abundance and species diversity increased with increasing bare area in wheat fields and with increasing amounts of non-cultivated lands and increasing patchiness in the surrounding landscape.We attributed the discrepancy between the response of these parasotoids and hyperparasotoids to landscape factors in relation to differences in mophological and life history characteristics.The specific composition of parasitoid and hyperparasitoid assemblaged significantly alters parasitism at different levels of agricultural landscape composition.Future studies should evaluate more plants in landscape,non-crop herbivore densities and allot more time to the impact of agricultural landscapes on diversity and function parasitoid and hyperparasitoid.Major areas of study covered the construction and design of agricultural structure in successive spatial-temporal scale,which enhanced activity and predation of natural enemies in agricultural landscape and suppressed the pest population to the greatest extent.