Natural enemies: Functional aspects of local management in agricultural landscapes
Abstract: Agricultural intensification has raised the global food production but also caused major concerns about environmental and health effects, including contamination by pesticides. Pesticide applications may induce toxicity not only on the target pest species but especially on non-target species. Hence we need to replace pesticides with management that promote organisms that suppress pest population. Natural enemies as parasitoids, spiders and ground and rove beetle can reduce pest population and contribute to the ecosystem service biological control. In this thesis I therefore study how agricultural management and land-use intensity as well as landscape composition influence the distribution of natural enemies and their functional aspects in agricultural landscapes. The studies were done in an agricultural region of southern Sweden dominated by annual crop production. Two study systems were considered. One consisting of oilseed rape, pollen beetles and parasitoid wasps that are natural enemies of pollen beetles. The other of three common agricultural lands uses, ranging from intensively managed sugar beet fields, winter wheat fields to grasslands and predatory arthropods. The results highlight that management intensity is an important factor explaining the spatiotemporal distribution of natural enemies and their biological control potential. In a functional perspective, increasing management intensity with insecticide application reduces the biological control of pollen beetles. Moreover insecticide treatment also reduce emergence of pollen beetle parasitoids the following spring. The most important factor explaining the distribution of adult parasitoids was the density of pollen beetle larva. Increased land-use intensity influenced the composition of arthropod communities. Spiders that are sensitive to disturbance were more commonly emerging in grasslands while omnivorus rove beetles and predacious macropterous ground beetles were most common in winter wheat fields. The phenology of arthropod emergence differed between the land uses and the overall dispersal tendency was higher for those emerging in crop fields compared to those emerging in grasslands. The land-use intensity also influenced the trait composition. The average body length of emerging ground beetle communities was lower in crop fields than grasslands while the average body length of actively moving communities did not differ between land uses. Further the proportion of ground beetles with good flight ability or a carnivorous diet was higher in crop fields than grasslands. To conclude, this thesis contributes to important knowledge concerning the distribution of natural enemies in the agricultural landscape in relation to management intensity. This knowledge base will be needed to develop more sustainable agricultural systems, as approaches focusing on reducing pesticide inputs as organic farming and integrated pest management (IPM) are dependent on natural enemies and their functions to be successful.
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