Biological rape pest control in spatio-temporally changing landscapes

Abstract

Community structure and trophic interactions depend on landscape context. We analysed trophic interactions of the pollen beetle (Meligethes aeneus) and its parasitoids on oilseed rape (Brassica napus) in 15 agricultural landscapes differing in structural complexity (∼50-100% arable land) and interannual changes of rape crop area (with ∼7% maximum expansion of rape crop area and ∼8% maximum contraction of rape crop area from year to year). A patch of potted rape plants was placed in the centre of each landscape for standardized measurement. Parasitism decreased and herbivory increased as the percentage of arable land in the surrounding landscape increased. Thus, semi-natural habitats appeared to support parasitoid populations contributing to the reduction of populations of the pollen beetle. In addition, parasitism decreased following rape crop expansion, and increased following rape crop contraction, indicating interannual dilution and concentration effects of the higher trophic level populations. When semi-natural habitat area dropped below a value of ∼20% of the landscape, or when the expansion of rape crop area between years exceeded ∼5% of the landscape, respectively, parasitism dropped below a threshold value of about 32-36%, below which success in classical biological control has never been reported. In a geographic scale analysis using five spatial scales ranging from 0.5 to 3 km diameter, parasitism and herbivory showed the best correlations with both the percentage of arable land and the percentage of interannually changing rape crops at the same spatial scales, i.e., landscape sectors of 1-2 km diameter, thereby suggesting that this 'functional spatial scale' indicates their dispersal abilities. © 2010 Springer Science+Business Media B.V.