Assessing the risks and benefits of flowering field edges. Strategic use of nectar sources to boost biological control

The intensification of agricultural production systems during the last decades hadaenormous impact on the landscape structure in agro-ecosystems. Landscape elements like hedges andvegetationalrichfieldmarginsdissapearedand crops are cultivated in large monocultures. To let beneficial insects play a role in these 'ecological deserts' and tofullfilltheir food requirements in form of pollen and nectar the establishment of flowering field edges gets increasingly popular.But not everything that flowers is naturally exclusively positive for beneficial insects.Pestinsects can profit from flowering field edges as well. In my PhD research Ianalyseda number of nectar plants with respect to their potential benefit for cabbage pests and/or their natural enemies. In field studies I observed theattractivityof nectar plants for pest insects and beneficial insects. In the laboratory I studied in how far the plant species differ in their nectar accessibility and their impact on insect longevity. In addition, I examined the impact of different nectar and honeydew sugars on the gustatory response and the longevity of the insects.Based on the results I found, I conducted field experiments with plants that provide food for either the herbivores or for the beneficial insects. I collected individuals of the diamondback mothPlutellaxylostella and its parasitoidDiadegmasemiclausum and tested them for their sugar content. The results indicated food uptake in the field for both species. I also could show in a field experiment, that suitable nectar plants, such as buckwheat, have an enormous positive impact on longevity and fecundity of the parasitoid D.semiclausum . In addition, I could demonstrate that nectar plants that selectively are used by herbivores, such asCentaureajacea by the cabbage whitePierisrapae , can lead to higher pest densities in adjacent cabbage plantings. My most important conclusion is therefore that a selective approach and a careful choice of plant species are necessary to achieve improved biological control by flowering field edges.