| The rapid expansion of intensive cultivation patterns and the homogenization of ecosystems have led to a decrease or even loss of biodiversity,and to a certain extent,have changed the living environment,food resources and pest control ability of farmland pests and their natural enemies.As an important forage crop in Ningxia,the yield and quality of alfalfa were restricted by the damage of aphid populations.There are many studies on the population dynamics of aphids and their natural enemies at the regional scale,but its ecological regulation mechanism at the large scale not clear.This study was conducted in an alfalfa grassland within the Ningxia Hui autonomous region,(1)the species of aphid parasitoids in alfalfa grassland within Ningxia were investigated and identified,to construct a quantitative aphid-parasitoids food web;(2)the occurrence of pests(aphids)and their natural enemies(predatory and parasitic natural enemies)were systematically monitored in alfalfa grassland;(3)the biological control efficiency of different natural enemy combinations was quantitatively evaluated by field cage experiments.Meanwhile,combined with multidisciplinary techniques such as 3S(RS,GIS,GPS)space technology,landscape ecology,and insect population ecology,the effects of agricultural landscape patterns on the structure of aphid-parasitoid food-web,the population density of aphids and their natural enemies(predatory and parasitic natural enemies),and the diversity of natural enemies,as well as the biological control efficiency of different natural enemy combinations were explored at four landscape scales(0.5,1.0,1.5 and 2.0 km).The main results of this study were listed as follows:1.The species composition of the main primary parasitoids and hyper-parasitoids of aphids in alfalfa grassland within Ningxia was clarified.The primary parasitoids parasitizing Acythosiphon pisum were Aphidius eadyi,Aphidius ervi(dominant species),Praon barbatum,and Praon volucre.The primary parasitoid parasitizing Therioaphis trifolii were Praon exsoletum and Trioxys complanatus(dominant species).Nine species of hyper-parasitoids were found,including Alloxysta brevis Thomson,Alloxysta circumscripta,Alloxysta fracticornis Thomson,Alloxysta victrix Westwood,Asaphes suspensus Nees,Asaphes vulgaris Walker,Dendrocerus sp.,Pachyneuron sp.,Pachyneuron aphidis Bouche(dominant species).2.The occurrence of aphids in alfalfa grassland within Ningxia was mainly caused by mixed aphid populations,but the dominant species of aphids in mountainous areas were mostly Therioaphis trifolii,and the dominant species of aphids in desert areas and oasis areas were mostly Acythosiphon pisum.This study found that the main predator groups in alfalfa grassland and surrounding weed strips in different regions of Ningxia were ladybeetles and lacewings.Six ladybeetles were found,including Hippodamia variegate Goeze,Propylaea japonica Thunberg,Harmonia axyridis Pallas,Coccinella septempunctata Linnaeus,Adalia bipunctata Linnaeus,and Coccinella trifasciata Linnaeus.2 lacewings were found in Ningxia,including Chrysoperla nipponensis Okamoto and Chrysopa phyllochroma Wesmael.The parasitism rate fluctuated with the aphid population,and the parasitism rates in mountainous areas,oasis areas,and desert areas were 69.3%,37.6%,and 42.7%,respectively.3.The proportion of alfalfa had a significant negative effect on the abundance of aphid populations.Landscape diversity had a significant positive effect on the abundance of predatory natural enemies,mainly in response to the number of ladybeetles(ladybeetles were positively correlated with landscape diversity).Staple crops had a significant positive effect on predatory natural enemies,and this relationship was strongest at a scale of 1.5 km.Landscape diversity had the most obvious effects on the abundance of parasitoids,the number of mummified aphids and the parasitism rate,and all of them were negatively correlated,most of which were the strongest at the scale of 1.5 km.The abundance of parasitoids,the number of mummified aphids and the parasitism rate were positively correlated with non-crop habitats,but negatively correlated with the proportion of alfalfa.Compared with the plain area(oasis area and desert area),the food web structure of aphid-primary parasitoid and the primary parasitoid-hyper parasitoid were more complex and stable in the mountainous area.4.The primary parasitoid-hyper parasitoid food web parameters(generality,interaction evenness,linkage density,and vulnerability)were positively correlated with non-crop habitats,and this relationship was strongest at a scale of 2.0 km.Food web parameters(generality,interaction evenness,linkage density,weighted connectance and vulnerability)were significantly positively correlated with the parasitism rate.With the increase of the proportion of non-crop habitats,the parasitism rate and hyper-parasitism rate of alfalfa fields increased.This correlation is consistent on all four scales.(5)The control efficiency of parasitic natural enemies was consistently higher in mountainous areas compared to the oasis area and desert area,The control efficiency of predatory natural enemies was consistently higher in desert areas,followed by the oasis area and mountainous area.The average control efficiency of parasitic,predatory,and mixed natural enemies against aphids was 67.7%,83.3%,and 92.2%,respectively.Non-crop habitats had significant effects on parasitoid abundance and their corresponding efficiency of biological control in the three regions.The habitat of maize had a significant effect on predators’abundance in oasis and desert areas.The habitat of maize had a significant effect on the biological control efficiency of predators in mountainous and desert areas.Edge density only had a significant effect on the biological control efficiency of mixed natural enemies in oasis areas.This study explored the response of the aphid-parasitoid food web,aphid population,natural enemy population,and biological control efficiency to agricultural landscape pattern in alfalfa grassland within Ningxia from the perspective of agricultural landscape pattern.The study found that staple crops and noncrop habitats around alfalfa play an important role in the conservation and pest control of natural enemies,which will optimize the agricultural landscape structure in the alfalfa farmland ecosystem in the future,promote the conservation and management of natural enemy communities,give full play to the ecosystem service function of natural enemies,and provide a scientific basis for the theory and model of biological control of crop pests. |
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